Copyright (C) 1989, 1991 Free Software Foundation, Inc. 675 Mass Ave, Cambridge, MA 02139, USA Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.
The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users. This General Public License applies to most of the Free Software Foundation's software and to any other program whose authors commit to using it. (Some other Free Software Foundation software is covered by the GNU Library General Public License instead.) You can apply it to your programs, too.
When we speak of free software, we are referring to freedom, not price. Our General Public Licenses are designed to make sure that you have the freedom to distribute copies of free software (and charge for this service if you wish), that you receive source code or can get it if you want it, that you can change the software or use pieces of it in new free programs; and that you know you can do these things.
To protect your rights, we need to make restrictions that forbid anyone to deny you these rights or to ask you to surrender the rights. These restrictions translate to certain responsibilities for you if you distribute copies of the software, or if you modify it.
For example, if you distribute copies of such a program, whether gratis or for a fee, you must give the recipients all the rights that you have. You must make sure that they, too, receive or can get the source code. And you must show them these terms so they know their rights.
We protect your rights with two steps: (1) copyright the software, and (2) offer you this license which gives you legal permission to copy, distribute and/or modify the software.
Also, for each author's protection and ours, we want to make certain that everyone understands that there is no warranty for this free software. If the software is modified by someone else and passed on, we want its recipients to know that what they have is not the original, so that any problems introduced by others will not reflect on the original authors' reputations.
Finally, any free program is threatened constantly by software patents. We wish to avoid the danger that redistributors of a free program will individually obtain patent licenses, in effect making the program proprietary. To prevent this, we have made it clear that any patent must be licensed for everyone's free use or not licensed at all.
The precise terms and conditions for copying, distribution and modification follow.
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This section is intended to make thoroughly clear what is believed to be a consequence of the rest of this License.
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If you develop a new program, and you want it to be of the greatest possible use to the public, the best way to achieve this is to make it free software which everyone can redistribute and change under these terms.
To do so, attach the following notices to the program. It is safest to attach them to the start of each source file to most effectively convey the exclusion of warranty; and each file should have at least the "copyright" line and a pointer to where the full notice is found.
one line to give the program's name and an idea of what it does. Copyright (C) 19yy name of author This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.
Also add information on how to contact you by electronic and paper mail.
If the program is interactive, make it output a short notice like this when it starts in an interactive mode:
Gnomovision version 69, Copyright (C) 19yy name of author Gnomovision comes with ABSOLUTELY NO WARRANTY; for details type `show w'. This is free software, and you are welcome to redistribute it under certain conditions; type `show c' for details.
The hypothetical commands `show w' and `show c' should show the appropriate parts of the General Public License. Of course, the commands you use may be called something other than `show w' and `show c'; they could even be mouse-clicks or menu items--whatever suits your program.
You should also get your employer (if you work as a programmer) or your school, if any, to sign a "copyright disclaimer" for the program, if necessary. Here is a sample; alter the names:
Yoyodyne, Inc., hereby disclaims all copyright interest in the program `Gnomovision' (which makes passes at compilers) written by James Hacker. signature of Ty Coon, 1 April 1989 Ty Coon, President of Vice
This General Public License does not permit incorporating your program into proprietary programs. If your program is a subroutine library, you may consider it more useful to permit linking proprietary applications with the library. If this is what you want to do, use the GNU Library General Public License instead of this License.
Most of the GNU Emacs text editor is written in the programming language called Emacs Lisp. You can write new code in Emacs Lisp and install it as an extension to the editor. However, Emacs Lisp is more than a mere "extension language"; it is a full computer programming language in its own right. You can use it as you would any other programming language.
Because Emacs Lisp is designed for use in an editor, it has special features for scanning and parsing text as well as features for handling files, buffers, displays, subprocesses, and so on. Emacs Lisp is closely integrated with the editing facilities; thus, editing commands are functions that can also conveniently be called from Lisp programs, and parameters for customization are ordinary Lisp variables.
This manual describes Emacs Lisp, presuming considerable familiarity with the use of Emacs for editing. (See The GNU Emacs Manual, for this basic information.) Generally speaking, the earlier chapters describe features of Emacs Lisp that have counterparts in many programming languages, and later chapters describe features that are peculiar to Emacs Lisp or relate specifically to editing.
This is edition 2.1.
This manual has gone through numerous drafts. It is nearly complete but not flawless. There are a few sections which are not included, either because we consider them secondary (such as most of the individual modes) or because they are yet to be written.
Because we are not able to deal with them completely, we have left out several parts intentionally. This includes most references to VMS and all information relating Sunview. (The Free Software Foundation expends no effort on support for Sunview, since we believe users should use the free X window system rather than proprietary window systems.)
The manual should be fully correct in what it does cover, and it is therefore open to criticism on anything it says--from specific examples and descriptive text, to the ordering of chapters and sections. If something is confusing, or you find that you have to look at the sources or experiment to learn something not covered in the manual, then perhaps the manual should be fixed. Please let us know.
As you use the manual, we ask that you mark pages with corrections so you can later look them up and send them in. If you think of a simple, real life example for a function or group of functions, please make an effort to write it up and send it in. Please reference any comments to the chapter name, section name, and function name, as appropriate, since page numbers and chapter and section numbers will change. Also state the number of the edition which you are criticizing.
Please mail comments and corrections to
bug-lisp-manual@prep.ai.mit.edu
--Bil Lewis, Dan LaLiberte, Richard Stallman
Lisp (LISt Processing language) was first developed in the late 1950s at the Massachusetts Institute of Technology for research in artificial intelligence. The great power of the Lisp language makes it superior for other purposes as well, such as writing editing commands.
Dozens of Lisp implementations have been built over the years, each with its own idiosyncrasies. Many of them were inspired by Maclisp, which was written in the 1960's at MIT's Project MAC. Eventually the implementors of the descendents of Maclisp came together and developed a standard for Lisp systems, called Common Lisp.
GNU Emacs Lisp is largely inspired by Maclisp, and a little by Common Lisp. If you know Common Lisp, you will notice many similarities. However, many of the features of Common Lisp have been omitted or simplified in order to reduce the memory requirements of GNU Emacs. Sometimes the simplifications are so drastic that a Common Lisp user might be very confused. We will occasionally point out how GNU Emacs Lisp differs from Common Lisp. If you don't know Common Lisp, don't worry about it; this manual is self-contained.
This section explains the notational conventions that are used in this manual. You may want to skip this section and refer back to it later.
Throughout this manual, the phrases "the Lisp reader" and "the Lisp printer" are used to refer to those routines in Lisp that convert textual representations of Lisp objects into actual objects, and vice versa. See section Printed Representation and Read Syntax, for more details. You, the person reading this manual, are thought of as "the programmer" and are addressed as "you". "The user" is the person who uses Lisp programs including those you write.
Examples of Lisp code appear in this font or form: (list 1 2
3). Names that represent arguments or metasyntactic variables appear
in this font or form: first-number.
nil and t
In Lisp, the symbol nil is overloaded with three meanings: it
is a symbol with the name `nil'; it is the logical truth value
false; and it is the empty list--the list of zero elements.
When used as a variable, nil always has the value nil.
As far as the Lisp reader is concerned, `()' and `nil' are
identical: they stand for the same object, the symbol nil. The
different ways of writing the symbol are intended entirely for human
readers. After the Lisp reader has read either `()' or `nil',
there is no way to determine which representation was actually written
by the programmer.
In this manual, we use () when we wish to emphasize that it
means the empty list, and we use nil when we wish to emphasize
that it means the truth value false. That is a good convention to use
in Lisp programs also.
(cons 'foo ()) ; Emphasize the empty list (not nil) ; Emphasize the truth value false
In contexts where a truth value is expected, any non-nil value
is considered to be true. However, t is the preferred way
to represent the truth value true. When you need to choose a
value which represents true, and there is no other basis for
choosing, use t. The symbol t always has value t.
In Emacs Lisp, nil and t are special symbols that always
evaluate to themselves. This is so that you do not need to quote them
to use them as constants in a program. An attempt to change their
values results in a setting-constant error. See section Accessing Variable Values.
A Lisp expression that you can evaluate is called a form. Evaluating a form always produces a result, which is a Lisp object. In the examples in this manual, this is indicated with `=>':
(car '(1 2))
=> 1
You can read this as "(car '(1 2)) evaluates to 1".
When a form is a macro call, it expands into a new form for Lisp to evaluate. We show the result of the expansion with `==>'. We may or may not show the actual result of the evaluation of the expanded form.
(third '(a b c))
==> (car (cdr (cdr '(a b c))))
=> c
Sometimes to help describe one form we show another form which produces identical results. The exact equivalence of two forms is indicated with `=='.
(make-sparse-keymap) == (list 'keymap)
Many of the examples in this manual print text when they are
evaluated. If you execute the code from an example in a Lisp
Interaction buffer (such as the buffer `*scratch*'), the printed
text is inserted into the buffer. If the example is executed by other
means (such as by evaluating the function eval-region), the text
printed is usually displayed in the echo area. You should be aware that
text displayed in the echo area is truncated to a single line.
In examples that print text, the printed text is indicated with
`-|', irrespective of how the form is executed. The value
returned by evaluating the form (here bar) follows on a separate
line.
(progn (print 'foo) (print 'bar))
-| foo
-| bar
=> bar
Some examples cause errors to be signaled. In them, the error message (which always appears in the echo area) is shown on a line starting with `error-->'. Note that `error-->' itself does not appear in the echo area.
(+ 23 'x) error--> Wrong type argument: integer-or-marker-p, x
Some examples show modifications to text in a buffer, with "before" and "after" versions of the text. In such cases, the entire contents of the buffer in question are included between two lines of dashes containing the buffer name. In addition, the location of point is shown as `-!-'. (The symbol for point, of course, is not part of the text in the buffer; it indicates the place between two characters where point is located.)
---------- Buffer: foo ----------
This is the -!-contents of foo.
---------- Buffer: foo ----------
(insert "changed ")
=> nil
---------- Buffer: foo ----------
This is the changed -!-contents of foo.
---------- Buffer: foo ----------
Functions, variables, macros, commands, user options, and special forms are described in this manual in a uniform format. The first line of a description contains the name of the item followed by its arguments, if any. The category--function, variable, or whatever--is printed next to the right margin. The description follows on succeeding lines, sometimes with examples.
In a function description, the name of the function being described appears first. It is followed on the same line by a list of parameters. The names used for the parameters are also used in the body of the description.
The appearance of the keyword &optional in the parameter list
indicates that the arguments for subsequent parameters may be omitted
(omitted parameters default to nil). Do not write
&optional when you call the function.
The keyword &rest (which will always be followed by a single
parameter) indicates that any number of arguments can follow. The value
of the single following parameter will be a list of all these arguments.
Do not write &rest when you call the function.
Here is a description of an imaginary function foo:
Function: foo integer1 &optional integer2 &rest integers
The function foo subtracts integer1 from integer2,
then adds all the rest of the arguments to the result. If integer2
is not supplied, then the number 19 is used by default.
(foo 1 5 3 9)
=> 16
(foo 5)
=> 14
More generally,
(foo w x y...) == (+ (- x w) y...)
Any parameter whose name contains the name of a type (e.g., integer, integer1 or buffer) is expected to be of that type. A plural of a type (such as buffers) often means a list of objects of that type. Parameters named object may be of any type. (See section Lisp Data Types, for a list of Emacs object types.) Parameters with other sorts of names (e.g., new-file) are discussed specifically in the description of the function. In some sections, features common to parameters of several functions are described at the beginning.
See section Lambda Expressions, for a more complete description of optional and rest arguments.
Command, macro, and special form descriptions have the same format, but the word `Function' is replaced by `Command', `Macro', or `Special Form', respectively. Commands are simply functions that may be called interactively; macros process their arguments differently from functions (the arguments are not evaluated), but are presented the same way.
Special form descriptions use a more complex notation to specify
optional and repeated parameters because they can break the argument
list down into separate arguments in more complicated ways.
`[optional-arg]' means that optional-arg is
optional and `repeated-args...' stands for zero or more
arguments. Parentheses are used when several arguments are grouped into
additional levels of list structure. Here is an example:
Special Form: count-loop (var [from to [inc]]) body...
This imaginary special form implements a loop that executes the body forms and then increments the variable var on each iteration. On the first iteration, the variable has the value from; on subsequent iterations, it is incremented by 1 (or by inc if that is given). The loop exits before executing body if var equals to. Here is an example:
(count-loop (i 0 10) (prin1 i) (princ " ") (prin1 (aref vector i)) (terpri))
If from and to are omitted, then var is bound to
nil before the loop begins, and the loop exits if var is
non-nil at the beginning of an iteration. Here is an example:
(count-loop (done)
(if (pending)
(fixit)
(setq done t)))
In this special form, the arguments from and to are optional, but must both be present or both absent. If they are present, inc may optionally be specified as well. These arguments are grouped with the argument var into a list, to distinguish them from body, which includes all remaining elements of the form.
A variable is a name that can hold a value. Although any variable can be set by the user, certain variables that exist specifically so that users can change them are called user options. Ordinary variables and user options are described using a format like that for functions except that there are no arguments.
Here is a description of the imaginary electric-future-map
variable.
The value of this variable is a full keymap used by electric command future mode. The functions in this map will allow you to edit commands you have not yet thought about executing.
User option descriptions have the same format, but `Variable' is replaced by `User Option'.
This manual was written by Robert Krawitz, Bil Lewis, Dan LaLiberte, Richard M. Stallman and Chris Welty, the volunteers of the GNU manual group, in an effort extending over several years. Robert J. Chassell helped to review and edit the manual, with the support of the Defense Advanced Research Projects Agency, ARPA Order 6082, arranged by Warren A. Hunt, Jr. of Computational Logic, Inc.
Corrections were supplied by Karl Berry, Jim Blandy, Bard Bloom, David Boyes, Alan Carroll, David A. Duff, Beverly Erlebacher, David Eckelkamp, Eirik Fuller, Eric Hanchrow, George Hartzell, Nathan Hess, Dan Jacobson, Jak Kirman, Bob Knighten, Frederick M. Korz, Joe Lammens, K. Richard Magill, Brian Marick, Roland McGrath, Skip Montanaro, John Gardiner Myers, Arnold D. Robbins, Raul Rockwell, Shinichirou Sugou, Kimmo Suominen, Edward Tharp, Bill Trost, Jean White, Matthew Wilding, Carl Witty, Dale Worley, Rusty Wright, and David D. Zuhn.
A Lisp object is a piece of data used and manipulated by Lisp programs. For our purposes, a type or data type is a set of possible objects.
Every object belongs to at least one type. Objects of the same type have similar structures and may usually be used in the same contexts. Types can overlap, and objects can belong to two or more types. Consequently, we can ask whether an object belongs to a particular type, but not for "the" type of an object.
A few fundamental object types are built into Emacs. These, from which all other types are constructed, are called primitive types. Each object belongs to one and only one primitive type. These types include integer, float, cons, symbol, string, vector, subr, byte-code function, and several special types, such as buffer, that are related to editing. (See section Editing Types.)
Each primitive type has a corresponding Lisp function that checks whether an object is a member of that type.
Note that Lisp is unlike many other languages in that Lisp objects are self-typing: the primitive type of the object is implicit in the object itself. For example, if an object is a vector, it cannot be treated as a number because Lisp knows it is a vector, not a number.
In most languages, the programmer must declare the data type of each variable, and the type is known by the compiler but not represented in the data. Such type declarations do not exist in Emacs Lisp. A Lisp variable can have any type of value, and remembers the type of any value you store in it.
This chapter describes the purpose, printed representation, and read syntax of each of the standard types in GNU Emacs Lisp. Details on how to use these types can be found in later chapters.
The printed representation of an object is the format of the
output generated by the Lisp printer (the function print) for
that object. The read syntax of an object is the format of the
input accepted by the Lisp reader (the function read) for that
object. Most objects have more than one possible read syntax. Some
types of object have no read syntax; except for these cases, the printed
representation of an object is also a read syntax for it.
In other languages, an expression is text; it has no other form. In Lisp, an expression is primarily a Lisp object and only secondarily the text that is the object's read syntax. Often there is no need to emphasize this distinction, but you must keep it in the back of your mind, or you will occasionally be very confused.
Every type has a printed representation. Some types have no read
syntax, since it may not make sense to enter objects of these types
directly in a Lisp program. For example, the buffer type does not have
a read syntax. Objects of these types are printed in hash
notation: the characters `#<' followed by a descriptive string
(typically the type name followed by the name of the object), and closed
with a matching `>'. Hash notation cannot be read at all, so the
Lisp reader signals the error invalid-read-syntax whenever a
`#<' is encountered.
(current-buffer)
=> #<buffer objects.texi>
When you evaluate an expression interactively, the Lisp interpreter
first reads the textual representation of it, producing a Lisp object,
and then evaluates that object (see section Evaluation). However,
evaluation and reading are separate activities. Reading returns the
Lisp object represented by the text that is read; the object may or may
not be evaluated later. See section Input Functions, for a description of
read, the basic function for reading objects.
A comment is text that is written in a program only for the sake of humans that read the program, and that has no effect on the meaning of the program. In Lisp, a comment starts with a semicolon (`;') if it is not within a string or character constant, and continues to the end of line. Comments are discarded by the Lisp reader, and do not become part of the Lisp objects which represent the program within the Lisp system.
See section Tips on Writing Comments, for conventions for formatting comments.
There are two general categories of types in Emacs Lisp: those having to do with Lisp programming, and those having to do with editing. The former are provided in many Lisp implementations, in one form or another. The latter are unique to Emacs Lisp.
Integers are the only kind of number in GNU Emacs Lisp, version 18.
The range of values for integers is -8388608 to
8388607 (24 bits; i.e.,
to
on most machines, but is 25 or 26 bits on some systems. It is important
to note that the Emacs Lisp arithmetic functions do not check for
overflow. Thus (1+ 8388607) is -8388608 on 24-bit
implementations.
The read syntax for numbers is a sequence of (base ten) digits with an optional sign. The printed representation produced by the Lisp interpreter never has a leading `+'.
-1 ; The integer -1.
1 ; The integer 1.
+1 ; Also the integer 1.
16777217 ; Also the integer 1!
; (on a 24-bit or 25-bit implementation)
See section Numbers, for more information.
Emacs version 19 supports floating point numbers, if compiled with the
macro LISP_FLOAT_TYPE defined. The precise range of floating
point numbers is machine-specific.
The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent.
See section Numbers, for more information.
A character in Emacs Lisp is nothing more than an integer. In other words, characters are represented by their character codes. For example, the character A is represented as the integer 65.
Individual characters are not often used in programs. It is far more common to work with strings, which are sequences composed of characters. See section String Type.
Characters in strings, buffers, and files are currently limited to the range of 0 to 255. If an arbitrary integer is used as a character for those purposes, only the lower eight bits are significant. Characters that represent keyboard input have a much wider range.
Since characters are really integers, the printed representation of a character is a decimal number. This is also a possible read syntax for a character, but writing characters that way in Lisp programs is a very bad idea. You should always use the special read syntax formats that Emacs Lisp provides for characters. These syntax formats start with a question mark.
The usual read syntax for alphanumeric characters is a question mark followed by the character; thus, `?A' for the character A, `?B' for the character B, and `?a' for the character a.
For example:
?Q => 81 ?q => 113
You can use the same syntax for punctuation characters, but it is often a good idea to add a `\' to prevent Lisp mode from getting confused. For example, `?\ ' is the way to write the space character. If the character is `\', you must use a second `\' to quote it: `?\\'.
You can express the characters control-g, backspace, tab, newline, vertical tab, formfeed, return, and escape as `?\a', `?\b', `?\t', `?\n', `?\v', `?\f', `?\r', `?\e', respectively. Those values are 7, 8, 9, 10, 11, 12, 13, and 27 in decimal. Thus,
?\a => 7 ; C-g ?\b => 8 ; backspace, BS, C-h ?\t => 9 ; tab, TAB, C-i ?\n => 10 ; newline, LFD, C-j ?\v => 11 ; vertical tab, C-k ?\f => 12 ; formfeed character, C-l ?\r => 13 ; carriage return, RET, C-m ?\e => 27 ; escape character, ESC, C-[ ?\\ => 92 ; backslash character, \
These sequences which start with backslash are also known as escape sequences, because backslash plays the role of an escape character, but they have nothing to do with the character ESC.
Control characters may be represented using yet another read syntax. This consists of a question mark followed by a backslash, caret, and the corresponding non-control character, in either upper or lower case. For example, either `?\^I' or `?\^i' may be used as the read syntax for the character C-i, the character whose value is 9.
Instead of the `^', you can use `C-'; thus, `?\C-i' is equivalent to `?\^I' and to `?\^i':
?\^I => 9
?\C-I => 9
For use in strings and buffers, you are limited to the control characters that exist in ASCII, but for keyboard input purposes, you can turn any character into a control character with `C-'. The character codes for these characters include the 2**22 bit as well as the code for the non-control character. Ordinary terminals have no way of generating non-ASCII control characters, but you can generate them straightforwardly using an X terminal.
The DEL key can be considered and written as Control-?:
?\^? => 127
?\C-? => 127
When you represent control characters to be found in files or strings, we recommend the `^' syntax; but when you refer to keyboard input, we prefer the `C-' syntax. This does not affect the meaning of the program, but may guide the understanding of people who read it.
A meta character is a character typed with the META key. The integer that represents such a character has the 2**23 bit set (which on most machines makes it a negative number). We use high bits for this and other modifiers to make possible a wide range of basic character codes.
In a string, the 2**7 bit indicates a meta character, so the meta characters that can fit in a string have codes in the range from 128 to 255, and are the meta versions of the ordinary ASCII characters. (In Emacs versions 18 and older, this convention was used for characters outside of strings as well.)
The read syntax for meta characters uses `\M-'. For example, `?\M-A' stands for M-A. You can use `\M-' together with octal codes, `\C-', or any other syntax for a character. Thus, you can write M-A as `?\M-A', or as `?\M-\101'. Likewise, you can write C-M-b as `?\M-\C-b', `?\C-\M-b', or `?\M-\002'.
The shift modifier is used in indicating the case of a character in special circumstances. The case of an ordinary letter is indicated by its character code as part of ASCII, but ASCII has no way to represent whether a control character is upper case or lower case. Emacs uses the 2**21 bit to indicate that the shift key was used for typing a control character. This distinction is possible only when you use X terminals or other special terminals; ordinary terminals do not indicate the distinction to the computer in any way.
The X Window system defines three other modifier bits that can be set in a character: hyper, super and alt. The syntaxes for these bits are `\H-', `\s-' and `\A-'. Thus, `?\H-\M-\A-x' represents Alt-Hyper-Meta-x. Numerically, the bit values are 2**18 for alt, 2**19 for super and 2**20 for hyper.
Finally, the most general read syntax consists of a question mark
followed by a backslash and the character code in octal (up to three
octal digits); thus, `?\101' for the character A,
`?\001' for the character C-a, and ?\002 for the
character C-b. Although this syntax can represent any ASCII
character, it is preferred only when the precise octal value is more
important than the ASCII representation.
?\012 => 10 ?\n => 10 ?\C-j => 10 ?\101 => 65 ?A => 65
A backslash is allowed, and harmless, preceding any character without a special escape meaning; thus, `?\A' is equivalent to `?A'. There is no reason to use a backslash before most such characters. However, any of the characters `()\|;'`"#.,' should be preceded by a backslash to avoid confusing the Emacs commands for editing Lisp code. Whitespace characters such as space, tab, newline and formfeed should also be preceded by a backslash. However, it is cleaner to use one of the easily readable escape sequences, such as `\t', instead of an actual control character such as a tab.
A sequence is a Lisp object that represents an ordered set of elements. There are two kinds of sequence in Emacs Lisp, lists and arrays. Thus, an object of type list or of type array is also considered a sequence.
Arrays are further subdivided into strings and vectors. Vectors can hold elements of any type, but string elements must be characters in the range from 0 to 255. However, the characters in a string can have text properties; vectors do not support text properties even when their elements happen to be characters.
Lists, strings and vectors are different, but they have important
similarities. For example, all have a length l, and all have
elements which can be indexed from zero to l minus one. Also,
several functions, called sequence functions, accept any kind of
sequence. For example, the function elt can be used to extract
an element of a sequence, given its index. See section Sequences, Arrays, and Vectors.
It is impossible to read the same sequence twice, in the sense of
eq (see section Equality Predicates), since sequences are always
created anew upon reading. There is one exception: the empty list
() always stands for the same object, nil.
A list is a series of cons cells, linked together. A cons cell is an object comprising two pointers named the CAR and the CDR. Each of them can point to any Lisp object, but when the cons cell is part of a list, the CDR points either to another cons cell or to the empty list. See section Lists, for functions that work on lists.
The names CAR and CDR have only historical meaning now. The
original Lisp implementation ran on an IBM 704 computer which
divided words into two parts, called the "address" part and the
"decrement"; CAR was an instruction to extract the contents of
the address part of a register, and CDR an instruction to extract
the contents of the decrement. By contrast, "cons cells" are named
for the function cons that creates them, which in turn is named
for its purpose, the construction of cells.
Because cons cells are so central to Lisp, we also have a word for "an object which is not a cons cell". These objects are called atoms.
The read syntax and printed representation for lists are identical, and consist of a left parenthesis, an arbitrary number of elements, and a right parenthesis.
Upon reading, any object inside the parentheses is made into an
element of the list. That is, a cons cell is made for each element.
The CAR of the cons cell points to the element, and its CDR
points to the next cons cell which holds the next element in the list.
The CDR of the last cons cell is set to point to nil.
A list can be illustrated by a diagram in which the cons cells are
shown as pairs of boxes. (The Lisp reader cannot read such an
illustration; unlike the textual notation, which can be understood both
humans and computers, the box illustrations can only be understood by
humans.) The following represents the three-element list (rose
violet buttercup):
___ ___ ___ ___ ___ ___
|___|___|--> |___|___|--> |___|___|--> nil
| | |
| | |
--> rose --> violet --> buttercup
In the diagram, each box represents a slot that can refer to any Lisp object. Each pair of boxes represents a cons cell. Each arrow is a reference to a Lisp object, either an atom or another cons cell.
In this example, the first box, the CAR of the first cons cell,
refers to or "contains" rose (a symbol). The second box, the
CDR of the first cons cell, refers to the next pair of boxes, the
second cons cell. The CAR of the second cons cell refers to
violet and the CDR refers to the third cons cell. The
CDR of the third (and last) cons cell refers to nil.
Here is another diagram of the same list, (rose violet
buttercup), sketched in a different manner:
--------------- ---------------- ------------------- | car | cdr | | car | cdr | | car | cdr | | rose | o-------->| violet | o-------->| buttercup | nil | | | | | | | | | | --------------- ---------------- -------------------
A list with no elements in it is the empty list; it is identical
to the symbol nil. In other words, nil is both a symbol
and a list.
Here are examples of lists written in Lisp syntax:
(A 2 "A") ; A list of three elements.
() ; A list of no elements (the empty list).
nil ; A list of no elements (the empty list).
("A ()") ; A list of one element: the string "A ()".
(A ()) ; A list of two elements: A and the empty list.
(A nil) ; Equivalent to the previous.
((A B C)) ; A list of one element
; (which is a list of three elements).
Here is the list (A ()), or equivalently (A nil),
depicted with boxes and arrows:
___ ___ ___ ___
|___|___|--> |___|___|--> nil
| |
| |
--> A --> nil
Dotted pair notation is an alternative syntax for cons cells
that represents the CAR and CDR explicitly. In this syntax,
(a . b) stands for a cons cell whose CAR is
the object a, and whose CDR is the object b. Dotted
pair notation is therefore more general than list syntax. In the dotted
pair notation, the list `(1 2 3)' is written as `(1 . (2 . (3
. nil)))'. For nil-terminated lists, the two notations produce
the same result, but list notation is usually clearer and more
convenient when it is applicable. When printing a list, the dotted pair
notation is only used if the CDR of a cell is not a list.
Box notation can also be used to illustrate what dotted pairs look
like. For example, (rose . violet) is diagrammed as follows:
___ ___
|___|___|--> violet
|
|
--> rose
Dotted pair notation can be combined with list notation to represent a
chain of cons cells with a non-nil final CDR. For example,
(rose violet . buttercup) is equivalent to (rose . (violet
. buttercup)). The object looks like this:
___ ___ ___ ___
|___|___|--> |___|___|--> buttercup
| |
| |
--> rose --> violet
These diagrams make it evident that (rose . violet .
buttercup) must have an invalid syntax since it would require that a
cons cell have three parts rather than two.
The list (rose violet) is equivalent to (rose . (violet))
and looks like this:
___ ___ ___ ___
|___|___|--> |___|___|--> nil
| |
| |
--> rose --> violet
Similarly, the three-element list (rose violet buttercup)
is equivalent to (rose . (violet . (buttercup))).
An association list or alist is a specially-constructed list whose elements are cons cells. In each element, the CAR is considered a key, and the CDR is considered an associated value. (In some cases, the associated value is stored in the CAR of the CDR.) Association lists are often used to implement stacks, since new associations may easily be added to or removed from the front of the list.
For example,
(setq alist-of-colors
'((rose . red) (lily . white) (buttercup . yellow)))
sets the variable alist-of-colors to an alist of three elements. In the
first element, rose is the key and red is the value.
See section Association Lists, for a further explanation of alists and for functions that work on alists.
An array is composed of an arbitrary number of other Lisp objects, arranged in a contiguous block of memory. Any element of an array may be accessed in constant time. In contrast, accessing an element of a list requires time proportional to the position of the element in the list. (Elements at the end of a list take longer to access than elements at the beginning of a list.)
Emacs defines two types of array, strings and vectors. A string is an array of characters and a vector is an array of arbitrary objects. Both are one-dimensional. (Most other programming languages support multidimensional arrays, but we don't think they are essential in Emacs Lisp.) Each type of array has its own read syntax; see section String Type, and section Vector Type.
An array may have any length up to the largest integer; but once created, it has a fixed size. The first element of an array has index zero, the second element has index 1, and so on. This is called zero-origin indexing. For example, an array of four elements has indices 0, 1, 2, and 3.
The array type is contained in the sequence type and contains both strings and vectors.
A string is an array of characters. Strings are used for many purposes in Emacs, as can be expected in a text editor; for example, as the names of Lisp symbols, as messages for the user, and to represent text extracted from buffers. Strings in Lisp are constants; evaluation of a string returns the same string.
The read syntax for strings is a double-quote, an arbitrary number of
characters, and another double-quote, "like this". The Lisp
reader accepts the same formats for reading the characters of a string
as it does for reading single characters (without the question mark that
begins a character literal). You can enter a nonprinting character such
as tab, C-a or M-C-A using the convenient escape sequences,
like this: "\t, \C-a, \M-\C-a". You can include a double-quote
in a string by preceding it with a backslash; thus, "\"" is a
string containing just a single double-quote character.
(See section Character Type, for a description of the read syntax for
characters.)
If you use the `\M-' syntax to indicate a meta character in a string constant, this sets the 2**7 bit of the character in the string. This is not the same representation that the meta modifier has in a character regarded as a simple integer. See section Character Type.
Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no others. They do not distinguish case in ASCII control characters.
In contrast with the C programming language, Emacs Lisp allows newlines in string literals. But an escaped newline--one that is preceded by `\'---does not become part of the string; i.e., the Lisp reader ignores an escaped newline in a string literal.
"It is useful to include newlines
in documentation strings,
but the newline is \
ignored if escaped."
=> "It is useful to include newlines
in documentation strings,
but the newline is ignored if escaped."
The printed representation of a string consists of a double-quote, the
characters it contains, and another double-quote. However, any
backslash or double-quote characters in the string are preceded with a
backslash like this: "this \" is an embedded quote".
A string can hold properties of the text it contains, in addition to the characters themselves. This enables programs that copy text between strings and buffers to preserve the properties with no special effort. See section Text Properties. Strings with text properties have a special read and print syntax:
#("characters" property-data...)
where property-data is zero or more elements in groups of three as follows:
beg end plist
The elements beg and end are integers, and together specify a portion of the string; plist is the property list for that portion.
See section Strings and Characters, for functions that work on strings.
A vector is a one-dimensional array of elements of any type. It takes a constant amount of time to access any element of a vector. (In a list, the access time of an element is proportional to the distance of the element from the beginning of the list.)
The printed representation of a vector consists of a left square bracket, the elements, and a right square bracket. This is also the read syntax. Like numbers and strings, vectors are considered constants for evaluation.
[1 "two" (three)] ; A vector of three elements.
=> [1 "two" (three)]
See section Vectors, for functions that work with vectors.
A symbol in GNU Emacs Lisp is an object with a name. The symbol name serves as the printed representation of the symbol. In ordinary use, the name is unique--no two symbols have the same name.
A symbol may be used in programs as a variable, as a function name, or to hold a list of properties. Or it may serve only to be distinct from all other Lisp objects, so that its presence in a data structure may be recognized reliably. In a given context, usually only one of these uses is intended.
A symbol name can contain any characters whatever. Most symbol names are written with letters, digits, and the punctuation characters `-+=*/'. Such names require no special punctuation; the characters of the name suffice as long as the name does not look like a number. (If it does, write a `\' at the beginning of the name to force interpretation as a symbol.) The characters `_~!@$%^&:<>{}' are less often used but also require no special punctuation. Any other characters may be included in a symbol's name by escaping them with a backslash. In contrast to its use in strings, however, a backslash in the name of a symbol quotes the single character that follows the backslash, without conversion. For example, in a string, `\t' represents a tab character; in the name of a symbol, however, `\t' merely quotes the letter t. To have a symbol with a tab character in its name, you must actually type an tab (preceded with a backslash). But you would hardly ever do such a thing.
Common Lisp note: in Common Lisp, lower case letters are always "folded" to upper case, unless they are explicitly escaped. This is in contrast to Emacs Lisp, in which upper case and lower case letters are distinct.
Here are several examples of symbol names. Note that the `+' in the fifth example is escaped to prevent it from being read as a number. This is not necessary in the last example because the rest of the name makes it invalid as a number.
foo ; A symbol named `foo'.
FOO ; A symbol named `FOO', different from `foo'.
char-to-string ; A symbol named `char-to-string'.
1+ ; A symbol named `1+'
; (not `+1', which is an integer).
\+1 ; A symbol named `+1'
; (not a very readable name).
\(*\ 1\ 2\) ; A symbol named `(* 1 2)' (a worse name).
+-*/_~!@$%^&=:<>{} ; A symbol named `+-*/_~!@$%^&=:<>{}'.
; These characters need not be escaped.
Just as functions in other programming languages are executable,
Lisp function objects are pieces of executable code. However,
functions in Lisp are primarily Lisp objects, and only secondarily the
text which represents them. These Lisp objects are lambda expressions:
lists whose first element is the symbol lambda (see section Lambda Expressions).
In most programming languages, it is impossible to have a function without a name. In Lisp, a function has no intrinsic name. A lambda expression is also called an anonymous function (see section Anonymous Functions). A named function in Lisp is actually a symbol with a valid function in its function cell (see section Defining Named Functions).
Most of the time, functions are called when their names are written in
Lisp expressions in Lisp programs. However, a function object found or
constructed at run time can be called and passed arguments with the
primitive functions funcall and apply. See section Calling Functions.
A Lisp macro is a user-defined construct that extends the Lisp
language. It is represented as an object much like a function, but with
different parameter-passing semantics. A Lisp macro has the form of a
list whose first element is the symbol macro and whose CDR
is a Lisp function object, including the lambda symbol.
Lisp macro objects are usually defined with the built-in
defmacro function, but any list that begins with macro is
a macro as far as Emacs is concerned. See section Macros, for an explanation
of how to write a macro.
A primitive function is a function callable from Lisp but written in the C programming language. Primitive functions are also called subrs or built-in functions. (The word "subr" is derived from "subroutine".) Most primitive functions evaluate all their arguments when they are called. A primitive function that does not evaluate all its arguments is called a special form (see section Special Forms).
It does not matter to the caller of a function whether the function is primitive. However, this does matter if you are trying to substitute a function written in Lisp for a primitive of the same name. The reason is that the primitive function may be called directly from C code. When the redefined function is called from Lisp, the new definition will be used; but calls from C code may still use the old definition.
The term function is used to refer to all Emacs functions, whether written in Lisp or C. See section Lisp Function Type, for information about the functions written in Lisp.
Primitive functions have no read syntax and print in hash notation with the name of the subroutine.
(symbol-function 'car) ; Access the function cell
; of the symbol.
=> #<subr car>
(subrp (symbol-function 'car)) ; Is this a primitive function?
=> t ; Yes.
The byte compiler produces byte-code function objects. Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. See section Byte Compilation, for information about the byte compiler.
The printed representation for a byte-code function object is like that for a vector, with an additional `#' before the opening `['.
An autoload object is a list whose first element is the symbol
autoload. It is stored as the function definition of a symbol to
say that a file of Lisp code should be loaded when necessary to find the
true definition of that symbol. The autoload object contains the name
of the file, plus some other information about the real definition.
After the file has been loaded, the symbol should have a new function definition that is not an autoload object. The new definition is then called as if it had been there to begin with. From the user's point of view, the function call works as expected, using the function definition in the loaded file.
An autoload object is usually created with the function
autoload, which stores the object in the function cell of a
symbol. See section Autoload, for more details.
The types in the previous section are common to many Lisp-like languages. But Emacs Lisp provides several additional data types for purposes connected with editing.
A buffer is an object that holds text that can be edited (see section Buffers). Most buffers hold the contents of a disk file (see section Files) so they can be edited, but some are used for other purposes. Most buffers are also meant to be seen by the user, and therefore displayed, at some time, in a window (see section Windows). But a buffer need not be displayed in a window.
The contents of a buffer are much like a string, but buffers are not used like strings in Emacs Lisp, and the available operations are different. For example, text can be inserted into a buffer very quickly, while "inserting" text into a string is accomplished by concatenation and the result is an entirely new string object.
Each buffer has a designated position called point (see section Positions). And one buffer is the current buffer. Most editing commands act on the contents of the current buffer in the neighborhood of point. Many other functions manipulate or test the characters in the current buffer and much of this manual is devoted to describing these functions (see section Text).
Several other data structures are associated with each buffer:
The local keymap and variable list contain entries which individually override global bindings or values. These are used to customize the behavior of programs in different buffers, without actually changing the programs.
Buffers have no read syntax. They print in hash notation with the buffer name.
(current-buffer)
=> #<buffer objects.texi>
A window describes the portion of the terminal screen that Emacs uses to display a buffer. Every window has one associated buffer, whose contents appear in the window. By contrast, a given buffer may appear in one window, no window, or several windows.
Though many windows may exist simultaneously, one window is designated the selected window. This is the window where the cursor is (usually) displayed when Emacs is ready for a command. The selected window usually displays the current buffer, but this is not necessarily the case.
Windows are grouped on the screen into frames; each window belongs to one and only one frame. See section Frame Type.
Windows have no read syntax. They print in hash notation, giving the window number and the name of the buffer being displayed. The window numbers exist to identify windows uniquely, since the buffer displayed in any given window can change frequently.
(selected-window)
=> #<window 1 on objects.texi>
See section Windows, for a description of the functions that work on windows.
A frame is a rectangle on the screen that contains one or more Emacs windows. A frame initially contains a single main window (plus perhaps a minibuffer window) which you can subdivide vertically or horizontally into smaller windows.
Frames have no read syntax. They print in hash notation, giving the frame's title, plus its address in core (useful to identify the frame uniquely).
(selected-frame)
=> #<frame xemacs@mole.gnu.ai.mit.edu 0xdac80>
See section Frames, for a description of the functions that work on frames.
A window configuration stores information about the positions and sizes of windows at the time the window configuration is created, so that the screen layout may be recreated later.
Window configurations do not have a read syntax. They print as `#<window-configuration>'. See section Window Configurations, for a description of several functions related to window configurations.
A marker denotes a position in a specific buffer. Markers therefore have two components: one for the buffer, and one for the position. The position value is changed automatically as necessary as text is inserted into or deleted from the buffer. This is to ensure that the marker always points between the same two characters in the buffer.
Markers have no read syntax. They print in hash notation, giving the current character position and the name of the buffer.
(point-marker)
=> #<marker at 10779 in objects.texi>
See section Markers, for information on how to test, create, copy, and move markers.
The word process means a running program. Emacs itself runs in a process of this sort. However, in Emacs Lisp, a process is a Lisp object that designates a subprocess created by Emacs process. External subprocesses, such as shells, GDB, ftp, and compilers, may be used to extend the processing capability of Emacs.
A process takes input from Emacs and returns output to Emacs for further manipulation. Both text and signals can be communicated between Emacs and a subprocess.
Processes have no read syntax. They print in hash notation, giving the name of the process:
(process-list)
=> (#<process shell>)
See section Processes, for information about functions that create, delete, return information about, send input or signals to, and receive output from processes.
A stream is an object that can be used as a source or sink for characters--either to supply characters for input or to accept them as output. Many different types can be used this way: markers, buffers, strings, and functions. Most often, input streams (character sources) obtain characters from the keyboard, a buffer, or a file, and output streams (character sinks) send characters to a buffer, such as a `*Help*' buffer, or to the echo area.
The object nil, in addition to its other meanings, may be used
as a stream. It stands for the value of the variable
standard-input or standard-output. Also, the object
t as a stream specifies input using the minibuffer
(see section Minibuffers) or output in the echo area (see section The Echo Area).
Streams have no special printed representation or read syntax, and print as whatever primitive type they are.
See section Reading and Printing Lisp Objects, for a description of various functions related to streams, including various parsing and printing functions.
A keymap maps keys typed by the user to functions. This mapping
controls how the user's command input is executed. A keymap is actually
a list whose CAR is the symbol keymap.
See section Keymaps, for information about creating keymaps, handling prefix keys, local as well as global keymaps, and changing key bindings.
A syntax table is a vector of 256 integers. Each element of the vector defines how one character is interpreted when it appears in a buffer. For example, in C mode (see section Major Modes), the `+' character is punctuation, but in Lisp mode it is a valid character in a symbol. These different interpretations are effected by changing the syntax table entry for `+', i.e., at index 43.
Syntax tables are only used for scanning text in buffers, not for reading Lisp expressions. The table the Lisp interpreter uses to read expressions is built into the Emacs source code and cannot be changed; thus, to change the list delimiters to be `{' and `}' instead of `(' and `)' would be impossible.
See section Syntax Tables, for details about syntax classes and how to make and modify syntax tables.
A display table specifies how to display each character code. Each buffer and each window can have its own display table. A display table is actually a vector of length 261. See section Display Tables.
An overlay specifies temporary alteration of the display appearance of a part of a buffer. It contains markers delimiting a range of the buffer, plus a property list (a list whose elements are alternating property names and values). Overlays are used to present parts of the buffer temporarily in a different display style.
See section Overlays, for how to create and use overlays.
The Emacs Lisp interpreter itself does not perform type checking on the actual arguments passed to functions when they are called. It could not do otherwise, since variables in Lisp are not declared to be of a certain type, as they are in other programming languages. It is therefore up to the individual function to test whether each actual argument belongs to a type that can be used by the function.
All built-in functions do check the types of their actual arguments
when appropriate and signal a wrong-type-argument error if an
argument is of the wrong type. For example, here is what happens if you
pass an argument to + which it cannot handle:
(+ 2 'a)
error--> Wrong type argument: integer-or-marker-p, a
Many functions, called type predicates, are provided to test whether an object is a member of a given type. (Following a convention of long standing, the names of most Emacs Lisp predicates end in `p'.)
Here is a table of predefined type predicates, in alphabetical order, with references to further information.
atom
arrayp
bufferp
byte-code-function-p
case-table-p
char-or-string-p
commandp
consp
floatp
frame-live-p
framep
integer-or-marker-p
integerp
keymapp
listp
markerp
natnump
nlistp
numberp
number-or-marker-p
overlayp
processp
sequencep
stringp
subrp
symbolp
syntax-table-p
user-variable-p
vectorp
window-configuration-p
window-live-p
windowp
Here we describe two functions that test for equality between any two objects. Other functions test equality between objects of specific types, e.g., strings. See the appropriate chapter describing the data type for these predicates.
This function returns t if object1 and object2 are
the same object, nil otherwise. The "same object" means that a
change in one will be reflected by the same change in the other.
eq returns t if object1 and object2 are
integers with the same value. Also, since symbol names are normally
unique, if the arguments are symbols with the same name, they are
eq. For other types (e.g., lists, vectors, strings), two
arguments with the same contents or elements are not necessarily
eq to each other: they are eq only if they are the same
object.
(The make-symbol function returns an uninterned symbol that is
not interned in the standard obarray. When uninterned symbols
are in use, symbol names are no longer unique. Distinct symbols with
the same name are not eq. See section Creating and Interning Symbols.)
(eq 'foo 'foo)
=> t
(eq 456 456)
=> t
(eq "asdf" "asdf")
=> nil
(eq '(1 (2 (3))) '(1 (2 (3))))
=> nil
(eq [(1 2) 3] [(1 2) 3])
=> nil
(eq (point-marker) (point-marker))
=> nil
Function: equal object1 object2
This function returns t if object1 and object2 have
equal components, nil otherwise. Whereas eq tests if its
arguments are the same object, equal looks inside nonidentical
arguments to see if their elements are the same. So, if two objects are
eq, they are equal, but the converse is not always true.
(equal 'foo 'foo)
=> t
(equal 456 456)
=> t
(equal "asdf" "asdf")
=> t
(eq "asdf" "asdf")
=> nil
(equal '(1 (2 (3))) '(1 (2 (3))))
=> t
(eq '(1 (2 (3))) '(1 (2 (3))))
=> nil
(equal [(1 2) 3] [(1 2) 3])
=> t
(eq [(1 2) 3] [(1 2) 3])
=> nil
(equal (point-marker) (point-marker))
=> t
(eq (point-marker) (point-marker))
=> nil
Comparison of strings is case-sensitive.
(equal "asdf" "ASDF")
=> nil
The test for equality is implemented recursively, and circular lists may therefore cause infinite recursion (leading to an error).
GNU Emacs supports two numeric data types: integers and floating point numbers. Integers are whole numbers such as -3, 0, 7, 13, and 511. Their values are exact. Floating point numbers are numbers with fractional parts, such as -4.5, 0.0, or 2.71828. They can also be expressed in an exponential notation as well: thus, 1.5e2 equals 150; in this example, `e2' stands for ten to the second power, and is multiplied by 1.5. Floating point values are not exact; they have a fixed, limited amount of precision.
Support for floating point numbers is a new feature in Emacs 19, and it is controlled by a separate compilation option, so you may encounter a site where Emacs does not support them.
The range of values for an integer depends on the machine. The range is -8388608 to 8388607 (24 bits; i.e., to ) on most machines, but on others it is -16777216 to 16777215 (25 bits), or -33554432 to 33554431 (26 bits). All of the examples shown below assume an integer has 24 bits.
The Lisp reader reads numbers as a sequence of digits with an optional sign.
1 ; The integer 1. +1 ; Also the integer 1. -1 ; The integer -1. 16777217 ; Also the integer 1, due to overflow. 0 ; The number 0. -0 ; The number 0. 1. ; The integer 1.
To understand how various functions work on integers, especially the bitwise operators (see section Bitwise Operations on Integers), it is often helpful to view the numbers in their binary form.
In 24 bit binary, the decimal integer 5 looks like this:
0000 0000 0000 0000 0000 0101
(We have inserted spaces between groups of 4 bits, and two spaces between groups of 8 bits, to make the binary integer easier to read.)
The integer -1 looks like this:
1111 1111 1111 1111 1111 1111
-1 is represented as 24 ones. (This is called two's complement notation.)
The negative integer, -5, is creating by subtracting 4 from -1. In binary, the decimal integer 4 is 100. Consequently, -5 looks like this:
1111 1111 1111 1111 1111 1011
In this implementation, the largest 24 bit binary integer is the decimal integer 8,388,607. In binary, this number looks like this:
0111 1111 1111 1111 1111 1111
Since the arithmetic functions do not check whether integers go outside their range, when you add 1 to 8,388,607, the value is negative integer -8,388,608:
(+ 1 8388607)
=> -8388608
=> 1000 0000 0000 0000 0000 0000
Many of the following functions accept markers for arguments as well as integers. (See section Markers.) More precisely, the actual parameters to such functions may be either integers or markers, which is why we often give these parameters the name int-or-marker. When the actual parameter is a marker, the position value of the marker is used and the buffer of the marker is ignored.
Emacs version 19 supports floating point numbers, if compiled with the
macro LISP_FLOAT_TYPE defined. The precise range of floating
point numbers is machine-specific; it is the same as the range of the C
data type double on the machine in question.
The printed representation for floating point numbers requires either a decimal point (with at least one digit following), an exponent, or both. For example, `1500.0', `15e2', `15.0e2', `1.5e3', and `.15e4' are five ways of writing a floating point number whose value is 1500. They are all equivalent. You can also use a minus sign to write negative floating point numbers, as in `-1.0'.
You can use logb to extract the binary exponent of a floating
point number (or estimate the logarithm of an integer):
This function returns the binary exponent of number. More precisely, the value is the logarithm of number base 2, rounded down to an integer.
The functions in this section test whether the argument is a number or
whether it is a certain sort of number. The functions integerp
and floatp can take any type of Lisp object as argument (the
predicates would not be of much use otherwise); but the zerop
predicate requires a number as its argument. See also
integer-or-marker-p and number-or-marker-p, in
section Predicates on Markers.
This predicate tests whether its argument is a floating point
number and returns t if so, nil otherwise.
floatp does not exist in Emacs versions 18 and earlier.
This predicate tests whether its argument is an integer, and returns
t if so, nil otherwise.
This predicate tests whether its argument is a number (either integer or
floating point), and returns t if so, nil otherwise.
The natnump predicate (whose name comes from the phrase
"natural-number-p") tests to see whether its argument is a nonnegative
integer, and returns t if so, nil otherwise. 0 is
considered non-negative.
Markers are not converted to integers, hence natnump of a marker
is always nil.
People have pointed out that this function is misnamed, because the term "natural number" is usually understood as excluding zero. We are open to suggestions for a better name to use in a future version.
This predicate tests whether its argument is zero, and returns t
if so, nil otherwise. The argument must be a number.
These two forms are equivalent: (zerop x) == (= x 0).
Floating point numbers in Emacs Lisp actually take up storage, and
there can be many distinct floating point number objects with the same
numeric value. If you use eq to compare them, then you test
whether two values are the same object. If you want to compare
just the numeric values, use =.
If you use eq to compare two integers, it always returns
t if they have the same value. This is sometimes useful, because
eq accepts arguments of any type and never causes an error,
whereas = signals an error if the arguments are not numbers or
markers. However, it is a good idea to use = if you can, even
for comparing integers, just in case we change the representation of
integers in a future Emacs version.
There is another wrinkle: because floating point arithmetic is not exact, it is often a bad idea to check for equality of two floating point values. Usually it is better to test for approximate equality. Here's a function to do this:
(defvar fuzz-factor 1.0e-6)
(defun approx-equal (x y)
(< (/ (abs (- x y))
(max (abs x) (abs y)))
fuzz-factor))
Common Lisp note: because of the way numbers are implemented in
Common Lisp, you generally need to use `=' to test for
equality between numbers of any kind.
Function: = number-or-marker1 number-or-marker2
This function tests whether its arguments are the same number, and
returns t if so, nil otherwise.
Function: /= number-or-marker1 number-or-marker2
This function tests whether its arguments are not the same number, and
returns t if so, nil otherwise.
Function: < number-or-marker1 number-or-marker2
This function tests whether its first argument is strictly less than
its second argument. It returns t if so, nil otherwise.
Function: <= number-or-marker1 number-or-marker2
This function tests whether its first argument is less than or equal
to its second argument. It returns t if so, nil
otherwise.
Function: > number-or-marker1 number-or-marker2
This function tests whether its first argument is strictly greater
than its second argument. It returns t if so, nil
otherwise.
Function: >= number-or-marker1 number-or-marker2
This function tests whether its first argument is greater than or
equal to its second argument. It returns t if so, nil
otherwise.
Function: max number-or-marker &rest numbers-or-markers
This function returns the largest of its arguments.
(max 20)
=> 20
(max 1 2)
=> 2
(max 1 3 2)
=> 3
Function: min number-or-marker &rest numbers-or-markers
This function returns the smallest of its arguments.
To convert an integer to floating point, use the function float.
This returns number converted to floating point.
If number is already a floating point number, float returns
it unchanged.
There are four functions to convert floating point numbers to integers; they differ in how they round. You can call these functions with an integer argument also; if you do, they return it without change.
This returns number, converted to an integer by rounding towards zero.
Function: floor number &optional divisor
This returns number, converted to an integer by rounding downward (towards negative infinity).
If divisor is specified, number is divided by divisor
before the floor is taken; this is the division operation that
corresponds to mod. An arith-error results if
divisor is 0.
This returns number, converted to an integer by rounding upward (towards positive infinity).
This returns number, converted to an integer by rounding towards the nearest integer.
Emacs Lisp provides the traditional four arithmetic operations: addition, subtraction, multiplication, and division. Remainder and modulus functions supplement the division functions. The functions to add or subtract 1 are provided because they are traditional in Lisp and commonly used.
All of these functions except % return a floating point value
if any argument is floating.
It is important to note that in GNU Emacs Lisp, arithmetic functions
do not check for overflow. Thus (1+ 8388607) may equal
-8388608, depending on your hardware.
This function returns number-or-marker plus 1. For example,
(setq foo 4)
=> 4
(1+ foo)
=> 5
This function is not analogous to the C operator ++---it does
not increment a variable. It just computes a sum. Thus,
foo
=> 4
If you want to increment the variable, you must use setq,
like this:
(setq foo (1+ foo))
=> 5
This function returns number-or-marker minus 1.
This returns the absolute value of number.
Function: + &rest numbers-or-markers
This function adds its arguments together. When given no arguments,
+ returns 0. It does not check for overflow.
(+)
=> 0
(+ 1)
=> 1
(+ 1 2 3 4)
=> 10
Function: - &optional number-or-marker &rest other-numbers-or-markers
The - function serves two purposes: negation and subtraction.
When - has a single argument, the value is the negative of the
argument. When there are multiple arguments, each of the
other-numbers-or-markers is subtracted from number-or-marker,
cumulatively. If there are no arguments, the result is 0. This
function does not check for overflow.
(- 10 1 2 3 4)
=> 0
(- 10)
=> -10
(-)
=> 0
Function: * &rest numbers-or-markers
This function multiplies its arguments together, and returns the
product. When given no arguments, * returns 1. It does
not check for overflow.
(*)
=> 1
(* 1)
=> 1
(* 1 2 3 4)
=> 24
Function: / dividend divisor &rest divisors
This function divides dividend by divisors and returns the quotient. If there are additional arguments divisors, then dividend is divided by each divisor in turn. Each argument may be a number or a marker.
If all the arguments are integers, then the result is an integer too.
This means the result has to be rounded. On most machines, the result
is rounded towards zero after each division, but some machines may round
differently with negative arguments. This is because the Lisp function
/ is implemented using the C division operator, which has the
same possibility for machine-dependent rounding. As a practical matter,
all known machines round in the standard fashion.
If you divide by 0, an arith-error error is signaled.
(See section Errors.)
(/ 6 2)
=> 3
(/ 5 2)
=> 2
(/ 25 3 2)
=> 4
(/ -17 6)
=> -2
Since the division operator in Emacs Lisp is implemented using the
division operator in C, the result of dividing negative numbers may in
principle vary from machine to machine, depending on how they round the
result. Thus, the result of (/ -17 6) could be -3 on some
machines. In practice, all known machines round the quotient towards
0.
This function returns the integer remainder after division of dividend by divisor. The arguments must be integers or markers.
For negative arguments, the value is in principle machine-dependent since the quotient is; but in practice, all known machines behave alike.
An arith-error results if divisor is 0.
(% 9 4)
=> 1
(% -9 4)
=> -1
(% 9 -4)
=> 1
(% -9 -4)
=> -1
For any two integers dividend and divisor,
(+ (% dividend divisor) (* (/ dividend divisor) divisor))
always equals dividend.
Function: mod dividend divisor
This function returns the value of dividend modulo divisor; in other words, the remainder after division of dividend by divisor, but with the same sign as divisor. The arguments must be numbers or markers.
Unlike %, the result is well-defined for negative arguments.
Also, floating point arguments are permitted.
An arith-error results if divisor is 0.
(mod 9 4)
=> 1
(mod -9 4)
=> 3
(mod 9 -4)
=> -3
(mod -9 -4)
=> -1
For any two numbers dividend and divisor,
(+ (mod dividend divisor) (* (floor dividend divisor) divisor))
always equals dividend, subject to rounding error if either argument is floating point.
In a computer, an integer is represented as a binary number, a sequence of bits (digits which are either zero or one). A bitwise operation acts on the individual bits of such a sequence. For example, shifting moves the whole sequence left or right one or more places, reproducing the same pattern "moved over".
The bitwise operations in Emacs Lisp apply only to integers.
lsh, which is an abbreviation for logical shift, shifts the
bits in integer1 to the left count places, or to the
right if count is negative. If count is negative,
lsh shifts zeros into the most-significant bit, producing a
positive result even if integer1 is negative. Contrast this with
ash, below.
Thus, the decimal number 5 is the binary number 00000101. Shifted once to the left, with a zero put in the one's place, the number becomes 00001010, decimal 10.
Here are two examples of shifting the pattern of bits one place to the
left. Since the contents of the rightmost place has been moved one
place to the left, a value has to be inserted into the rightmost place.
With lsh, a zero is placed into the rightmost place. (These
examples show only the low-order eight bits of the binary pattern; the
rest are all zero.)
(lsh 5 1)
=> 10
;; Decimal 5 becomes decimal 10.
00000101 => 00001010
(lsh 7 1)
=> 14
;; Decimal 7 becomes decimal 14.
00000111 => 00001110
As the examples illustrate, shifting the pattern of bits one place to the left produces a number that is twice the value of the previous number.
Note, however that functions do not check for overflow, and a returned value may be negative (and in any case, no more than a 24 bit value) when an integer is sufficiently left shifted.
For example, left shifting 8,388,607 produces -2:
(lsh 8388607 1) ; left shift
=> -2
In binary, in the 24 bit implementation, the numbers looks like this:
;; Decimal 8,388,607 0111 1111 1111 1111 1111 1111
which becomes the following when left shifted:
;; Decimal -2 1111 1111 1111 1111 1111 1110
Shifting the pattern of bits two places to the left produces results like this (with 8-bit binary numbers):
(lsh 3 2)
=> 12
;; Decimal 3 becomes decimal 12.
00000011 => 00001100
On the other hand, shifting the pattern of bits one place to the right looks like this:
(lsh 6 -1)
=> 3
;; Decimal 6 becomes decimal 3.
00000110 => 00000011
(lsh 5 -1)
=> 2
;; Decimal 5 becomes decimal 2.
00000101 => 00000010
As the example illustrates, shifting the pattern of bits one place to the right divides the value of the binary number by two, rounding downward.
ash (arithmetic shift) shifts the bits in integer1
to the left count places, or to the right if count
is negative.
ash gives the same results as lsh except when
integer1 and count are both negative. In that case,
ash puts a one in the leftmost position, while lsh puts
a zero in the leftmost position.
Thus, with ash, shifting the pattern of bits one place to the right
looks like this:
(ash -6 -1)
=> -3
;; Decimal -6
;; becomes decimal -3.
1111 1111 1111 1111 1111 1010
=>
1111 1111 1111 1111 1111 1101
In contrast, shifting the pattern of bits one place to the right with
lsh looks like this:
(lsh -6 -1)
=> 8388605
;; Decimal -6
;; becomes decimal 8,388,605.
1111 1111 1111 1111 1111 1010
=>
0111 1111 1111 1111 1111 1101
In this case, the 1 in the leftmost position is shifted one place to the right, and a zero is shifted into the leftmost position.
Here are other examples:
; 24-bit binary values
(lsh 5 2) ; 5 = 0000 0000 0000 0000 0000 0101
=> 20 ; 20 = 0000 0000 0000 0000 0001 0100
(ash 5 2)
=> 20
(lsh -5 2) ; -5 = 1111 1111 1111 1111 1111 1011
=> -20 ; -20 = 1111 1111 1111 1111 1110 1100
(ash -5 2)
=> -20
(lsh 5 -2) ; 5 = 0000 0000 0000 0000 0000 0101
=> 1 ; 1 = 0000 0000 0000 0000 0000 0001
(ash 5 -2)
=> 1
(lsh -5 -2) ; -5 = 1111 1111 1111 1111 1111 1011
=> 4194302 ; 0011 1111 1111 1111 1111 1110
(ash -5 -2) ; -5 = 1111 1111 1111 1111 1111 1011
=> -2 ; -2 = 1111 1111 1111 1111 1111 1110
Function: logand &rest ints-or-markers
This function returns the "logical and" of the arguments: the nth bit is set in the result if, and only if, the nth bit is set in all the arguments. ("Set" means that the value of the bit is 1 rather than 0.)
For example, using 4-bit binary numbers, the "logical and" of 13 and 12 is 12: 1101 combined with 1100 produces 1100.
In both the binary numbers, the leftmost two bits are set (i.e., they are 1's), so the leftmost two bits of the returned value are set. However, for the rightmost two bits, each is zero in at least one of the arguments, so the rightmost two bits of the returned value are 0's.
Therefore,
(logand 13 12)
=> 12
If logand is not passed any argument, it returns a value of
-1. This number is an identity element for logand
because its binary representation consists entirely of ones. If
logand is passed just one argument, it returns that argument.
; 24-bit binary values
(logand 14 13) ; 14 = 0000 0000 0000 0000 0000 1110
; 13 = 0000 0000 0000 0000 0000 1101
=> 12 ; 12 = 0000 0000 0000 0000 0000 1100
(logand 14 13 4) ; 14 = 0000 0000 0000 0000 0000 1110
; 13 = 0000 0000 0000 0000 0000 1101
; 4 = 0000 0000 0000 0000 0000 0100
=> 4 ; 4 = 0000 0000 0000 0000 0000 0100
(logand)
=> -1 ; -1 = 1111 1111 1111 1111 1111 1111
Function: logior &rest ints-or-markers
This function returns the "inclusive or" of its arguments: the nth bit
is set in the result if, and only if, the nth bit is set in at least
one of the arguments. If there are no arguments, the result is zero,
which is an identity element for this operation. If logior is
passed just one argument, it returns that argument.
; 24-bit binary values
(logior 12 5) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
=> 13 ; 13 = 0000 0000 0000 0000 0000 1101
(logior 12 5 7) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
; 7 = 0000 0000 0000 0000 0000 0111
=> 15 ; 15 = 0000 0000 0000 0000 0000 1111
Function: logxor &rest ints-or-markers
This function returns the "exclusive or" of its arguments: the
nth bit is set in the result if, and only if, the nth bit
is set in an odd number of the arguments. If there are no arguments,
the result is 0. If logxor is passed just one argument, it returns
that argument.
; 24-bit binary values
(logxor 12 5) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
=> 9 ; 9 = 0000 0000 0000 0000 0000 1001
(logxor 12 5 7) ; 12 = 0000 0000 0000 0000 0000 1100
; 5 = 0000 0000 0000 0000 0000 0101
; 7 = 0000 0000 0000 0000 0000 0111
=> 14 ; 14 = 0000 0000 0000 0000 0000 1110
This function returns the logical complement of its argument: the nth bit is one in the result if, and only if, the nth bit is zero in integer, and vice-versa.
;; 5 = 0000 0000 0000 0000 0000 0101
;; becomes
;; -6 = 1111 1111 1111 1111 1111 1010
(lognot 5)
=> -6
These mathematical functions are available if floating point is supported. They allow integers as well as floating point numbers as arguments.
These are the ordinary trigonometric functions, with argument measured in radians.
The value of (asin arg) is a number between - pi / 2
and pi / 2 (inclusive) whose sine is arg; if, however, arg
is out of range (outside [-1, 1]), then the result is a NaN.
The value of (acos arg) is a number between 0 and pi
(inclusive) whose cosine is arg; if, however, arg
is out of range (outside [-1, 1]), then the result is a NaN.
The value of (atan arg) is a number between - pi / 2
and pi / 2 (exclusive) whose tangent is arg.
This is the exponential function; it returns e to the power arg.
Function: log arg &optional base
This function returns the logarithm of arg, with base base. If you don't specify base, the base e is used. If arg is negative, the result is a NaN.
This function returns the logarithm of arg, with base 10. If arg is negative, the result is a NaN.
This function returns x raised to power y.
This returns the square root of arg.
In a computer, a series of pseudo-random numbers is generated in a deterministic fashion. The numbers are not truly random, but they have certain properties that mimic a random series. For example, all possible values occur equally often in a pseudo-random series.
In Emacs, pseudo-random numbers are generated from a "seed" number.
Starting from any given seed, the random function always
generates the same sequence of numbers. Emacs always starts with the
same seed value, so the sequence of values of random is actually
the same in each Emacs run! For example, in one operating system, the
first call to (random) after you start Emacs always returns
-1457731, and the second one always returns -7692030. This is helpful
for debugging.
If you want truly unpredictable random numbers, execute (random
t). This chooses a new seed based on the current time of day and on
Emacs' process ID number.
Function: random &optional limit
This function returns a pseudo-random integer. When called more than once, it returns a series of pseudo-random integers.
If limit is nil, then the value may in principle be any
integer. If limit is a positive integer, the value is chosen to
be nonnegative and less than limit (only in Emacs 19).
If limit is t, it means to choose a new seed based on the
current time of day and on Emacs's process ID number.
On some machines, any integer representable in Lisp may be the result
of random. On other machines, the result can never be larger
than a certain maximum or less than a certain (negative) minimum.
A string in Emacs Lisp is an array that contains an ordered sequence of characters. Strings are used as names of symbols, buffers, and files, to send messages to users, to hold text being copied between buffers, and for many other purposes. Because strings are so important, many functions are provided expressly for manipulating them. Emacs Lisp programs use strings more often than individual characters.
See section Putting Keyboard Events in Strings, for special considerations when using strings of keyboard character events.
Strings in Emacs Lisp are arrays that contain an ordered sequence of characters. Characters are represented in Emacs Lisp as integers; whether an integer was intended as a character or not is determined only by how it is used. Thus, strings really contain integers.
The length of a string (like any array) is fixed and independent of the string contents, and cannot be altered. Strings in Lisp are not terminated by a distinguished character code. (By contrast, strings in C are terminated by a character with ASCII code 0.) This means that any character, including the null character (ASCII code 0), is a valid element of a string.
Since strings are considered arrays, you can operate on them with the
general array functions. (See section Sequences, Arrays, and Vectors.) For
example, you can access or change individual characters in a string
using the functions aref and aset (see section Functions that Operate on Arrays).
Each character in a string is stored in a single byte. Therefore, numbers not in the range 0 to 255 are truncated when stored into a string. This means that a string takes up much less memory than a vector of the same length.
Sometimes key sequences are represented as strings. When a string is a key sequence, string elements in the range 128 to 255 represent meta characters (which are extremely large integers) rather than keyboard events in the range 128 to 255.
Strings cannot hold characters that have the hyper, super or alt modifiers; they can hold ASCII control characters, but no others. They do not distinguish case in ASCII control characters. See section Character Type, for more information about representation of meta and other modifiers for keyboard input characters.
Like a buffer, a string can contain text properties for the characters in it, as well as the characters themselves. See section Text Properties.
See section Text, for information about functions that display strings or copy them into buffers. See section Character Type, and section String Type, for information about the syntax of characters and strings.
For more information about general sequence and array predicates, see section Sequences, Arrays, and Vectors, and section Arrays.
This function returns t if object is a string, nil
otherwise.
Function: char-or-string-p object
This function returns t if object is a string or a
character (i.e., an integer), nil otherwise.
The following functions create strings, either from scratch, or by putting strings together, or by taking them apart.
Function: make-string count character
This function returns a string made up of count repetitions of character. If count is negative, an error is signaled.
(make-string 5 ?x)
=> "xxxxx"
(make-string 0 ?x)
=> ""
Other functions to compare with this one include char-to-string
(see section Conversion of Characters and Strings), make-vector (see section Vectors), and
make-list (see section Building Cons Cells and Lists).
Function: substring string start &optional end
This function returns a new string which consists of those characters from string in the range from (and including) the character at the index start up to (but excluding) the character at the index end. The first character is at index zero.
(substring "abcdefg" 0 3)
=> "abc"
Here the index for `a' is 0, the index for `b' is 1, and the index for `c' is 2. Thus, three letters, `abc', are copied from the full string. The index 3 marks the character position up to which the substring is copied. The character whose index is 3 is actually the fourth character in the string.
A negative number counts from the end of the string, so that -1 signifies the index of the last character of the string. For example:
(substring "abcdefg" -3 -1)
=> "ef"
In this example, the index for `e' is -3, the index for `f' is -2, and the index for `g' is -1. Therefore, `e' and `f' are included, and `g' is excluded.
When nil is used as an index, it falls after the last character
in the string. Thus:
(substring "abcdefg" -3 nil)
=> "efg"
Omitting the argument end is equivalent to specifying nil.
It follows that (substring string 0) returns a copy of all
of string.
(substring "abcdefg" 0)
=> "abcdefg"
But we recommend copy-sequence for this purpose (see section Sequences).
A wrong-type-argument error is signaled if either start
or end are non-integers. An args-out-of-range error is
signaled if start indicates a character following end, or if
either integer is out of range for string.
Contrast this function with buffer-substring (see section Examining Buffer Contents), which returns a string containing a portion of the text in
the current buffer. The beginning of a string is at index 0, but the
beginning of a buffer is at index 1.
Function: concat &rest sequences
This function returns a new string consisting of the characters in the
arguments passed to it. The arguments may be strings, lists of numbers,
or vectors of numbers; they are not themselves changed. If no arguments
are passed to concat, it returns an empty string.
(concat "abc" "-def")
=> "abc-def"
(concat "abc" (list 120 (+ 256 121)) [122])
=> "abcxyz"
(concat "The " "quick brown " "fox.")
=> "The quick brown fox."
(concat)
=> ""
The second example above shows how characters stored in strings are taken modulo 256. In other words, each character in the string is stored in one byte.
The concat function always constructs a new string that is
not eq to any existing string.
When an argument is an integer (not a sequence of integers), it is
converted to a string of digits making up the decimal printed
representation of the integer. This special case exists for
compatibility with Mocklisp, and we don't recommend you take advantage
of it. If you want to convert an integer in this way, use format
(see section Formatting Strings) or int-to-string (see section Conversion of Characters and Strings).
(concat 137)
=> "137"
(concat 54 321)
=> "54321"
For information about other concatenation functions, see the
description of mapconcat in section Mapping Functions,
vconcat in section Vectors, and append in section Building Cons Cells and Lists.
Function: char-equal character1 character2
This function returns t if the arguments represent the same
character, nil otherwise. This function ignores differences
in case if case-fold-search is non-nil.
(char-equal ?x ?x)
=> t
(char-to-string (+ 256 ?x))
=> "x"
(char-equal ?x (+ 256 ?x))
=> t
Function: string= string1 string2
This function returns t if the characters of the two strings
match exactly; case is significant.
(string= "abc" "abc")
=> t
(string= "abc" "ABC")
=> nil
(string= "ab" "ABC")
=> nil
Function: string-equal string1 string2
string-equal is another name for string=.
Function: string< string1 string2
This function compares two strings a character at a time. First it
scans both the strings at once to find the first pair of corresponding
characters that do not match. If the lesser character of those two is
the character from string1, then string1 is less, and this
function returns t. If the lesser character is the one from
string2, then string1 is greater, and this function returns
nil. If the two strings match entirely, the value is nil.
Pairs of characters are compared by their ASCII codes. Keep in mind that lower case letters have higher numeric values in the ASCII character set than their upper case counterparts; numbers and many punctuation characters have a lower numeric value than upper case letters.
(string< "abc" "abd")
=> t
(string< "abd" "abc")
=> nil
(string< "123" "abc")
=> t
When the strings have different lengths, and they match
up to the length of string1, then the result is t. If they
match up to the length of string2, the result is nil.
A string without any characters in it is the smallest possible string.
(string< "" "abc")
=> t
(string< "ab" "abc")
=> t
(string< "abc" "")
=> nil
(string< "abc" "ab")
=> nil
(string< "" "")
=> nil
Function: string-lessp string1 string2
string-lessp is another name for string<.
See compare-buffer-substrings in section Comparing Text, for a
way to compare text in buffers.
Characters and strings may be converted into each other and into
integers. format and prin1-to-string
(see section Output Functions) may also be used to convert Lisp objects into
strings. read-from-string (see section Input Functions) may be used
to "convert" a string representation of a Lisp object into an object.
See section Documentation, for a description of functions which return a
string representing the Emacs standard notation of the argument
character (single-key-description and
text-char-description). These functions are used primarily for
printing help messages.
Function: char-to-string character
This function returns a new string with a length of one character. The value of character, modulo 256, is used to initialize the element of the string.
This function is similar to make-string with an integer argument
of 1. (See section Creating Strings.) This conversion can also be done with
format using the `%c' format specification.
(See section Formatting Strings.)
(char-to-string ?x)
=> "x"
(char-to-string (+ 256 ?x))
=> "x"
(make-string 1 ?x)
=> "x"
Function: string-to-char string
This function returns the first character in string. If the string is empty, the function returns 0. The value is also 0 when the first character of string is the null character, ASCII code 0.
(string-to-char "ABC")
=> 65
(string-to-char "xyz")
=> 120
(string-to-char "")
=> 0
(string-to-char "\000")
=> 0
This function may be eliminated in the future if it does not seem useful enough to retain.
Function: number-to-string number
Function: int-to-string number
This function returns a string consisting of the printed representation of number, which may be an integer or a floating point number. The value starts with a sign if the argument is negative.
(int-to-string 256)
=> "256"
(int-to-string -23)
=> "-23"
(int-to-string -23.5)
=> "-23.5"
See also the function format in section Formatting Strings.
Function: string-to-number string
Function: string-to-int string
This function returns the integer value of the characters in string, read as a number in base ten. It skips spaces at the beginning of string, then reads as much of string as it can interpret as a number. (On some systems it ignores other whitespace at the beginning, not just spaces.) If the first character after the ignored whitespace is not a digit or a minus sign, this function returns 0.
(string-to-number "256")
=> 256
(string-to-number "25 is a perfect square.")
=> 25
(string-to-number "X256")
=> 0
(string-to-number "-4.5")
=> -4.5
Formatting means constructing a string by substitution of computed values at various places in a constant string. This string controls how the other values are printed as well as where they appear; it is called a format string.
Formatting is often useful for computing messages to be displayed. In
fact, the functions message and error provide the same
formatting feature described here; they differ from format only
in how they use the result of formatting.
Function: format string &rest objects
This function returns a new string that is made by copying string and then replacing any format specification in the copy with encodings of the corresponding objects. The arguments objects are the computed values to be formatted.
A format specification is a sequence of characters beginning with a
`%'. Thus, if there is a `%d' in string, the
format function replaces it with the printed representation of
one of the values to be formatted (one of the arguments objects).
For example:
(format "The value of fill-column is %d." fill-column)
=> "The value of fill-column is 72."
If string contains more than one format specification, the format specifications are matched with successive values from objects. Thus, the first format specification in string is matched with the first such value, the second format specification is matched with the second such value, and so on. Any extra format specifications (those for which there are no corresponding values) cause unpredictable behavior. Any extra values to be formatted will be ignored.
Certain format specifications require values of particular types. However, no error is signaled if the value actually supplied fails to have the expected type. Instead, the output is likely to be meaningless.
Here is a table of the characters that can follow `%' to make up a format specification:
If there is no corresponding object, the empty string is used.
If there is no corresponding object, the empty string is used.
(format "%%
%d" 30) returns "% 30".
Any other format character results in an `Invalid format operation' error.
Here are several examples:
(format "The name of this buffer is %s." (buffer-name))
=> "The name of this buffer is strings.texi."
(format "The buffer object prints as %s." (current-buffer))
=> "The buffer object prints as #<buffer strings.texi>."
(format "The octal value of 18 is %o,
and the hex value is %x." 18 18)
=> "The octal value of 18 is 22,
and the hex value is 12."
All the specification characters allow an optional numeric prefix between the `%' and the character. The optional numeric prefix defines the minimum width for the object. If the printed representation of the object contains fewer characters than this, then it is padded. The padding is on the left if the prefix is positive (or starts with zero) and on the right if the prefix is negative. The padding character is normally a space, but if the numeric prefix starts with a zero, zeros are used for padding.
(format "%06d will be padded on the left with zeros" 123)
=> "000123 will be padded on the left with zeros"
(format "%-6d will be padded on the right" 123)
=> "123 will be padded on the right"
format never truncates an object's printed representation, no
matter what width you specify. Thus, you can use a numeric prefix to
specify a minimum spacing between columns with no risk of losing
information.
In the following three examples, `%7s' specifies a minimum width
of 7. In the first case, the string inserted in place of `%7s' has
only 3 letters, so 4 blank spaces are inserted for padding. In the
second case, the string "specification" is 13 letters wide but is
not truncated. In the third case, the padding is on the right.
(format "The word `%7s' actually has %d letters in it." "foo"
(length "foo"))
=> "The word ` foo' actually has 3 letters in it."
(format "The word `%7s' actually has %d letters in it."
"specification"
(length "specification"))
=> "The word `specification' actually has 13 letters in it."
(format "The word `%-7s' actually has %d letters in it." "foo"
(length "foo"))
=> "The word `foo ' actually has 3 letters in it."
The character case functions change the case of single characters or of the contents of strings. The functions convert only alphabetic characters (the letters `A' through `Z' and `a' through `z'); other characters are not altered. The functions do not modify the strings that are passed to them as arguments.
The examples below use the characters `X' and `x' which have ASCII codes 88 and 120 respectively.
Function: downcase string-or-char
This function converts a character or a string to lower case.
When the argument to downcase is a string, the function creates
and returns a new string in which each letter in the argument that is
upper case is converted to lower case. When the argument to
downcase is a character, downcase returns the
corresponding lower case character. This value is an integer. If the
original character is lower case, or is not a letter, then the value
equals the original character.
(downcase "The cat in the hat")
=> "the cat in the hat"
(downcase ?X)
=> 120
Function: upcase string-or-char
This function converts a character or a string to upper case.
When the argument to upcase is a string, the function creates
and returns a new string in which each letter in the argument that is
lower case is converted to upper case.
When the argument to upcase is a character, upcase
returns the corresponding upper case character. This value is an integer.
If the original character is upper case, or is not a letter, then the
value equals the original character.
(upcase "The cat in the hat")
=> "THE CAT IN THE HAT"
(upcase ?x)
=> 88
Function: capitalize string-or-char
This function capitalizes strings or characters. If string-or-char is a string, the function creates and returns a new string, whose contents are a copy of string-or-char in which each word has been capitalized. This means that the first character of each word is converted to upper case, and the rest are converted to lower case.
The definition of a word is any sequence of consecutive characters that are assigned to the word constituent category in the current syntax table (See section Table of Syntax Classes).
When the argument to capitalize is a character, capitalize
has the same result as upcase.
(capitalize "The cat in the hat")
=> "The Cat In The Hat"
(capitalize "THE 77TH-HATTED CAT")
=> "The 77th-Hatted Cat"
(capitalize ?x)
=> 88
You can customize case conversion by installing a special case table. A case table specifies the mapping between upper case and lower case letters. It affects both the string and character case conversion functions (see the previous section) and those that apply to text in the buffer (see section Case Changes). Use case table if you are using a language which has letters that are not the standard ASCII letters.
A case table is a list of this form:
(downcase upcase canonicalize equivalences)
where each element is either nil or a string of length 256. The
element downcase says how to map each character to its lower-case
equivalent. The element upcase maps each character to its
upper-case equivalent. If lower and upper case characters are in
one-to-one correspondence, use nil for upcase; then Emacs
deduces the upcase table from downcase.
For some languages, upper and lower case letters are not in one-to-one correspondence. There may be two different lower case letters with the same upper case equivalent. In these cases, you need to specify the maps for both directions.
The element canonicalize maps each character to a canonical equivalent; any two characters that are related by case-conversion have the same canonical equivalent character.
The element equivalences is a map that cyclicly permutes each equivalence class (of characters with the same canonical equivalent). (For ordinary ASCII, this would map `a' into `A' and `A' into `a', and likewise for each set of equivalent characters.)
You can provide nil for both canonicalize and
equivalences, in which case both are deduced from downcase
and upcase. Normally, that's what you should do, when you
construct a case table. Alternatively, you can provide suitable strings
for both canonicalize and equivalences. When you look at
the case table that's in use, you will find non-nil values for
those components. Do not try to make just one of these components
nil; that is not meaningful.
Each buffer has a case table. Emacs also has a standard case table which is copied into each buffer when you create the buffer. (Changing the standard case table doesn't affect any existing buffers.)
Here are the functions for working with case tables:
This predicate returns non-nil if object is a valid case
table.
Function: set-standard-case-table table
This function makes table the standard case table, so that it will apply to any buffers created subsequently.
This returns the standard case table.
This function returns the current buffer's case table.
Function: set-case-table table
This sets the current buffer's case table to table.
The following three functions are convenient subroutines for packages that define non-ASCII character sets. They modify a string downcase-table provided as an argument; this should be a string to be used as the downcase part of a case table. They also modify two syntax tables, the standard syntax table and the Text mode syntax table. (See section Syntax Tables.)
Function: set-case-syntax-pair uc lc downcase-table
This function specifies a pair of corresponding letters, one upper case and one lower case.
Function: set-case-syntax-delims l r downcase-table
This function makes characters l and r a matching pair of case-invariant delimiters.
Function: set-case-syntax char syntax downcase-table
This function makes char case-invariant, with syntax syntax.
Command: describe-buffer-case-table
This command displays a description of the contents of the current buffer's case table.
You can load the library `iso-syntax' to set up the syntax and case table for the 256 bit ISO Latin 1 character set.
A list represents a sequence of zero or more elements (which may be any Lisp objects). The important difference between lists and vectors is that two or more lists can share part of their structure; in addition, you can insert or delete elements in a list without copying the whole list.
Lists in Lisp are not a primitive data type; they are built up from cons cells. A cons cell is a data object which represents an ordered pair. It records two Lisp objects, one labeled as the CAR, and the other labeled as the CDR. (These names are traditional.)
A list is made by chaining cons cells together, one cons cell per
element. By convention, the CARs of the cons cells are the
elements of the list, and the CDRs are used to chain the list: the
CDR of each cons cell is the following cons cell. The CDR of
the last cons cell is nil. This asymmetry between the CAR
and the CDR is entirely a matter of convention; at the level of
cons cells, the CAR and CDR slots have the same
characteristics.
The symbol nil is considered a list as well as a symbol; it is
the list with no elements. For convenience, the symbol nil is
considered to have nil as its CDR (and also as its
CAR).
The CDR of any nonempty list l is a list containing all the elements of l except the first.
A cons cell can be illustrated as a pair of boxes. The first box
represents the CAR and the second box represents the CDR.
Here is an illustration of the two-element list, (tulip lily),
made from two cons cells:
--------------- --------------- | car | cdr | | car | cdr | | tulip | o---------->| lily | nil | | | | | | | --------------- ---------------
Each pair of boxes represents a cons cell. Each box "refers to",
"points to" or "contains" a Lisp object. (These terms are
synonymous.) The first box, which is the CAR of the first cons
cell, contains the symbol tulip. The arrow from the CDR of
the first cons cell to the second cons cell indicates that the CDR
of the first cons cell points to the second cons cell.
The same list can be illustrated in a different sort of box notation like this:
___ ___ ___ ___
|___|___|--> |___|___|--> nil
| |
| |
--> tulip --> lily
Here is a more complex illustration, this time of the three-element
list, ((pine needles) oak maple), the first element of which is
a two-element list:
___ ___ ___ ___ ___ ___
|___|___|--> |___|___|--> |___|___|--> nil
| | |
| | |
| --> oak --> maple
|
| ___ ___ ___ ___
--> |___|___|--> |___|___|--> nil
| |
| |
--> pine --> needles
The same list is represented in the first box notation like this:
-------------- -------------- --------------
| car | cdr | | car | cdr | | car | cdr |
| o | o------->| oak | o------->| maple | nil |
| | | | | | | | | |
-- | --------- -------------- --------------
|
|
| -------------- ----------------
| | car | cdr | | car | cdr |
------>| pine | o------->| needles | nil |
| | | | | |
-------------- ----------------
See section List Type, for the read and print syntax of lists, and for more "box and arrow" illustrations of lists.
The following predicates test whether a Lisp object is an atom, is a cons
cell or is a list, or whether it is the distinguished object nil.
(Many of these tests can be defined in terms of the others, but they are
used so often that it is worth having all of them.)
This function returns t if object is a cons cell, nil
otherwise. nil is not a cons cell, although it is a list.
This function returns t if object is an atom, nil
otherwise. All objects except cons cells are atoms. The symbol
nil is an atom and is also a list; it is the only Lisp object
which is both.
(atom object) == (not (consp object))
This function returns t if object is a cons cell or
nil. Otherwise, it returns nil.
(listp '(1))
=> t
(listp '())
=> t
This function is the opposite of listp: it returns t if
object is not a list. Otherwise, it returns nil.
(listp object) == (not (nlistp object))
This function returns t if object is nil, and
returns nil otherwise. This function is identical to not,
but as a matter of clarity we use null when object is
considered a list and not when it is considered a truth value
(see not in section Constructs for Combining Conditions).
(null '(1))
=> nil
(null '())
=> t
This function returns the value pointed to by the first pointer of the cons cell cons-cell. Expressed another way, this function returns the CAR of cons-cell.
As a special case, if cons-cell is nil, then car
is defined to return nil; therefore, any list is a valid argument
for car. An error is signaled if the argument is not a cons cell
or nil.
(car '(a b c))
=> a
(car '())
=> nil
This function returns the value pointed to by the second pointer of the cons cell cons-cell. Expressed another way, this function returns the CDR of cons-cell.
As a special case, if cons-cell is nil, then cdr
is defined to return nil; therefore, any list is a valid argument
for cdr. An error is signaled if the argument is not a cons cell
or nil.
(cdr '(a b c))
=> (b c)
(cdr '())
=> nil
This function lets you take the CAR of a cons cell while avoiding
errors for other data types. It returns the CAR of object if
object is a cons cell, nil otherwise. This is in contrast
to car, which signals an error if object is not a list.
(car-safe object)
==
(let ((x object))
(if (consp x)
(car x)
nil))
This function lets you take the CDR of a cons cell while
avoiding errors for other data types. It returns the CDR of
object if object is a cons cell, nil otherwise.
This is in contrast to cdr, which signals an error if
object is not a list.
(cdr-safe object)
==
(let ((x object))
(if (consp x)
(cdr x)
nil))
This function returns the nth element of list. Elements
are numbered starting with zero, so the CAR of list is
element number zero. If the length of list is n or less,
the value is nil.
If n is less than zero, then the first element is returned.
(nth 2 '(1 2 3 4))
=> 3
(nth 10 '(1 2 3 4))
=> nil
(nth -3 '(1 2 3 4))
=> 1
(nth n x) == (car (nthcdr n x))
This function returns the nth cdr of list. In other words, it removes the first n links of list and returns what follows.
If n is less than or equal to zero, then all of list is
returned. If the length of list is n or less, the value is
nil.
(nthcdr 1 '(1 2 3 4))
=> (2 3 4)
(nthcdr 10 '(1 2 3 4))
=> nil
(nthcdr -3 '(1 2 3 4))
=> (1 2 3 4)
Many functions build lists, as lists reside at the very heart of Lisp.
cons is the fundamental list-building function; however, it is
interesting to note that list is used more times in the source
code for Emacs than cons.
Function: cons object1 object2
This function is the fundamental function used to build new list structure. It creates a new cons cell, making object1 the CAR, and object2 the CDR. It then returns the new cons cell. The arguments object1 and object2 may be any Lisp objects, but most often object2 is a list.
(cons 1 '(2))
=> (1 2)
(cons 1 '())
=> (1)
(cons 1 2)
=> (1 . 2)
cons is often used to add a single element to the front of a
list. This is called consing the element onto the list. For
example:
(setq list (cons newelt list))
Note that there is no conflict between the variable named list
used in this example and the function named list described below;
any symbol can serve both functions.
This function creates a list with objects as its elements. The
resulting list is always nil-terminated. If no objects
are given, the empty list is returned.
(list 1 2 3 4 5)
=> (1 2 3 4 5)
(list 1 2 '(3 4 5) 'foo)
=> (1 2 (3 4 5) foo)
(list)
=> nil
Function: make-list length object
This function creates a list of length length, in which all the
elements have the identical value object. Compare
make-list with make-string (see section Creating Strings).
(make-list 3 'pigs)
=> (pigs pigs pigs)
(make-list 0 'pigs)
=> nil
Function: append &rest sequences
This function returns a list containing all the elements of sequences. The sequences may be lists, vectors, strings, or integers. All arguments except the last one are copied, so none of them are altered.
The final argument to append may be any object but it is
typically a list. The final argument is not copied or converted; it
becomes part of the structure of the new list.
Here is an example:
(setq trees '(pine oak))
=> (pine oak)
(setq more-trees (append '(maple birch) trees))
=> (maple birch pine oak)
trees
=> (pine oak)
more-trees
=> (maple birch pine oak)
(eq trees (cdr (cdr more-trees)))
=> t
You can see what happens by looking at a box diagram. The variable
trees is set to the list (pine oak) and then the variable
more-trees is set to the list (maple birch pine oak).
However, the variable trees continues to refer to the original
list:
more-trees trees
| |
| ___ ___ ___ ___ -> ___ ___ ___ ___
--> |___|___|--> |___|___|--> |___|___|--> |___|___|--> nil
| | | |
| | | |
--> maple -->birch --> pine --> oak
An empty sequence contributes nothing to the value returned by
append. As a consequence of this, a final nil argument
forces a copy of the previous argument.
trees
=> (pine oak)
(setq wood (append trees ()))
=> (pine oak)
wood
=> (pine oak)
(eq wood trees)
=> nil
This once was the standard way to copy a list, before the function
copy-sequence was invented. See section Sequences, Arrays, and Vectors.
With the help of apply, we can append all the lists in a list of
lists:
(apply 'append '((a b c) nil (x y z) nil))
=> (a b c x y z)
If no sequences are given, nil is returned:
(append)
=> nil
In the special case where one of the sequences is an integer
(not a sequence of integers), it is first converted to a string of
digits making up the decimal print representation of the integer. This
special case exists for compatibility with Mocklisp, and we don't
recommend you take advantage of it. If you want to convert an integer
in this way, use format (see section Formatting Strings) or
number-to-string (see section Conversion of Characters and Strings).
(setq trees '(pine oak))
=> (pine oak)
(char-to-string 54)
=> "6"
(setq longer-list (append trees 6 '(spruce)))
=> (pine oak 54 spruce)
(setq x-list (append trees 6 6))
=> (pine oak 54 . 6)
See nconc in section Functions that Rearrange Lists, for another way to join lists
without copying.
This function creates a new list whose elements are the elements of list, but in reverse order. The original argument list is not altered.
(setq x '(1 2 3 4))
=> (1 2 3 4)
(reverse x)
=> (4 3 2 1)
x
=> (1 2 3 4)
You can modify the CAR and CDR contents of a cons cell with the
primitives setcar and setcdr.
Common Lisp note: Common Lisp uses functionsrplacaandrplacdto alter list structure; they change structure the same way assetcarandsetcdr, but the Common Lisp functions return the cons cell whilesetcarandsetcdrreturn the new CAR or CDR.
setcar
Changing the CAR of a cons cell is done with setcar and
replaces one element of a list with a different element.
This function stores object as the new CAR of cons, replacing its previous CAR. It returns the value object. For example:
(setq x '(1 2))
=> (1 2)
(setcar x '4)
=> 4
x
=> (4 2)
When a cons cell is part of the shared structure of several lists, storing a new CAR into the cons changes one element of each of these lists. Here is an example:
;; Create two lists that are partly shared.
(setq x1 '(a b c))
=> (a b c)
(setq x2 (cons 'z (cdr x1)))
=> (z b c)
;; Replace the CAR of a shared link.
(setcar (cdr x1) 'foo)
=> foo
x1 ; Both lists are changed.
=> (a foo c)
x2
=> (z foo c)
;; Replace the CAR of a link that is not shared.
(setcar x1 'baz)
=> baz
x1 ; Only one list is changed.
=> (baz foo c)
x2
=> (z foo c)
Here is a graphical depiction of the shared structure of the two lists
x1 and x2, showing why replacing b changes them both:
___ ___ ___ ___ ___ ___
x1---> |___|___|----> |___|___|--> |___|___|--> nil
| --> | |
| | | |
--> a | --> b --> c
|
___ ___ |
x2--> |___|___|--
|
|
--> z
Here is an alternative form of box diagram, showing the same relationship:
x1:
-------------- -------------- --------------
| car | cdr | | car | cdr | | car | cdr |
| a | o------->| b | o------->| c | nil |
| | | -->| | | | | |
-------------- | -------------- --------------
|
x2: |
-------------- |
| car | cdr | |
| z | o----
| | |
--------------
The lowest-level primitive for modifying a CDR is setcdr:
This function stores object into the cdr of cons. The value returned is object, not cons.
Here is an example of replacing the CDR of a list with a different list. All but the first element of the list are removed in favor of a different sequence of elements. The first element is unchanged, because it resides in the CAR of the list, and is not reached via the CDR.
(setq x '(1 2 3))
=> (1 2 3)
(setcdr x '(4))
=> (4)
x
=> (1 4)
You can delete elements from the middle of a list by altering the
CDRs of the cons cells in the list. For example, here we delete
the second element, b, from the list (a b c), by changing
the CDR of the first cell:
(setq x1 '(a b c))
=> (a b c)
(setcdr x1 (cdr (cdr x1)))
=> (c)
x1
=> (a c)
Here is the result in box notation:
--------------------
| |
-------------- | -------------- | --------------
| car | cdr | | | car | cdr | -->| car | cdr |
| a | o----- | b | o-------->| c | nil |
| | | | | | | | |
-------------- -------------- --------------
The second cons cell, which previously held the element b, still
exists and its CAR is still b, but it no longer forms part
of this list.
It is equally easy to insert a new element by changing CDRs:
(setq x1 '(a b c))
=> (a b c)
(setcdr x1 (cons 'd (cdr x1)))
=> (d b c)
x1
=> (a d b c)
Here is this result in box notation:
-------------- ------------- -------------
| car | cdr | | car | cdr | | car | cdr |
| a | o | -->| b | o------->| c | nil |
| | | | | | | | | | |
--------- | -- | ------------- -------------
| |
----- --------
| |
| --------------- |
| | car | cdr | |
-->| d | o------
| | |
---------------
Here are some functions that rearrange lists "destructively" by modifying the CDRs of their component cons cells. We call these functions "destructive" because the original lists passed as arguments to them are chewed up to produce a new list that is subsequently returned.
This function returns a list containing all the elements of lists.
Unlike append (see section Building Cons Cells and Lists), the lists are
not copied. Instead, the last CDR of each of the
lists is changed to refer to the following list. The last of the
lists is not altered. For example:
(setq x '(1 2 3))
=> (1 2 3)
(nconc x '(4 5))
=> (1 2 3 4 5)
x
=> (1 2 3 4 5)
Since the last argument of nconc is not itself modified, it is
reasonable to use a constant list, such as '(4 5), as is done in
the above example. For the same reason, the last argument need not be a
list:
(setq x '(1 2 3))
=> (1 2 3)
(nconc x 'z)
=> (1 2 3 . z)
x
=> (1 2 3 . z)
A common pitfall is to use a quoted constant list as a non-last
argument to nconc. If you do this, your program will change
each time you run it! Here is what happens:
(defun add-foo (x) ; This function should add
(nconc '(foo) x)) ; foo to the front of its arg.
(symbol-function 'add-foo)
=> (lambda (x) (nconc (quote (foo)) x))
(setq xx (add-foo '(1 2))) ; It seems to work.
=> (foo 1 2)
(setq xy (add-foo '(3 4))) ; What happened?
=> (foo 1 2 3 4)
(eq xx xy)
=> t
(symbol-function 'add-foo)
=> (lambda (x) (nconc (quote (foo 1 2 3 4) x)))
This function reverses the order of the elements of list.
Unlike reverse, nreverse alters its argument destructively
by reversing the CDRs in the cons cells forming the list. The cons
cell which used to be the last one in list becomes the first cell
of the value.
For example:
(setq x '(1 2 3 4))
=> (1 2 3 4)
x
=> (1 2 3 4)
(nreverse x)
=> (4 3 2 1)
;; The cell that was first is now last.
x
=> (1)
To avoid confusion, we usually store the result of nreverse
back in the same variable which held the original list:
(setq x (nreverse x))
Here is the nreverse of our favorite example, (a b c),
presented graphically:
Original list head: Reversed list:
------------- ------------- ------------
| car | cdr | | car | cdr | | car | cdr |
| a | nil |<-- | b | o |<-- | c | o |
| | | | | | | | | | | | |
------------- | --------- | - | -------- | -
| | | |
------------- ------------
This function sorts list stably, though destructively, and returns the sorted list. It compares elements using predicate. A stable sort is one in which elements with equal sort keys maintain their relative order before and after the sort. Stability is important when successive sorts are used to order elements according to different criteria.
The argument predicate must be a function that accepts two
arguments. It is called with two elements of list. To get an
increasing order sort, the predicate should return t if the
first element is "less than" the second, or nil if not.
The destructive aspect of sort is that it rearranges the cons
cells forming list by changing CDRs. A nondestructive sort
function would create new cons cells to store the elements in their
sorted order. If you wish to sort a list without destroying the
original, copy it first with copy-sequence.
The CARs of the cons cells are not changed; the cons cell that
originally contained the element a in list still has
a in its CAR after sorting, but it now appears in a
different position in the list due to the change of CDRs. For
example:
(setq nums '(1 3 2 6 5 4 0))
=> (1 3 2 6 5 4 0)
(sort nums '<)
=> (0 1 2 3 4 5 6)
nums
=> (1 2 3 4 5 6)
Note that the list in nums no longer contains 0; this is the same
cons cell that it was before, but it is no longer the first one in the
list. Don't assume a variable that formerly held the argument now holds
the entire sorted list! Instead, save the result of sort and use
that. Most often we store the result back into the variable that held
the original list:
(setq nums (sort nums '<))
See section Sorting Text, for more functions that perform sorting.
See documentation in section Access to Documentation Strings, for a
useful example of sort.
The function delq in the following section is another example
of destructive list manipulation.
A list can represent an unordered mathematical set--simply consider a
value an element of a set if it appears in the list, and ignore the
order of the list. To form the union of two sets, use append (as
long as you don't mind having duplicate elements). Other useful
functions for sets include memq and delq, and their
equal versions, member and delete.
Common Lisp note: Common Lisp has functionsunion(which avoids duplicate elements) andintersectionfor set operations, but GNU Emacs Lisp does not have them. You can write them in Lisp if you wish.
This function tests to see whether object is a member of
list. If it is, memq returns a list starting with the
first occurrence of object. Otherwise, it returns nil.
The letter `q' in memq says that it uses eq to
compare object against the elements of the list. For example:
(memq 2 '(1 2 3 2 1))
=> (2 3 2 1)
(memq '(2) '((1) (2))) ; (2) and (2) are not eq.
=> nil
This function removes all elements eq to object from
list. The letter `q' in delq says that it uses
eq to compare object against the elements of the list, like
memq.
When delq deletes elements from the front of the list, it does so
simply by advancing down the list and returning a sublist that starts
after those elements:
(delq 'a '(a b c)) == (cdr '(a b c))
When an element to be deleted appears in the middle of the list, removing it involves changing the CDRs (see section Altering the CDR of a List).
(setq sample-list '(1 2 3 (4)))
=> (1 2 3 (4))
(delq 1 sample-list)
=> (2 3 (4))
sample-list
=> (1 2 3 (4))
(delq 2 sample-list)
=> (1 3 (4))
sample-list
=> (1 3 (4))
Note that (delq 2 sample-list) modifies sample-list to
splice out the second element, but (delq 1 sample-list) does not
splice anything--it just returns a shorter list. Don't assume that a
variable which formerly held the argument list now has fewer
elements, or that it still holds the original list! Instead, save the
result of delq and use that. Most often we store the result back
into the variable that held the original list:
(setq flowers (delq 'rose flowers))
In the following example, the (4) that delq attempts to match
and the (4) in the sample-list are not eq:
(delq '(4) sample-list)
=> (1 3 (4))
The following two functions are like memq and delq but use
equal rather than eq to compare elements. They are new in
Emacs 19.
The function member tests to see whether object is a member
of list, comparing members with object using equal.
If object is a member, member returns a list starting with
its first occurrence in list. Otherwise, it returns nil.
Compare this with memq:
(member '(2) '((1) (2))) ;(2)and(2)areequal. => ((2)) (memq '(2) '((1) (2))) ;(2)and(2)are noteq. => nil ;; Two strings with the same contents areequal. (member "foo" '("foo" "bar")) => ("foo" "bar")
This function removes all elements equal to object from
list. It is to delq as member is to memq: it
uses equal to compare elements with object, like
member; when it finds an element that matches, it removes the
element just as delq would. For example:
(delete '(2) '((2) (1) (2)))
=> '((1))
Common Lisp note: The functionsmemberanddeletein GNU Emacs Lisp are derived from Maclisp, not Common Lisp. The Common Lisp versions do not useequalto compare elements.
An association list, or alist for short, records a mapping from keys to values. It is a list of cons cells called associations: the CAR of each cell is the key, and the CDR is the associated value. (This usage of "key" is not related to the term "key sequence"; it means any object which can be looked up in a table.)
Here is an example of an alist. The key pine is associated with
the value cones; the key oak is associated with
acorns; and the key maple is associated with seeds.
'((pine . cones) (oak . acorns) (maple . seeds))
The associated values in an alist may be any Lisp objects; so may the
keys. For example, in the following alist, the symbol a is
associated with the number 1, and the string "b" is
associated with the list (2 3), which is the CDR of
the alist element:
((a . 1) ("b" 2 3))
Sometimes it is better to design an alist to store the associated value in the CAR of the CDR of the element. Here is an example:
'((rose red) (lily white) (buttercup yellow)))
Here we regard red as the value associated with rose. One
advantage of this method is that you can store other related
information--even a list of other items--in the CDR of the
CDR. One disadvantage is that you cannot use rassq (see
below) to find the element containing a given value. When neither of
these considerations is important, the choice is a matter of taste, as
long as you are consistent about it for any given alist.
Note that the same alist shown above could be regarded as having the
associated value in the CDR of the element; the value associated
with rose would be the list (red).
Association lists are often used to record information that you might otherwise keep on a stack, since new associations may be added easily to the front of the list. When searching an association list for an association with a given key, the first one found is returned, if there is more than one.
In Emacs Lisp, it is not an error if an element of an association list is not a cons cell. The alist search functions simply ignore such elements. Many other versions of Lisp signal errors in such cases.
Note that property lists are similar to association lists in several respects. A property list behaves like an association list in which each key can occur only once. See section Property Lists, for a comparison of property lists and association lists.
This function returns the first association for key in
alist. It compares key against the alist elements using
equal (see section Equality Predicates). It returns nil if no
association in alist has a CAR equal to key.
For example:
(setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
=> ((pine . cones) (oak . acorns) (maple . seeds))
(assoc 'oak trees)
=> (oak . acorns)
(cdr (assoc 'oak trees))
=> acorns
(assoc 'birch trees)
=> nil
Here is another example in which the keys and values are not symbols:
(setq needles-per-cluster
'((2 . ("Austrian Pine" "Red Pine"))
(3 . "Pitch Pine")
(5 . "White Pine")))
(cdr (assoc 3 needles-per-cluster))
=> "Pitch Pine"
(cdr (assoc 2 needles-per-cluster))
=> ("Austrian Pine" "Red Pine")
This function is like assoc in that it returns the first
association for key in alist, but it makes the comparison
using eq instead of equal. assq returns nil
if no association in alist has a CAR eq to key.
This function is used more often than assoc, since eq is
faster than equal and most alists use symbols as keys.
See section Equality Predicates.
(setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
(assq 'pine trees)
=> (pine . cones)
On the other hand, assq is not usually useful in alists where the
keys may not be symbols:
(setq leaves
'(("simple leaves" . oak)
("compound leaves" . horsechestnut)))
(assq "simple leaves" leaves)
=> nil
(assoc "simple leaves" leaves)
=> ("simple leaves" . oak)
This function returns the first association with value value in
alist. It returns nil if no association in alist has
a CDR eq to value.
rassq is like assq except that the CDR of the
alist associations is tested instead of the CAR. You can
think of this as "reverse assq", finding the key for a given
value.
For example:
(setq trees '((pine . cones) (oak . acorns) (maple . seeds)))
(rassq 'acorns trees)
=> (oak . acorns)
(rassq 'spores trees)
=> nil
Note that rassq cannot be used to search for a value stored in
the CAR of the CDR of an element:
(setq colors '((rose red) (lily white) (buttercup yellow)))
(rassq 'white colors)
=> nil
In this case, the CDR of the association (lily white) is not
the symbol white, but rather the list (white). This can
be seen more clearly if the association is written in dotted pair
notation:
(lily white) == (lily . (white))
This function returns a two-level deep copy of alist: it creates a new copy of each association, so that you can alter the associations of the new alist without changing the old one.
(setq needles-per-cluster
'((2 . ("Austrian Pine" "Red Pine"))
(3 . "Pitch Pine")
(5 . "White Pine")))
=>
((2 "Austrian Pine" "Red Pine")
(3 . "Pitch Pine")
(5 . "White Pine"))
(setq copy (copy-alist needles-per-cluster))
=>
((2 "Austrian Pine" "Red Pine")
(3 . "Pitch Pine")
(5 . "White Pine"))
(eq needles-per-cluster copy)
=> nil
(equal needles-per-cluster copy)
=> t
(eq (car needles-per-cluster) (car copy))
=> nil
(cdr (car (cdr needles-per-cluster)))
=> "Pitch Pine"
(eq (cdr (car (cdr needles-per-cluster)))
(cdr (car (cdr copy))))
=> t
Recall that the sequence type is the union of three other Lisp types: lists, vectors, and strings. In other words, any list is a sequence, any vector is a sequence, and any string is a sequence. The common property that all sequences have is that each is an ordered collection of elements.
An array is a single primitive object directly containing all its elements. Therefore, all the elements are accessible in constant time. The length of an existing array cannot be changed. Both strings and vectors are arrays. A list is a sequence of elements, but it is not a single primitive object; it is made of cons cells, one cell per element. Therefore, elements farther from the beginning of the list take longer to access, but it is possible to add elements to the list or remove elements. The elements of vectors and lists may be any Lisp objects. The elements of strings are all characters.
The following diagram shows the relationship between these types:
___________________________________
| |
| Sequence |
| ______ ______________________ |
| | | | | |
| | List | | Array | |
| | | | ________ _______ | |
| |______| | | | | | | |
| | | String | | Vector| | |
| | |________| |_______| | |
| |______________________| |
|___________________________________|
The Relationship between Sequences, Arrays, and Vectors
In Emacs Lisp, a sequence is either a list, a vector or a string. The common property that all sequences have is that each is an ordered collection of elements. This section describes functions that accept any kind of sequence.
Returns t if object is a list, vector, or
string, nil otherwise.
Function: copy-sequence sequence
Returns a copy of sequence. The copy is the same type of object as the original sequence, and it has the same elements in the same order.
Storing a new element into the copy does not affect the original
sequence, and vice versa. However, the elements of the new
sequence are not copies; they are identical (eq) to the elements
of the original. Therefore, changes made within these elements, as
found via the copied sequence, are also visible in the original
sequence.
If the sequence is a string with text properties, the property list in the copy is itself a copy, not shared with the original's property list. However, the actual values of the properties are shared. See section Text Properties.
See also append in section Building Cons Cells and Lists, concat in
section Creating Strings, and vconcat in section Vectors, for others
ways to copy sequences.
(setq bar '(1 2))
=> (1 2)
(setq x (vector 'foo bar))
=> [foo (1 2)]
(setq y (copy-sequence x))
=> [foo (1 2)]
(eq x y)
=> nil
(equal x y)
=> t
(eq (elt x 1) (elt y 1))
=> t
;; Replacing an element of one sequence.
(aset x 0 'quux)
x => [quux (1 2)]
y => [foo (1 2)]
;; Modifying the inside of a shared element.
(setcar (aref x 1) 69)
x => [quux (69 2)]
y => [foo (69 2)]
Returns the number of elements in sequence. If sequence is
a cons cell that is not a list (because the final CDR is not
nil), a wrong-type-argument error is signaled.
(length '(1 2 3))
=> 3
(length ())
=> 0
(length "foobar")
=> 6
(length [1 2 3])
=> 3
This function returns the element of sequence indexed by
index. Legitimate values of index are integers ranging from
0 up to one less than the length of sequence. If sequence
is a list, then out-of-range values of index return nil;
otherwise, they produce an args-out-of-range error.
(elt [1 2 3 4] 2)
=> 3
(elt '(1 2 3 4) 2)
=> 3
(char-to-string (elt "1234" 2))
=> "3"
(elt [1 2 3 4] 4)
error-->Args out of range: [1 2 3 4], 4
(elt [1 2 3 4] -1)
error-->Args out of range: [1 2 3 4], -1
This function duplicates aref (see section Functions that Operate on Arrays) and
nth (see section Accessing Elements of Lists), except that it works for any kind of
sequence.
An array object refers directly to a number of other Lisp objects, called the elements of the array. Any element of an array may be accessed in constant time. In contrast, an element of a list requires access time that is proportional to the position of the element in the list.
When you create an array, you must specify how many elements it has. The amount of space allocated depends on the number of elements. Therefore, it is impossible to change the size of an array once it is created. You cannot add or remove elements. However, you can replace an element with a different value.
Emacs defines two types of array, both of which are one-dimensional: strings and vectors. A vector is a general array; its elements can be any Lisp objects. A string is a specialized array; its elements must be characters (i.e., integers between 0 and 255). Each type of array has its own read syntax. See section String Type, and section Vector Type.
Both kinds of arrays share these characteristics:
aref and aset, respectively (see section Functions that Operate on Arrays).
In principle, if you wish to have an array of characters, you could use either a string or a vector. In practice, we always choose strings for such applications, for four reasons:
In this section, we describe the functions that accept both strings and vectors.
This function returns t if object is an array (i.e., either a
vector or a string).
(arrayp [a]) => t (arrayp "asdf") => t
This function returns the indexth element of array. The first element is at index zero.
(setq primes [2 3 5 7 11 13])
=> [2 3 5 7 11 13]
(aref primes 4)
=> 11
(elt primes 4)
=> 11
(aref "abcdefg" 1)
=> 98 ; `b' is ASCII code 98.
See also the function elt, in section Sequences.
Function: aset array index object
This function sets the indexth element of array to be object. It returns object.
(setq w [foo bar baz])
=> [foo bar baz]
(aset w 0 'fu)
=> fu
w
=> [fu bar baz]
(setq x "asdfasfd")
=> "asdfasfd"
(aset x 3 ?Z)
=> 90
x
=> "asdZasfd"
If array is a string and object is not a character, a
wrong-type-argument error results.
Function: fillarray array object
This function fills the array array with pointers to object, replacing any previous values. It returns array.
(setq a [a b c d e f g])
=> [a b c d e f g]
(fillarray a 0)
=> [0 0 0 0 0 0 0]
a
=> [0 0 0 0 0 0 0]
(setq s "When in the course")
=> "When in the course"
(fillarray s ?-)
=> "------------------"
If array is a string and object is not a character, a
wrong-type-argument error results.
The general sequence functions copy-sequence and length
are often useful for objects known to be arrays. See section Sequences.
Arrays in Lisp, like arrays in most languages, are blocks of memory whose elements can be accessed in constant time. A vector is a general-purpose array; its elements can be any Lisp objects. (The other kind of array provided in Emacs Lisp is the string, whose elements must be characters.) The main uses of vectors in Emacs are as syntax tables (vectors of integers) and keymaps (vectors of commands). They are also used internally as part of the representation of a byte-compiled function; if you print such a function, you will see a vector in it.
The indices of the elements of a vector are numbered starting with zero in Emacs Lisp.
Vectors are printed with square brackets surrounding the elements
in their order. Thus, a vector containing the symbols a,
b and c is printed as [a b c]. You can write
vectors in the same way in Lisp input.
A vector, like a string or a number, is considered a constant for evaluation: the result of evaluating it is the same vector. The elements of the vector are not evaluated. See section Self-Evaluating Forms.
Here are examples of these principles:
(setq avector [1 two '(three) "four" [five]])
=> [1 two (quote (three)) "four" [five]]
(eval avector)
=> [1 two (quote (three)) "four" [five]]
(eq avector (eval avector))
=> t
Here are some functions that relate to vectors:
This function returns t if object is a vector.
(vectorp [a])
=> t
(vectorp "asdf")
=> nil
Function: vector &rest objects
This function creates and returns a vector whose elements are the arguments, objects.
(vector 'foo 23 [bar baz] "rats")
=> [foo 23 [bar baz] "rats"]
(vector)
=> []
Function: make-vector integer object
This function returns a new vector consisting of integer elements, each initialized to object.
(setq sleepy (make-vector 9 'Z))
=> [Z Z Z Z Z Z Z Z Z]
Function: vconcat &rest sequences
This function returns a new vector containing all the elements of the sequences. The arguments sequences may be lists, vectors, or strings. If no sequences are given, an empty vector is returned.
The value is a newly constructed vector that is not eq to any
existing vector.
(setq a (vconcat '(A B C) '(D E F)))
=> [A B C D E F]
(eq a (vconcat a))
=> nil
(vconcat)
=> []
(vconcat [A B C] "aa" '(foo (6 7)))
=> [A B C 97 97 foo (6 7)]
When an argument is an integer (not a sequence of integers), it is
converted to a string of digits making up the decimal printed
representation of the integer. This special case exists for
compatibility with Mocklisp, and we don't recommend you take advantage
of it. If you want to convert an integer in this way, use format
(see section Formatting Strings) or int-to-string (see section Conversion of Characters and Strings).
For other concatenation functions, see mapconcat in section Mapping Functions, concat in section Creating Strings, and append
in section Building Cons Cells and Lists.
The append function may be used to convert a vector into a list
with the same elements (see section Building Cons Cells and Lists):
(setq avector [1 two (quote (three)) "four" [five]])
=> [1 two (quote (three)) "four" [five]]
(append avector nil)
=> (1 two (quote (three)) "four" [five])
A symbol is an object with a unique name. This chapter describes symbols, their components, and how they are created and interned. Property lists are also described. The uses of symbols as variables and as function names are described in separate chapters; see section Variables, and section Functions. For the precise syntax for symbols, see section Symbol Type.
You can test whether an arbitrary Lisp object is a symbol
with symbolp:
This function returns t if object is a symbol, nil
otherwise.
Each symbol has four components (or "cells"), each of which references another object:
symbol-name in section Creating and Interning Symbols.
symbol-value in
section Accessing Variable Values.
symbol-function in section Accessing Function Cell Contents.
symbol-plist in section Property Lists.
The print name cell always holds a string, and cannot be changed. The other three cells can be set individually to any specified Lisp object.
The print name cell holds the string that is the name of the symbol. Since symbols are represented textually by their names, it is important not to have two symbols with the same name. The Lisp reader ensures this: every time it reads a symbol, it looks for an existing symbol with the specified name before it creates a new one. (In GNU Emacs Lisp, this is done with a hashing algorithm that uses an obarray; see section Creating and Interning Symbols.)
In normal usage, the function cell usually contains a function or
macro, as that is what the Lisp interpreter expects to see there
(see section Evaluation). Keyboard macros (see section Keyboard Macros),
keymaps (see section Keymaps) and autoload objects (see section Autoloading) are
also sometimes stored in the function cell of symbols. We often refer
to "the function foo" when we really mean the function stored
in the function cell of the symbol foo. We make the distinction
only when necessary.
Similarly, the property list cell normally holds a correctly formatted property list (see section Property Lists), as a number of functions expect to see a property list there.
The function cell or the value cell may be void, which means
that the cell does not reference any object. (This is not the same
thing as holding the symbol void, nor the same as holding the
symbol nil.) Examining the value of a cell which is void results
in an error, such as `Symbol's value as variable is void'.
The four functions symbol-name, symbol-value,
symbol-plist, and symbol-function return the contents of
the four cells. Here as an example we show the contents of the four
cells of the symbol buffer-file-name:
(symbol-name 'buffer-file-name)
=> "buffer-file-name"
(symbol-value 'buffer-file-name)
=> "/gnu/elisp/symbols.texi"
(symbol-plist 'buffer-file-name)
=> (variable-documentation 29529)
(symbol-function 'buffer-file-name)
=> #<subr buffer-file-name>
Because this symbol is the variable which holds the name of the file
being visited in the current buffer, the value cell contents we see are
the name of the source file of this chapter of the Emacs Lisp Manual.
The property list cell contains the list (variable-documentation
29529) which tells the documentation functions where to find
documentation about buffer-file-name in the `DOC' file.
(29529 is the offset from the beginning of the `DOC' file where the
documentation for the function begins.) The function cell contains the
function for returning the name of the file. buffer-file-name
names a primitive function, which has no read syntax and prints in hash
notation (see section Primitive Function Type). A symbol naming a function
written in Lisp would have a lambda expression (or a byte-code object)
in this cell.
A definition in Lisp is a special form that announces your intention to use a certain symbol in a particular way. In Emacs Lisp, you can define a symbol as a variable, or define it as a function (or macro), or both independently.
A definition construct typically specifies a value or meaning for the symbol for one kind of use, plus documentation for its meaning when used in this way. Thus, when you define a symbol as a variable, you can supply an initial value for the variable, plus documentation for the variable.
defvar and defconst are special forms that define a
symbol as a global variable. They are documented in detail in
section Defining Global Variables.
defun defines a symbol as a function, creating a lambda
expression and storing it in the function cell of the symbol. This
lambda expression thus becomes the function definition of the symbol.
(The term "function definition", meaning the contents of the function
cell, is derived from the idea that defun gives the symbol its
definition as a function.) See section Functions.
defmacro defines a symbol as a macro. It creates a macro
object and stores it in the function cell of the symbol. Note that a
given symbol can be a macro or a function, but not both at once, because
both macro and function definitions are kept in the function cell, and
that cell can hold only one Lisp object at any given time.
See section Macros.
In GNU Emacs Lisp, a definition is not required in order to use a
symbol as a variable or function. Thus, you can make a symbol a global
variable with setq, whether you define it first or not. The real
purpose of definitions is to guide programmers and programming tools.
They inform programmers who read the code that certain symbols are
intended to be used as variables, or as functions. In addition,
utilities such as `etags' and `make-docfile' can recognize
definitions, and add the appropriate information to tag tables and the
`emacs/etc/DOC-version' file. See section Access to Documentation Strings.
To understand how symbols are created in GNU Emacs Lisp, you must know how Lisp reads them. Lisp must ensure that it finds the same symbol every time it reads the same set of characters. Failure to do so would cause complete confusion.
When the Lisp reader encounters a symbol, it reads all the characters of the name. Then it "hashes" those characters to find an index in a table called an obarray. Hashing is an efficient method of looking something up. For example, instead of searching a telephone book cover to cover when looking up Jan Jones, you start with the J's and go from there. That is a simple version of hashing. Each element of the obarray is a bucket which holds all the symbols with a given hash code; to look for a given name, it is sufficient to look through all the symbols in the bucket for that name's hash code.
If a symbol with the desired name is found, then it is used. If no such symbol is found, then a new symbol is created and added to the obarray bucket. Adding a symbol to an obarray is called interning it, and the symbol is then called an interned symbol. In Emacs Lisp, a symbol may be interned in only one obarray--if you try to intern the same symbol in more than one obarray, you will get unpredictable results.
It is possible for two different symbols to have the same name in
different obarrays; these symbols are not eq or equal.
However, this normally happens only as part of abbrev definition
(see section Abbrevs And Abbrev Expansion).
Common Lisp note: in Common Lisp, a symbol may be interned in several obarrays at once.
If a symbol is not in the obarray, then there is no way for Lisp to find it when its name is read. Such a symbol is called an uninterned symbol relative to the obarray. An uninterned symbol has all the other characteristics of symbols.
In Emacs Lisp, an obarray is represented as a vector. Each element of
the vector is a bucket; its value is either an interned symbol whose
name hashes to that bucket, or 0 if the bucket is empty. Each interned
symbol has an internal link (invisible to the user) to the next symbol
in the bucket. Because these links are invisible, there is no way to
scan the symbols in an obarray except using mapatoms (below).
The order of symbols in a bucket is not significant.
In an empty obarray, every element is 0, and you can create an obarray
with (make-vector length 0). This is the only
valid way to create an obarray. Prime numbers as lengths tend
to result in good hashing; lengths one less than a power of two are also
good.
Do not try to create an obarray that is not empty. This
does not work--only intern can enter a symbol in an obarray
properly. Also, don't try to put into an obarray of your own
a symbol that is already interned in the main obarray, because in
Emacs Lisp a symbol cannot be in two obarrays at once.
Most of the functions below take a name and sometimes an obarray as
arguments. A wrong-type-argument error is signaled if the name
is not a string, or if the obarray is not a vector.
This function returns the string that is symbol's name. For example:
(symbol-name 'foo)
=> "foo"
Changing the string by substituting characters, etc, does change the name of the symbol, but fails to update the obarray, so don't do it!
This function returns a newly-allocated, uninterned symbol whose name is
name (which must be a string). Its value and function definition
are void, and its property list is nil. In the example below,
the value of sym is not eq to foo because it is a
distinct uninterned symbol whose name is also `foo'.
(setq sym (make-symbol "foo"))
=> foo
(eq sym 'foo)
=> nil
Function: intern name &optional obarray
This function returns the interned symbol whose name is name. If
there is no such symbol in the obarray, a new one is created, added to
the obarray, and returned. If obarray is supplied, it specifies
the obarray to use; otherwise, the value of the global variable
obarray is used.
(setq sym (intern "foo"))
=> foo
(eq sym 'foo)
=> t
(setq sym1 (intern "foo" other-obarray))
=> foo
(eq sym 'foo)
=> nil
Function: intern-soft name &optional obarray
This function returns the symbol whose name is name, or nil
if a symbol with that name is not found in the obarray. Therefore, you
can use intern-soft to test whether a symbol with a given name is
interned. If obarray is supplied, it specifies the obarray to
use; otherwise the value of the global variable obarray is used.
(intern-soft "frazzle") ; No such symbol exists.
=> nil
(make-symbol "frazzle") ; Create an uninterned one.
=> frazzle
(intern-soft "frazzle") ; That one cannot be found.
=> nil
(setq sym (intern "frazzle")) ; Create an interned one.
=> frazzle
(intern-soft "frazzle") ; That one can be found!
=> frazzle
(eq sym 'frazzle) ; And it is the same one.
=> t
This variable is the standard obarray for use by intern and
read.
Function: mapatoms function &optional obarray
This function applies function to every symbol in obarray.
It returns nil. If obarray is not supplied, it defaults to
the value of obarray, the standard obarray for ordinary symbols.
(setq count 0)
=> 0
(defun count-syms (s)
(setq count (1+ count)))
=> count-syms
(mapatoms 'count-syms)
=> nil
count
=> 1871
See documentation in section Access to Documentation Strings, for another
example using mapatoms.
A property list (plist for short) is a list of paired elements stored in the property list cell of a symbol. Each of the pairs associates a property name (usually a symbol) with a property or value. Property lists are generally used to record information about a symbol, such as how to compile it, the name of the file where it was defined, or perhaps even the grammatical class of the symbol (representing a word) in a language understanding system.
Character positions in a string or buffer can also have property lists. See section Text Properties.
The property names and values in a property list can be any Lisp
objects, but the names are usually symbols. They are compared using
eq. Here is an example of a property list, found on the symbol
progn when the compiler is loaded:
(lisp-indent-function 0 byte-compile byte-compile-progn)
Here lisp-indent-function and byte-compile are property
names, and the other two elements are the corresponding values.
Association lists (see section Association Lists) are very similar to property lists. In contrast to association lists, the order of the pairs in the property list is not significant since the property names must be distinct.
Property lists are better than association lists when it is necessary
to attach information to various Lisp function names or variables. If
all the pairs are recorded in one association list, the program will
need to search that entire list each time a function or variable is to
be operated on. By contrast, if the information is recorded in the
property lists of the function names or variables themselves, each
search will scan only the length of one property list, which is usually
short. For this reason, the documentation for a variable is recorded in
a property named variable-documentation. The byte compiler
likewise uses properties to record those functions needing special
treatment.
However, association lists have their own advantages. Depending on your application, it may be faster to add an association to the front of an association list than to update a property. All properties for a symbol are stored in the same property list, so there is a possibility of a conflict between different uses of a property name. (For this reason, it is a good idea to use property names that are probably unique, such as by including the name of the library in the property name.) An association list may be used like a stack where associations are pushed on the front of the list and later discarded; this is not possible with a property list.
This function returns the property list of symbol.
Function: setplist symbol plist
This function sets symbol's property list to plist. Normally, plist should be a well-formed property list, but this is not enforced.
(setplist 'foo '(a 1 b (2 3) c nil))
=> (a 1 b (2 3) c nil)
(symbol-plist 'foo)
=> (a 1 b (2 3) c nil)
For symbols in special obarrays, which are not used for ordinary purposes, it may make sense to use the property list cell in a nonstandard fashion; in fact, the abbrev mechanism does so (see section Abbrevs And Abbrev Expansion).
This function finds the value of the property named property in
symbol's property list. If there is no such property, nil
is returned. Thus, there is no distinction between a value of
nil and the absence of the property.
The name property is compared with the existing property names
using eq, so any object is a legitimate property.
See put for an example.
Function: put symbol property value
This function puts value onto symbol's property list under the property name property, replacing any previous value.
(put 'fly 'verb 'transitive)
=>'transitive
(put 'fly 'noun '(a buzzing little bug))
=> (a buzzing little bug)
(get 'fly 'verb)
=> transitive
(symbol-plist 'fly)
=> (verb transitive noun (a buzzing little bug))
The evaluation of expressions in Emacs Lisp is performed by the
Lisp interpreter---a program that receives a Lisp object as input
and computes its value as an expression. The value is computed in
a fashion that depends on the data type of the object, following rules
described in this chapter. The interpreter runs automatically
to evaluate portions of your program, but can also be called explicitly
via the Lisp primitive function eval.
A Lisp object which is intended for evaluation is called an expression or a form. The fact that expressions are data objects and not merely text is one of the fundamental differences between Lisp-like languages and typical programming languages. Any object can be evaluated, but in practice only numbers, symbols, lists and strings are evaluated very often.
It is very common to read a Lisp expression and then evaluate the
expression, but reading and evaluation are separate activities, and
either can be performed alone. Reading per se does not evaluate
anything; it converts the printed representation of a Lisp object to the
object itself. It is up to the caller of read whether this
object is a form to be evaluated, or serves some entirely different
purpose. See section Input Functions.
Do not confuse evaluation with command key interpretation. The
editor command loop translates keyboard input into a command (an
interactively callable function) using the active keymaps, and then
uses call-interactively to invoke the command. The execution of
the command itself involves evaluation if the command is written in
Lisp, but that is not a part of command key interpretation itself.
See section Command Loop.
Evaluation is a recursive process. That is, evaluation of a form may
cause eval to be called again in order to evaluate parts of the
form. For example, evaluation of a function call first evaluates each
argument of the function call, and then evaluates each form in the
function body. Consider evaluation of the form (car x): the
subform x must first be evaluated recursively, so that its value
can be passed as an argument to the function car.
The evaluation of forms takes place in a context called the environment, which consists of the current values and bindings of all Lisp variables.(1) Whenever the form refers to a variable without creating a new binding for it, the value of the binding in the current environment is used. See section Variables.
Evaluation of a form may create new environments for recursive
evaluation by binding variables (see section Local Variables). These
environments are temporary and will be gone by the time evaluation of
the form is complete. The form may also make changes that persist;
these changes are called side effects. An example of a form that
produces side effects is (setq foo 1).
Finally, evaluation of one particular function call, byte-code,
invokes the byte-code interpreter on its arguments. Although the
byte-code interpreter is not the same as the Lisp interpreter, it uses
the same environment as the Lisp interpreter, and may on occasion invoke
the Lisp interpreter. (See section Byte Compilation.)
The details of what evaluation means for each kind of form are described below (see section Kinds of Forms).
Most often, forms are evaluated automatically, by virtue of their
occurrence in a program being run. On rare occasions, you may need to
write code that evaluates a form that is computed at run time, such as
after reading a form from text being edited or getting one from a
property list. On these occasions, use the eval function.
The functions and variables described in this section evaluate forms, specify limits to the evaluation process, or record recently returned values. Loading a file also does evaluation (see section Loading).
This is the basic function for performing evaluation. It evaluates form in the current environment and returns the result. How the evaluation proceeds depends on the type of the object (see section Kinds of Forms).
Since eval is a function, the argument expression that appears
in a call to eval is evaluated twice: once as preparation before
eval is called, and again by the eval function itself.
Here is an example:
(setq foo 'bar)
=> bar
(setq bar 'baz)
=> baz
;; eval receives argument bar, which is the value of foo
(eval foo)
=> baz
The number of currently active calls to eval is limited to
max-lisp-eval-depth (see below).
Command: eval-current-buffer &optional stream
This function evaluates the forms in the current buffer. It reads
forms from the buffer and calls eval on them until the end of the
buffer is reached, or until an error is signaled and not handled.
If stream is supplied, the variable standard-output is
bound to stream during the evaluation (see section Output Functions).
eval-current-buffer always returns nil.
Command: eval-region start end &optional stream
This function evaluates the forms in the current buffer in the region
defined by the positions start and end. It reads forms from
the region and calls eval on them until the end of the region is
reached, or until an error is signaled and not handled.
If stream is supplied, standard-output is bound to it
for the duration of the command.
eval-region always returns nil.
This variable defines the maximum depth allowed in calls to
eval, apply, and funcall before an error is
signaled (with error message "Lisp nesting exceeds
max-lisp-eval-depth"). eval is called recursively to evaluate
the arguments of Lisp function calls and to evaluate bodies of
functions.
This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function.
The default value of this variable is 200. If you set it to a value less than 100, Lisp will reset it to 100 if the given value is reached.
max-specpdl-size provides another limit on nesting.
See section Local Variables.
The value of this variable is a list of values returned by all expressions which were read from buffers (including the minibuffer), evaluated, and printed. The elements are in order, most recent first.
(setq x 1)
=> 1
(list 'A (1+ 2) auto-save-default)
=> (A 3 t)
values
=> ((A 3 t) 1 ...)
This variable is useful for referring back to values of forms recently
evaluated. It is generally a bad idea to print the value of
values itself, since this may be very long. Instead, examine
particular elements, like this:
;; Refer to the most recent evaluation result.
(nth 0 values)
=> (A 3 t)
;; That put a new element on,
;; so all elements move back one.
(nth 1 values)
=> (A 3 t)
;; This gets the element that was next-to-last
;; before this example.
(nth 3 values)
=> 1
A Lisp object that is intended to be evaluated is called a form. How Emacs evaluates a form depends on its data type. Emacs has three different kinds of form that are evaluated differently: symbols, lists, and "all other types". All three kinds are described in this section, starting with "all other types" which are self-evaluating forms.
A self-evaluating form is any form that is not a list or symbol.
Self-evaluating forms evaluate to themselves: the result of evaluation
is the same object that was evaluated. Thus, the number 25 evaluates to
25, and the string "foo" evaluates to the string "foo".
Likewise, evaluation of a vector does not cause evaluation of the
elements of the vector--it returns the same vector with its contents
unchanged.
'123 ; An object, shown without evaluation.
=> 123
123 ; Evaluated as usual--result is the same.
=> 123
(eval '123) ; Evaluated "by hand"---result is the same.
=> 123
(eval (eval '123)) ; Evaluating twice changes nothing.
=> 123
It is common to write numbers, characters, strings, and even vectors in Lisp code, taking advantage of the fact that they self-evaluate. However, it is quite unusual to do this for types that lack a read syntax, because it is inconvenient and not very useful; however, it is possible to put them inside Lisp programs when they are constructed from subexpressions rather than read. Here is an example:
;; Build such an expression.
(setq buffer (list 'print (current-buffer)))
=> (print #<buffer eval.texi>)
;; Evaluate it.
(eval buffer)
-| #<buffer eval.texi>
=> #<buffer eval.texi>
When a symbol is evaluated, it is treated as a variable. The result is the variable's value, if it has one. If it has none (if its value cell is void), an error is signaled. For more information on the use of variables, see section Variables.
In the following example, we set the value of a symbol with
setq. When the symbol is later evaluated, that value is
returned.
(setq a 123)
=> 123
(eval 'a)
=> 123
a
=> 123
The symbols nil and t are treated specially, so that the
value of nil is always nil, and the value of t is
always t. Thus, these two symbols act like self-evaluating
forms, even though eval treats them like any other symbol.
A form that is a nonempty list is either a function call, a macro call, or a special form, according to its first element. These three kinds of forms are evaluated in different ways, described below. The rest of the list consists of arguments for the function, macro or special form.
The first step in evaluating a nonempty list is to examine its first element. This element alone determines what kind of form the list is and how the rest of the list is to be processed. The first element is not evaluated, as it would be in some Lisp dialects including Scheme.
If the first element of the list is a symbol then evaluation examines the symbol's function cell, and uses its contents instead of the original symbol. If the contents are another symbol, this process, called symbol function indirection, is repeated until a non-symbol is obtained. See section Naming a Function, for more information about using a symbol as a name for a function stored in the function cell of the symbol.
One possible consequence of this process is an infinite loop, in the
event that a symbol's function cell refers to the same symbol. Or a
symbol may have a void function cell, causing a void-function
error. But if neither of these things happens, we eventually obtain a
non-symbol, which ought to be a function or other suitable object.
More precisely, we should now have a Lisp function (a lambda
expression), a byte-code function, a primitive function, a Lisp macro, a
special form, or an autoload object. Each of these types is a case
described in one of the following sections. If the object is not one of
these types, the error invalid-function is signaled.
The following example illustrates the symbol indirection process. We
use fset to set the function cell of a symbol and
symbol-function to get the function cell contents
(see section Accessing Function Cell Contents). Specifically, we store the symbol car
into the function cell of first, and the symbol first into
the function cell of erste.
;; Build this function cell linkage: ;; ------------- ----- ------- ------- ;; | #<subr car> | <-- | car | <-- | first | <-- | erste | ;; ------------- ----- ------- -------
(symbol-function 'car)
=> #<subr car>
(fset 'first 'car)
=> car
(fset 'erste 'first)
=> first
(erste '(1 2 3)) ; Call the function referenced by erste.
=> 1
By contrast, the following example calls a function without any symbol function indirection, because the first element is an anonymous Lisp function, not a symbol.
((lambda (arg) (erste arg))
'(1 2 3))
=> 1
After that function is called, its body is evaluated; this does
involve symbol function indirection when calling erste.
The built-in function indirect-function provides an easy way to
perform symbol function indirection explicitly.
Function: indirect-function function
This function returns the meaning of function as a function. If function is a symbol, then it finds function's function definition and starts over with that value. If function is not a symbol, then it returns function itself.
Here is how you could define indirect-function in Lisp:
(defun indirect-function (function)
(if (symbolp function)
(indirect-function (symbol-function function))
function))
If the first element of a list being evaluated is a Lisp function
object, byte-code object or primitive function object, then that list is
a function call. For example, here is a call to the function
+:
(+ 1 x)
When a function call is evaluated, the first step is to evaluate the
remaining elements of the list in the order they appear. The results
are the actual argument values, one argument from each element. Then
the function is called with this list of arguments, effectively using
the function apply (see section Calling Functions). If the function
is written in Lisp, the arguments are used to bind the argument
variables of the function (see section Lambda Expressions); then the forms
in the function body are evaluated in order, and the result of the last
one is used as the value of the function call.
If the first element of a list being evaluated is a macro object, then the list is a macro call. When a macro call is evaluated, the elements of the rest of the list are not initially evaluated. Instead, these elements themselves are used as the arguments of the macro. The macro definition computes a replacement form, called the expansion of the macro, which is evaluated in place of the original form. The expansion may be any sort of form: a self-evaluating constant, a symbol or a list. If the expansion is itself a macro call, this process of expansion repeats until some other sort of form results.
Normally, the argument expressions are not evaluated as part of computing the macro expansion, but instead appear as part of the expansion, so they are evaluated when the expansion is evaluated.
For example, given a macro defined as follows:
(defmacro cadr (x) (list 'car (list 'cdr x)))
an expression such as (cadr (assq 'handler list)) is a macro
call, and its expansion is:
(car (cdr (assq 'handler list)))
Note that the argument (assq 'handler list) appears in the
expansion.
See section Macros, for a complete description of Emacs Lisp macros.
A special form is a primitive function specially marked so that its arguments are not all evaluated. Special forms define control structures or perform variable bindings--things which functions cannot do.
Each special form has its own rules for which arguments are evaluated and which are used without evaluation. Whether a particular argument is evaluated may depend on the results of evaluating other arguments.
Here is a list, in alphabetical order, of all of the special forms in Emacs Lisp with a reference to where each is described.
and
catch
catch and throw
cond
condition-case
defconst
defmacro
defun
defvar
function
if
interactive
let
let*
or
prog1
prog2
progn
quote
save-excursion
save-restriction
save-window-excursion
setq
setq-default
track-mouse
unwind-protect
while
with-output-to-temp-buffer
Common Lisp note: here are some comparisons of special forms in GNU Emacs Lisp and Common Lisp.setq,if, andcatchare special forms in both Emacs Lisp and Common Lisp.defunis a special form in Emacs Lisp, but a macro in Common Lisp.save-excursionis a special form in Emacs Lisp, but doesn't exist in Common Lisp.throwis a special form in Common Lisp (because it must be able to throw multiple values), but it is a function in Emacs Lisp (which doesn't have multiple values).
The autoload feature allows you to call a function or macro whose function definition has not yet been loaded into Emacs. When an autoload object appears as a symbol's function definition and that symbol is used as a function, Emacs will automatically install the real definition (plus other associated code) and then call that definition. (See section Autoload.)
The special form quote returns its single argument
"unchanged".
This special form returns object, without evaluating it. This allows symbols and lists, which would normally be evaluated, to be included literally in a program. (It is not necessary to quote numbers, strings, and vectors since they are self-evaluating.)
Because quote is used so often in programs, Lisp provides a
convenient read syntax for it. An apostrophe character (`'')
followed by a Lisp object (in read syntax) expands to a list whose first
element is quote, and whose second element is the object. Thus,
the read syntax 'x is an abbreviation for (quote x).
Here are some examples of expressions that use quote:
(quote (+ 1 2))
=> (+ 1 2)
(quote foo)
=> foo
'foo
=> foo
"foo
=> (quote foo)
'(quote foo)
=> (quote foo)
['foo]
=> [(quote foo)]
Other quoting constructs include function (see section Anonymous Functions), which causes an anonymous lambda expression written in Lisp
to be compiled, and ` (see section Backquote), which is used to quote
only part of a list, while computing and substituting other parts.
A Lisp program consists of expressions or forms (see section Kinds of Forms). We control the order of execution of the forms by enclosing them in control structures. Control structures are special forms which control when, whether, or how many times to execute the forms they contain.
The simplest control structure is sequential execution: first form a, then form b, and so on. This is what happens when you write several forms in succession in the body of a function, or at top level in a file of Lisp code--the forms are executed in the order they are written. We call this textual order. For example, if a function body consists of two forms a and b, evaluation of the function evaluates first a and then b, and the function's value is the value of b.
Naturally, Emacs Lisp has many kinds of control structures, including other varieties of sequencing, function calls, conditionals, iteration, and (controlled) jumps. The built-in control structures are special forms since their subforms are not necessarily evaluated. You can use macros to define your own control structure constructs (see section Macros).
Evaluating forms in the order they are written is the most common
control structure. Sometimes this happens automatically, such as in a
function body. Elsewhere you must use a control structure construct to
do this: progn, the simplest control construct of Lisp.
A progn special form looks like this:
(progn a b c ...)
and it says to execute the forms a, b, c and so on, in
that order. These forms are called the body of the progn form.
The value of the last form in the body becomes the value of the entire
progn.
When Lisp was young, progn was the only way to execute two or
more forms in succession and use the value of the last of them. But
programmers found they often needed to use a progn in the body of
a function, where (at that time) only one form was allowed. So the body
of a function was made into an "implicit progn": several forms
are allowed just as in the body of an actual progn. Many other
control structures likewise contain an implicit progn. As a
result, progn is not used as often as it used to be. It is
needed now most often inside of an unwind-protect, and, or
or.
This special form evaluates all of the forms, in textual order, returning the result of the final form.
(progn (print "The first form")
(print "The second form")
(print "The third form"))
-| "The first form"
-| "The second form"
-| "The third form"
=> "The third form"
Two other control constructs likewise evaluate a series of forms but return a different value:
Special Form: prog1 form1 forms...
This special form evaluates form1 and all of the forms, in textual order, returning the result of form1.
(prog1 (print "The first form")
(print "The second form")
(print "The third form"))
-| "The first form"
-| "The second form"
-| "The third form"
=> "The first form"
Here is a way to remove the first element from a list in the variable
x, then return the value of that former element:
(prog1 (car x) (setq x (cdr x)))
Special Form: prog2 form1 form2 forms...
This special form evaluates form1, form2, and all of the following forms, in textual order, returning the result of form2.
(prog2 (print "The first form")
(print "The second form")
(print "The third form"))
-| "The first form"
-| "The second form"
-| "The third form"
=> "The second form"
Conditional control structures choose among alternatives. Emacs Lisp
has two conditional forms: if, which is much the same as in other
languages, and cond, which is a generalized case statement.
Special Form: if condition then-form else-forms...
if chooses between the then-form and the else-forms
based on the value of condition. If the evaluated condition is
non-nil, then-form is evaluated and the result returned.
Otherwise, the else-forms are evaluated in textual order, and the
value of the last one is returned. (The else part of if is
an example of an implicit progn. See section Sequencing.)
If condition has the value nil, and no else-forms are
given, if returns nil.
if is a special form because the branch which is not selected is
never evaluated--it is ignored. Thus, in the example below,
true is not printed because print is never called.
(if nil
(print 'true)
'very-false)
=> very-false
cond chooses among an arbitrary number of alternatives. Each
clause in the cond must be a list. The CAR of this
list is the condition; the remaining elements, if any, the
body-forms. Thus, a clause looks like this:
(condition body-forms...)
cond tries the clauses in textual order, by evaluating the
condition of each clause. If the value of condition is
non-nil, the body-forms are evaluated, and the value of the
last of body-forms becomes the value of the cond. The
remaining clauses are ignored.
If the value of condition is nil, the clause "fails", so
the cond moves on to the following clause, trying its
condition.
If every condition evaluates to nil, so that every clause
fails, cond returns nil.
A clause may also look like this:
(condition)
Then, if condition is non-nil when tested, the value of
condition becomes the value of the cond form.
The following example has four clauses, which test for the cases where
the value of x is a number, string, buffer and symbol,
respectively:
(cond ((numberp x) x)
((stringp x) x)
((bufferp x)
(setq temporary-hack x) ; multiple body-forms
(buffer-name x)) ; in one clause
((symbolp x) (symbol-value x)))
Often we want the last clause to be executed whenever none of the
previous clauses was successful. To do this, we use t as the
condition of the last clause, like this: (t
body-forms). The form t evaluates to t, which
is never nil, so this clause never fails, provided the
cond gets to it at all.
For example,
(cond ((eq a 1) 'foo)
(t "default"))
=> "default"
This expression is a cond which returns foo if the value
of a is 1, and returns the string "default" otherwise.
Both cond and if can usually be written in terms of the
other. Therefore, the choice between them is a matter of taste and
style. For example:
(if a b c) == (cond (a b) (t c))
This section describes three constructs that are often used together
with if and cond to express complicated conditions. The
constructs and and or can also be used individually as
kinds of multiple conditional constructs.
This function tests for the falsehood of condition. It returns
t if condition is nil, and nil otherwise.
The function not is identical to null, and we recommend
using null if you are testing for an empty list.
Special Form: and conditions...
The and special form tests whether all the conditions are
true. It works by evaluating the conditions one by one in the
order written.
If any of the conditions evaluates to nil, then the result
of the and must be nil regardless of the remaining
conditions; so the remaining conditions are ignored and the
and returns right away.
If all the conditions turn out non-nil, then the value of
the last of them becomes the value of the and form.
Here is an example. The first condition returns the integer 1, which is
not nil. Similarly, the second condition returns the integer 2,
which is not nil. The third condition is nil, so the
remaining condition is never evaluated.
(and (print 1) (print 2) nil (print 3))
-| 1
-| 2
=> nil
Here is a more realistic example of using and:
(if (and (consp foo) (eq (car foo) 'x))
(message "foo is a list starting with x"))
Note that (car foo) is not executed if (consp foo) returns
nil, thus avoiding an error.
and can be expressed in terms of either if or cond.
For example:
(and arg1 arg2 arg3) == (if arg1 (if arg2 arg3)) == (cond (arg1 (cond (arg2 arg3))))
Special Form: or conditions...
The or special form tests whether at least one of the
conditions is true. It works by evaluating all the
conditions one by one in the order written.
If any of the conditions evaluates to a non-nil value, then
the result of the or must be non-nil; so the remaining
conditions are ignored and the or returns right away. The
value it returns is the non-nil value of the condition just
evaluated.
If all the conditions turn out nil, then the or
expression returns nil.
For example, this expression tests whether x is either 0 or
nil:
(or (eq x nil) (= x 0))
Like the and construct, or can be written in terms of
cond. For example:
(or arg1 arg2 arg3)
==
(cond (arg1)
(arg2)
(arg3))
You could almost write or in terms of if, but not quite:
(if arg1 arg1
(if arg2 arg2
arg3))
This is not completely equivalent because it can evaluate arg1 or
arg2 twice. By contrast, (or arg1 arg2
arg3) never evaluates any argument more than once.
Iteration means executing part of a program repetitively. For example,
you might want to repeat some expressions once for each element of a list,
or once for each integer from 0 to n. You can do this in Emacs Lisp
with the special form while:
Special Form: while condition forms...
while first evaluates condition. If the result is
non-nil, it evaluates forms in textual order. Then it
reevaluates condition, and if the result is non-nil, it
evaluates forms again. This process repeats until condition
evaluates to nil.
There is no limit on the number of iterations that may occur. The loop
will continue until either condition evaluates to nil or
until an error or throw jumps out of it (see section Nonlocal Exits).
The value of a while form is always nil.
(setq num 0)
=> 0
(while (< num 4)
(princ (format "Iteration %d." num))
(setq num (1+ num)))
-| Iteration 0.
-| Iteration 1.
-| Iteration 2.
-| Iteration 3.
=> nil
If you would like to execute something on each iteration before the
end-test, put it together with the end-test in a progn as the
first argument of while, as shown here:
(while (progn
(forward-line 1)
(not (looking-at "^$"))))
This moves forward one line and continues moving by lines until an empty line is reached.
A nonlocal exit is a transfer of control from one point in a program to another remote point. Nonlocal exits can occur in Emacs Lisp as a result of errors; you can also use them under explicit control. Nonlocal exits unbind all variable bindings made by the constructs being exited.
catch and throw
Most control constructs affect only the flow of control within the
construct itself. The function throw is the exception to this
rule for of normal program execution: it performs a nonlocal exit on
request. (There are other exceptions, but they are for error handling
only.) throw is used inside a catch, and jumps back to
that catch. For example:
(catch 'foo
(progn
...
(throw 'foo t)
...))
The throw transfers control straight back to the corresponding
catch, which returns immediately. The code following the
throw is not executed. The second argument of throw is used
as the return value of the catch.
The throw and the catch are matched through the first
argument: throw searches for a catch whose first argument
is eq to the one specified. Thus, in the above example, the
throw specifies foo, and the catch specifies the
same symbol, so that catch is applicable. If there is more than
one applicable catch, the innermost one takes precedence.
All Lisp constructs between the catch and the throw,
including function calls, are exited automatically along with the
catch. When binding constructs such as let or function
calls are exited in this way, the bindings are unbound, just as they are
when these constructs are exited normally (see section Local Variables).
Likewise, the buffer and position saved by save-excursion
(see section Excursions) are restored, and so is the narrowing status
saved by save-restriction and the window selection saved by
save-window-excursion (see section Window Configurations). Any
cleanups established with the unwind-protect special form are
executed if the unwind-protect is exited with a throw.
The throw need not appear lexically within the catch
that it jumps to. It can equally well be called from another function
called within the catch. As long as the throw takes place
chronologically after entry to the catch, and chronologically
before exit from it, it has access to that catch. This is why
throw can be used in commands such as exit-recursive-edit
which throw back to the editor command loop (see section Recursive Editing).
Common Lisp note: most other versions of Lisp, including Common Lisp, have several ways of transferring control nonsequentially:return,return-from, andgo, for example. Emacs Lisp has onlythrow.
Special Form: catch tag body...
catch establishes a return point for the throw function. The
return point is distinguished from other such return points by tag,
which may be any Lisp object. The argument tag is evaluated normally
before the return point is established.
With the return point in effect, the forms of the body are evaluated
in textual order. If the forms execute normally, without error or nonlocal
exit, the value of the last body form is returned from the catch.
If a throw is done within body specifying the same value
tag, the catch exits immediately; the value it returns is
whatever was specified as the second argument of throw.
The purpose of throw is to return from a return point previously
established with catch. The argument tag is used to choose
among the various existing return points; it must be eq to the value
specified in the catch. If multiple return points match tag,
the innermost one is used.
The argument value is used as the value to return from that
catch.
If no return point is in effect with tag tag, then a no-catch
error is signaled with data (tag value).
catch and throw
One way to use catch and throw is to exit from a doubly
nested loop. (In most languages, this would be done with a "go to".)
Here we compute (foo i j) for i and j
varying from 0 to 9:
(defun search-foo ()
(catch 'loop
(let ((i 0))
(while (< i 10)
(let ((j 0))
(while (< j 10)
(if (foo i j)
(throw 'loop (list i j)))
(setq j (1+ j))))
(setq i (1+ i))))))
If foo ever returns non-nil, we stop immediately and return a
list of i and j. If foo always returns nil, the
catch returns normally, and the value is nil, since that
is the result of the while.
Here are two tricky examples, slightly different, showing two
return points at once. First, two return points with the same tag,
hack:
(defun catch2 (tag)
(catch tag
(throw 'hack 'yes)))
=> catch2
(catch 'hack
(print (catch2 'hack))
'no)
-| yes
=> no
Since both return points have tags that match the throw, it goes to
the inner one, the one established in catch2. Therefore,
catch2 returns normally with value yes, and this value is
printed. Finally the second body form in the outer catch, which is
'no, is evaluated and returned from the outer catch.
Now let's change the argument given to catch2:
(defun catch2 (tag)
(catch tag
(throw 'hack 'yes)))
=> catch2
(catch 'hack
(print (catch2 'quux))
'no)
=> yes
We still have two return points, but this time only the outer one has the
tag hack; the inner one has the tag quux instead. Therefore,
the throw returns the value yes from the outer return point.
The function print is never called, and the body-form 'no is
never evaluated.
When Emacs Lisp attempts to evaluate a form that, for some reason, cannot be evaluated, it signals an error.
When an error is signaled, Emacs's default reaction is to print an error message and terminate execution of the current command. This is the right thing to do in most cases, such as if you type C-f at the end of the buffer.
In complicated programs, simple termination may not be what you want.
For example, the program may have made temporary changes in data
structures, or created temporary buffers which should be deleted before
the program is finished. In such cases, you would use
unwind-protect to establish cleanup expressions to be
evaluated in case of error. Occasionally, you may wish the program to
continue execution despite an error in a subroutine. In these cases,
you would use condition-case to establish error handlers to
recover control in case of error.
Resist the temptation to use error handling to transfer control from
one part of the program to another; use catch and throw.
See section Explicit Nonlocal Exits: catch and throw.
Most errors are signaled "automatically" within Lisp primitives
which you call for other purposes, such as if you try to take the
CAR of an integer or move forward a character at the end of the
buffer; you can also signal errors explicitly with the functions
error and signal.
Quitting, which happens when the user types C-g, is not considered an error, but it handled almost like an error. See section Quitting.
Function: error format-string &rest args
This function signals an error with an error message constructed by
applying format (see section Conversion of Characters and Strings) to
format-string and args.
Typical uses of error is shown in the following examples:
(error "You have committed an error.
Try something else.")
error--> You have committed an error.
Try something else.
(error "You have committed %d errors." 10)
error--> You have committed 10 errors.
error works by calling signal with two arguments: the
error symbol error, and a list containing the string returned by
format.
If you want to use a user-supplied string as an error message verbatim,
don't just write (error string). If string contains
`%', it will be interpreted as a format specifier, with undesirable
results. Instead, use (error "%s" string).
Function: signal error-symbol data
This function signals an error named by error-symbol. The argument data is a list of additional Lisp objects relevant to the circumstances of the error.
The argument error-symbol must be an error symbol---a symbol
bearing a property error-conditions whose value is a list of
condition names. This is how different sorts of errors are classified.
The number and significance of the objects in data depends on
error-symbol. For example, with a wrong-type-arg error,
there are two objects in the list: a predicate which describes the type
that was expected, and the object which failed to fit that type.
See section Error Symbols and Condition Names, for a description of error symbols.
Both error-symbol and data are available to any error
handlers which handle the error: a list (error-symbol .
data) is constructed to become the value of the local variable
bound in the condition-case form (see section Writing Code to Handle Errors). If
the error is not handled, both of them are used in printing the error
message.
The function signal never returns (though in older Emacs versions
it could sometimes return).
(signal 'wrong-number-of-arguments '(x y))
error--> Wrong number of arguments: x, y
(signal 'no-such-error '("My unknown error condition."))
error--> peculiar error: "My unknown error condition."
Common Lisp note: Emacs Lisp has nothing like the Common Lisp concept of continuable errors.
When an error is signaled, Emacs searches for an active handler
for the error. A handler is a specially marked place in the Lisp code
of the current function or any of the functions by which it was called.
If an applicable handler exists, its code is executed, and control
resumes following the handler. The handler executes in the environment
of the condition-case which established it; all functions called
within that condition-case have already been exited, and the
handler cannot return to them.
If no applicable handler is in effect in your program, the current command is terminated and control returns to the editor command loop, because the command loop has an implicit handler for all kinds of errors. The command loop's handler uses the error symbol and associated data to print an error message.
When an error is not handled explicitly, it may cause the Lisp debugger
to be called. The debugger is enabled if the variable
debug-on-error (see section Entering the Debugger on an Error) is non-nil.
Unlike error handlers, the debugger runs in the environment of the
error, so that you can examine values of variables precisely as they
were at the time of the error.
The usual effect of signaling an error is to terminate the command that
is running and return immediately to the Emacs editor command loop.
You can arrange to trap errors occurring in a part of your program by
establishing an error handler with the special form
condition-case. A simple example looks like this:
(condition-case nil
(delete-file filename)
(error nil))
This deletes the file named filename, catching any error and
returning nil if an error occurs.
The second argument of condition-case is called the
protected form. (In the example above, the protected form is a
call to delete-file.) The error handlers go into effect when
this form begins execution and are deactivated when this form returns.
They remain in effect for all the intervening time. In particular, they
are in effect during the execution of subroutines called by this form,
and their subroutines, and so on. This is a good thing, since, strictly
speaking, errors can be signaled only by Lisp primitives (including
signal and error) called by the protected form, not by the
protected form itself.
The arguments after the protected form are handlers. Each handler
lists one or more condition names (which are symbols) to specify
which errors it will handle. The error symbol specified when an error
is signaled also defines a list of condition names. A handler applies
to an error if they have any condition names in common. In the example
above, there is one handler, and it specifies one condition name,
error, which covers all errors.
The search for an applicable handler checks all the established handlers
starting with the most recently established one. Thus, if two nested
condition-case forms try to handle the same error, the inner of
the two will actually handle it.
When an error is handled, control returns to the handler. Before this
happens, Emacs unbinds all variable bindings made by binding constructs
that are being exited and executes the cleanups of all
unwind-protect forms that are exited. Once control arrives at
the handler, the body of the handler is executed.
After execution of the handler body, execution continues by returning
from the condition-case form. Because the protected form is
exited completely before execution of the handler, the handler cannot
resume execution at the point of the error, nor can it examine variable
bindings that were made within the protected form. All it can do is
clean up and proceed.
condition-case is often used to trap errors that are
predictable, such as failure to open a file in a call to
insert-file-contents. It is also used to trap errors that are
totally unpredictable, such as when the program evaluates an expression
read from the user.
Error signaling and handling have some resemblance to throw and
catch, but they are entirely separate facilities. An error
cannot be caught by a catch, and a throw cannot be handled
by an error handler (though using throw when there is no suitable
catch signals an error which can be handled).
Special Form: condition-case var protected-form handlers...
This special form establishes the error handlers handlers around
the execution of protected-form. If protected-form executes
without error, the value it returns becomes the value of the
condition-case form; in this case, the condition-case has
no effect. The condition-case form makes a difference when an
error occurs during protected-form.
Each of the handlers is a list of the form (conditions
body...). conditions is an error condition name to be
handled, or a list of condition names; body is one or more Lisp
expressions to be executed when this handler handles an error. Here are
examples of handlers:
(error nil) (arith-error (message "Division by zero")) ((arith-error file-error) (message "Either division by zero or failure to open a file"))
Each error that occurs has an error symbol which describes what
kind of error it is. The error-conditions property of this
symbol is a list of condition names (see section Error Symbols and Condition Names). Emacs
searches all the active condition-case forms for a handler which
specifies one or more of these names; the innermost matching
condition-case handles the error. The handlers in this
condition-case are tested in the order in which they appear.
The body of the handler is then executed, and the condition-case
returns normally, using the value of the last form in the body as the
overall value.
The argument var is a variable. condition-case does not
bind this variable when executing the protected-form, only when it
handles an error. At that time, var is bound locally to a list of
the form (error-symbol . data), giving the
particulars of the error. The handler can refer to this list to decide
what to do. For example, if the error is for failure opening a file,
the file name is the second element of data---the third element of
var.
If var is nil, that means no variable is bound. Then the
error symbol and associated data are not made available to the handler.
Here is an example of using condition-case to handle the error
that results from dividing by zero. The handler prints out a warning
message and returns a very large number.
(defun safe-divide (dividend divisor)
(condition-case err
;; Protected form.
(/ dividend divisor)
;; The handler.
(arith-error ; Condition.
(princ (format "Arithmetic error: %s" err))
1000000)))
=> safe-divide
(safe-divide 5 0)
-| Arithmetic error: (arith-error)
=> 1000000
The handler specifies condition name arith-error so that it will handle only division-by-zero errors. Other kinds of errors will not be handled, at least not by this condition-case. Thus,
(safe-divide nil 3)
error--> Wrong type argument: integer-or-marker-p, nil
Here is a condition-case that catches all kinds of errors,
including those signaled with error:
(setq baz 34)
=> 34
(condition-case err
(if (eq baz 35)
t
;; This is a call to the function error.
(error "Rats! The variable %s was %s, not 35." 'baz baz))
;; This is the handler; it is not a form.
(error (princ (format "The error was: %s" err))
2))
-| The error was: (error "Rats! The variable baz was 34, not 35.")
=> 2
When you signal an error, you specify an error symbol to specify the kind of error you have in mind. Each error has one and only one error symbol to categorize it. This is the finest classification of errors defined by the Lisp language.
These narrow classifications are grouped into a hierarchy of wider
classes called error conditions, identified by condition
names. The narrowest such classes belong to the error symbols
themselves: each error symbol is also a condition name. There are also
condition names for more extensive classes, up to the condition name
error which takes in all kinds of errors. Thus, each error has
one or more condition names: error, the error symbol if that
is distinct from error, and perhaps some intermediate
classifications.
In order for a symbol to be usable as an error symbol, it must have an
error-conditions property which gives a list of condition names.
This list defines the conditions which this kind of error belongs to.
(The error symbol itself, and the symbol error, should always be
members of this list.) Thus, the hierarchy of condition names is
defined by the error-conditions properties of the error symbols.
In addition to the error-conditions list, the error symbol
should have an error-message property whose value is a string to
be printed when that error is signaled but not handled. If the
error-message property exists, but is not a string, the error
message `peculiar error' is used.
Here is how we define a new error symbol, new-error:
(put 'new-error
'error-conditions
'(error my-own-errors new-error))
=> (error my-own-errors new-error)
(put 'new-error 'error-message "A new error")
=> "A new error"
This error has three condition names: new-error, the narrowest
classification; my-own-errors, which we imagine is a wider
classification; and error, which is the widest of all.
Naturally, Emacs will never signal a new-error on its own; only
an explicit call to signal (see section Errors) in your code can do
this:
(signal 'new-error '(x y))
error--> A new error: x, y
This error can be handled through any of the three condition names.
This example handles new-error and any other errors in the class
my-own-errors:
(condition-case foo
(bar nil t)
(my-own-errors nil))
The significant way that errors are classified is by their condition
names--the names used to match errors with handlers. An error symbol
serves only as a convenient way to specify the intended error message
and list of condition names. If signal were given a list of
condition names rather than one error symbol, that would be cumbersome.
By contrast, using only error symbols without condition names would
seriously decrease the power of condition-case. Condition names
make it possible to categorize errors at various levels of generality
when you write an error handler. Using error symbols alone would
eliminate all but the narrowest level of classification.
See section Standard Errors, for a list of all the standard error symbols and their conditions.
The unwind-protect construct is essential whenever you
temporarily put a data structure in an inconsistent state; it permits
you to ensure the data are consistent in the event of an error or throw.
Special Form: unwind-protect body cleanup-forms...
unwind-protect executes the body with a guarantee that the
cleanup-forms will be evaluated if control leaves body, no
matter how that happens. The body may complete normally, or
execute a throw out of the unwind-protect, or cause an
error; in all cases, the cleanup-forms will be evaluated.
Only the body is actually protected by the unwind-protect.
If any of the cleanup-forms themselves exit nonlocally (e.g., via
a throw or an error), it is not guaranteed that the rest
of them will be executed. If the failure of one of the
cleanup-forms has the potential to cause trouble, then it should
be protected by another unwind-protect around that form.
The number of currently active unwind-protect forms counts,
together with the number of local variable bindings, against the limit
max-specpdl-size (see section Local Variables).
For example, here we make an invisible buffer for temporary use, and make sure to kill it before finishing:
(save-excursion
(let ((buffer (get-buffer-create " *temp*")))
(set-buffer buffer)
(unwind-protect
body
(kill-buffer buffer))))
You might think that we could just as well write (kill-buffer
(current-buffer)) and dispense with the variable buffer.
However, the way shown above is safer, if body happens to get an
error after switching to a different buffer! (Alternatively, you could
write another save-excursion around the body, to ensure that the
temporary buffer becomes current in time to kill it.)
Here is an actual example taken from the file `ftp.el'. It creates
a process (see section Processes) to try to establish a connection to a remote
machine. As the function ftp-login is highly susceptible to
numerous problems which the writer of the function cannot anticipate, it is
protected with a form that guarantees deletion of the process in the event
of failure. Otherwise, Emacs might fill up with useless subprocesses.
(let ((win nil))
(unwind-protect
(progn
(setq process (ftp-setup-buffer host file))
(if (setq win (ftp-login process host user password))
(message "Logged in")
(error "Ftp login failed")))
(or win (and process (delete-process process)))))
This example actually has a small bug: if the user types C-g to
quit, and the quit happens immediately after the function
ftp-setup-buffer returns but before the variable process is
set, the process will not be killed. There is no easy way to fix this bug,
but at least it is very unlikely.
A variable is a name used in a program to stand for a value. Nearly all programming languages have variables of some sort. In the text for a Lisp program, variables are written using the syntax for symbols.
In Lisp, unlike most programming languages, programs are represented primarily as Lisp objects and only secondarily as text. The Lisp objects used for variables are symbols: the symbol name is the variable name, and the variable's value is stored in the value cell of the symbol. The use of a symbol as a variable is independent of whether the same symbol has a function definition. See section Symbol Components.
The textual form of a program is determined by its Lisp object representation; it is the read syntax for the Lisp object which constitutes the program. This is why a variable in a textual Lisp program is written as the read syntax for the symbol that represents the variable.
The simplest way to use a variable is globally. This means that the variable has just one value at a time, and this value is in effect (at least for the moment) throughout the Lisp system. The value remains in effect until you specify a new one. When a new value replaces the old one, no trace of the old value remains in the variable.
You specify a value for a symbol with setq. For example,
(setq x '(a b))
gives the variable x the value (a b). Note that the
first argument of setq, the name of the variable, is not
evaluated, but the second argument, the desired value, is evaluated
normally.
Once the variable has a value, you can refer to it by using the symbol by itself as an expression. Thus,
x
=> (a b)
assuming the setq form shown above has already been executed.
If you do another setq, the new value replaces the old one:
x
=> (a b)
(setq x 4)
=> 4
x
=> 4
Emacs Lisp has two special symbols, nil and t, that
always evaluate to themselves. These symbols cannot be rebound, nor can
their value cells be changed. An attempt to change the value of
nil or t signals a setting-constant error.
nil == 'nil
=> nil
(setq nil 500)
error--> Attempt to set constant symbol: nil
Global variables are given values that last until explicitly superseded with new values. Sometimes it is useful to create variable values that exist temporarily--only while within a certain part of the program. These values are called local, and the variables so used are called local variables.
For example, when a function is called, its argument variables receive
new local values which last until the function exits. Similarly, the
let special form explicitly establishes new local values for
specified variables; these last until exit from the let form.
When a local value is established, the previous value (or lack of one) of the variable is saved away. When the life span of the local value is over, the previous value is restored. In the mean time, we say that the previous value is shadowed and not visible. Both global and local values may be shadowed (see section Scope).
If you set a variable (such as with setq) while it is local,
this replaces the local value; it does not alter the global value, or
previous local values that are shadowed. To model this behavior, we
speak of a local binding of the variable as well as a local value.
The local binding is a conceptual place that holds a local value.
Entry to a function, or a special form such as let, creates the
local binding; exit from the function or from the let removes the
local binding. As long as the local binding lasts, the variable's value
is stored within it. Use of setq or set while there is a
local binding stores a different value into the local binding; it does
not create a new binding.
We also speak of the global binding, which is where (conceptually) the global value is kept.
A variable can have more than one local binding at a time (for
example, if there are nested let forms that bind it). In such a
case, the most recently created local binding that still exists is the
current binding of the variable. (This is called dynamic
scoping; see section Scoping Rules for Variable Bindings.) If there are no local bindings,
the variable's global binding is its current binding. We also call the
current binding the most-local existing binding, for emphasis.
Ordinary evaluation of a symbol always returns the value of its current
binding.
The special forms let and let* exist to create
local bindings.
Special Form: let (bindings...) forms...
This function binds variables according to bindings and then
evaluates all of the forms in textual order. The let-form
returns the value of the last form in forms.
Each of the bindings is either (i) a symbol, in which case
that symbol is bound to nil; or (ii) a list of the form
(symbol value-form), in which case symbol is
bound to the result of evaluating value-form. If value-form
is omitted, nil is used.
All of the value-forms in bindings are evaluated in the
order they appear and before any of the symbols are bound. Here
is an example of this: Z is bound to the old value of Y,
which is 2, not the new value, 1.
(setq Y 2)
=> 2
(let ((Y 1)
(Z Y))
(list Y Z))
=> (1 2)
Special Form: let* (bindings...) forms...
This special form is like let, except that each symbol in
bindings is bound as soon as its new value is computed, before the
computation of the values of the following local bindings. Therefore,
an expression in bindings may reasonably refer to the preceding
symbols bound in this let* form. Compare the following example
with the example above for let.
(setq Y 2)
=> 2
(let* ((Y 1)
(Z Y)) ; Use the just-established value of Y.
(list Y Z))
=> (1 1)
Here is a complete list of the other facilities which create local bindings:
condition-case (see section Errors).
This variable defines the limit on the total number of local variable
bindings and unwind-protect cleanups (see section Nonlocal Exits)
that are allowed before signaling an error (with data "Variable
binding depth exceeds max-specpdl-size").
This limit, with the associated error when it is exceeded, is one way that Lisp avoids infinite recursion on an ill-defined function.
The default value is 600.
max-lisp-eval-depth provides another limit on depth of nesting.
See section Eval.
If you have never given a symbol any value as a global variable, we
say that that symbol's global value is void. In other words, the
symbol's value cell does not have any Lisp object in it. If you try to
evaluate the symbol, you get a void-variable error rather than
a value.
Note that a value of nil is not the same as void. The symbol
nil is a Lisp object and can be the value of a variable just as any
other object can be; but it is a value. A void variable does not
have any value.
After you have given a variable a value, you can make it void once more
using makunbound.
This function makes the current binding of symbol void. This
causes any future attempt to use this symbol as a variable to signal the
error void-variable, unless or until you set it again.
makunbound returns symbol.
(makunbound 'x) ; Make the global value
; of x void.
=> x
x
error--> Symbol's value as variable is void: x
If symbol is locally bound, makunbound affects the most
local existing binding. This is the only way a symbol can have a void
local binding, since all the constructs that create local bindings
create them with values. In this case, the voidness lasts at most as
long as the binding does; when the binding is removed due to exit from
the construct that made it, the previous or global binding is reexposed
as usual, and the variable is no longer void unless the newly reexposed
binding was void all along.
(setq x 1) ; Put a value in the global binding.
=> 1
(let ((x 2)) ; Locally bind it.
(makunbound 'x) ; Void the local binding.
x)
error--> Symbol's value as variable is void: x
x ; The global binding is unchanged.
=> 1
(let ((x 2)) ; Locally bind it.
(let ((x 3)) ; And again.
(makunbound 'x) ; Void the innermost-local binding.
x)) ; And refer: it's void.
error--> Symbol's value as variable is void: x
(let ((x 2))
(let ((x 3))
(makunbound 'x)) ; Void inner binding, then remove it.
x) ; Now outer let binding is visible.
=> 2
A variable that has been made void with makunbound is
indistinguishable from one that has never received a value and has
always been void.
You can use the function boundp to test whether a variable is
currently void.
boundp returns t if variable (a symbol) is not void;
more precisely, if its current binding is not void. It returns
nil otherwise.
(boundp 'abracadabra) ; Starts out void.
=> nil
(let ((abracadabra 5)) ; Locally bind it.
(boundp 'abracadabra))
=> t
(boundp 'abracadabra) ; Still globally void.
=> nil
(setq abracadabra 5) ; Make it globally nonvoid.
=> 5
(boundp 'abracadabra)
=> t
You may announce your intention to use a symbol as a global variable
with a definition, using defconst or defvar.
In Emacs Lisp, definitions serve three purposes. First, they inform
the user who reads the code that certain symbols are intended to be
used as variables. Second, they inform the Lisp system of these things,
supplying a value and documentation. Third, they provide information to
utilities such as etags and make-docfile, which create data
bases of the functions and variables in a program.
The difference between defconst and defvar is primarily
a matter of intent, serving to inform human readers of whether programs
will change the variable. Emacs Lisp does not restrict the ways in
which a variable can be used based on defconst or defvar
declarations. However, it also makes a difference for initialization:
defconst unconditionally initializes the variable, while
defvar initializes it only if it is void.
One would expect user option variables to be defined with
defconst, since programs do not change them. Unfortunately, this
has bad results if the definition is in a library that is not preloaded:
defconst would override any prior value when the library is
loaded. Users would like to be able to set the option in their init
files, and override the default value given in the definition. For this
reason, user options must be defined with defvar.
Special Form: defvar symbol [value [doc-string]]
This special form informs a person reading your code that symbol
will be used as a variable that the programs are likely to set or
change. It is also used for all user option variables except in the
preloaded parts of Emacs. Note that symbol is not evaluated;
the symbol to be defined must appear explicitly in the
defvar.
If symbol already has a value (i.e., it is not void), value is not even evaluated, and symbol's value remains unchanged. If symbol is void and value is specified, it is evaluated and symbol is set to the result. (If value is not specified, the value of symbol is not changed in any case.)
If symbol has a buffer-local binding in the current buffer,
defvar sets the default value, not the local value.
If the doc-string argument appears, it specifies the documentation
for the variable. (This opportunity to specify documentation is one of
the main benefits of defining the variable.) The documentation is
stored in the symbol's variable-documentation property. The
Emacs help functions (see section Documentation) look for this property.
If the first character of doc-string is `*', it means that
this variable is considered to be a user option. This affects commands
such as set-variable and edit-options.
For example, this form defines foo but does not set its value:
(defvar foo)
=> foo
The following example sets the value of bar to 23, and
gives it a documentation string:
(defvar bar 23
"The normal weight of a bar.")
=> bar
The following form changes the documentation string for bar,
making it a user option, but does not change the value, since bar
already has a value. (The addition (1+ 23) is not even
performed.)
(defvar bar (1+ 23)
"*The normal weight of a bar.")
=> bar
bar
=> 23
Here is an equivalent expression for the defvar special form:
(defvar symbol value doc-string)
==
(progn
(if (not (boundp 'symbol))
(setq symbol value))
(put 'symbol 'variable-documentation 'doc-string)
'symbol)
The defvar form returns symbol, but it is normally used
at top level in a file where its value does not matter.
Special Form: defconst symbol [value [doc-string]]
This special form informs a person reading your code that symbol
has a global value, established here, that will not normally be changed
or locally bound by the execution of the program. The user, however,
may be welcome to change it. Note that symbol is not evaluated;
the symbol to be defined must appear explicitly in the defconst.
defconst always evaluates value and sets the global value
of symbol to the result, provided value is given. If
symbol has a buffer-local binding in the current buffer,
defconst sets the default value, not the local value.
Please note: don't use defconst for user option
variables in libraries that are not normally loaded. The user should be
able to specify a value for such a variable in the `.emacs' file,
so that it will be in effect if and when the library is loaded later.
Here, pi is a constant that presumably ought not to be changed
by anyone (attempts by the Indiana State Legislature notwithstanding).
As the second form illustrates, however, this is only advisory.
(defconst pi 3 "Pi to one place.")
=> pi
(setq pi 4)
=> pi
pi
=> 4
Function: user-variable-p variable
This function returns t if variable is a user option,
intended to be set by the user for customization, nil otherwise.
(Variables other than user options exist for the internal purposes of
Lisp programs, and users need not know about them.)
User option variables are distinguished from other variables by the
first character of the variable-documentation property. If the
property exists and is a string, and its first character is `*',
then the variable is a user option.
Note that if the defconst and defvar special forms are
used while the variable has a local binding, the local binding's value
is set, and the global binding is not changed. This would be confusing.
But the normal way to use these special forms is at top level in a file,
where no local binding should be in effect.
The usual way to reference a variable is to write the symbol which
names it (see section Symbol Forms). This requires you to specify the
variable name when you write the program. Usually that is exactly what
you want to do. Occasionally you need to choose at run time which
variable to reference; then you can use symbol-value.
This function returns the value of symbol. This is the value in the innermost local binding of the symbol, or its global value if it has no local bindings.
(setq abracadabra 5)
=> 5
(setq foo 9)
=> 9
;; Here the symbol abracadabra
;; is the symbol whose value is examined.
(let ((abracadabra 'foo))
(symbol-value 'abracadabra))
=> foo
;; Here the value of abracadabra,
;; which is foo,
;; is the symbol whose value is examined.
(let ((abracadabra 'foo))
(symbol-value abracadabra))
=> 9
(symbol-value 'abracadabra)
=> 5
A void-variable error is signaled if symbol has neither a
local binding nor a global value.
The usual way to change the value of a variable is with the special
form setq. When you need to compute the choice of variable at
run time, use the function set.
Special Form: setq [symbol form]...
This special form is the most common method of changing a variable's value. Each symbol is given a new value, which is the result of evaluating the corresponding form. The most-local existing binding of the symbol is changed.
The value of the setq form is the value of the last form.
(setq x (1+ 2))
=> 3
x ; x now has a global value.
=> 3
(let ((x 5))
(setq x 6) ; The local binding of x is set.
x)
=> 6
x ; The global value is unchanged.
=> 3
Note that the first form is evaluated, then the first symbol is set, then the second form is evaluated, then the second symbol is set, and so on:
(setq x 10 ; Notice thatxis set before y (1+ x)) ; the value ofyis computed. => 11
This function sets symbol's value to value, then
returns value. Since set is a function, the expression
written for symbol is evaluated to obtain the symbol to be
set.
The most-local existing binding of the variable is the binding that is
set; shadowed bindings are not affected. If symbol is not
actually a symbol, a wrong-type-argument error is signaled.
(set one 1)
error--> Symbol's value as variable is void: one
(set 'one 1)
=> 1
(set 'two 'one)
=> one
(set two 2) ; two evaluates to symbol one.
=> 2
one ; So it is one that was set.
=> 2
(let ((one 1)) ; This binding of one is set,
(set 'one 3) ; not the global value.
one)
=> 3
one
=> 2
Logically speaking, set is a more fundamental primitive that
setq. Any use of setq can be trivially rewritten to use
set; setq could even be defined as a macro, given the
availability of set. However, set itself is rarely used;
beginners hardly need to know about it. It is needed only when the
choice of variable to be set is made at run time. For example, the
command set-variable, which reads a variable name from the user
and then sets the variable, needs to use set.
Common Lisp note: in Common Lisp,setalways changes the symbol's special value, ignoring any lexical bindings. In Emacs Lisp, all variables and all bindings are special, sosetalways affects the most local existing binding.
A given symbol foo may have several local variable bindings,
established at different places in the Lisp program, as well as a global
binding. The most recently established binding takes precedence over
the others.
Local bindings in Emacs Lisp have indefinite scope and dynamic extent. Scope refers to where textually in the source code the binding can be accessed. Indefinite scope means that any part of the program can potentially access the variable binding. Extent refers to when, as the program is executing, the binding exists. Dynamic extent means that the binding lasts as long as the activation of the construct that established it.
The combination of dynamic extent and indefinite scope is called dynamic scoping. By contrast, most programming languages use lexical scoping, in which references to a local variable must be textually within the function or block that binds the variable.
Common Lisp note: variables declared "special" in Common Lisp are dynamically scoped like variables in Emacs Lisp.
Emacs Lisp uses indefinite scope for local variable bindings. This means that any function anywhere in the program text might access a given binding of a variable. Consider the following function definitions:
(defun binder (x) ;xis bound inbinder. (foo 5)) ;foois some other function. (defun user () ;xis used inuser. (list x))
In a lexically scoped language, the binding of x from
binder would never be accessible in user, because
user is not textually contained within the function
binder. However, in dynamically scoped Emacs Lisp, user
may or may not refer to the binding of x established in
binder, depending on circumstances:
user directly without calling binder at all,
then whatever binding of x is found, it cannot come from
binder.
foo as follows and call binder, then the
binding made in binder will be seen in user:
(defun foo (lose) (user))
foo as follows and call binder, then the
binding made in binder will not be seen in user:
(defun foo (x) (user))
Here, when foo is called by binder, it binds x.
(The binding in foo is said to shadow the one made in
binder.) Therefore, user will access the x bound
by foo instead of the one bound by binder.
Extent refers to the time during program execution that a variable name is valid. In Emacs Lisp, a variable is valid only while the form that bound it is executing. This is called dynamic extent. "Local" or "automatic" variables in most languages, including C and Pascal, have dynamic extent.
One alternative to dynamic extent is indefinite extent. This means that a variable binding can live on past the exit from the form that made the binding. Common Lisp and Scheme, for example, support this, but Emacs Lisp does not.
To illustrate this, the function below, make-add, returns a
function that purports to add n to its own argument m.
This would work in Common Lisp, but it does not work as intended in
Emacs Lisp, because after the call to make-add exits, the
variable n is no longer bound to the actual argument 2.
(defun make-add (n)
(function (lambda (m) (+ n m)))) ; Return a function.
=> make-add
(fset 'add2 (make-add 2)) ; Define function add2
; with (make-add 2).
=> (lambda (m) (+ n m))
(add2 4) ; Try to add 2 to 4.
error--> Symbol's value as variable is void: n
A simple sample implementation (which is not how Emacs Lisp actually works) may help you understand dynamic binding. This technique is called deep binding and was used in early Lisp systems.
Suppose there is a stack of bindings: variable-value pairs. At entry
to a function or to a let form, we can push bindings on the stack
for the arguments or local variables created there. We can pop those
bindings from the stack at exit from the binding construct.
We can find the value of a variable by searching the stack from top to bottom for a binding for that variable; the value from that binding is the value of the variable. To set the variable, we search for the current binding, then store the new value into that binding.
As you can see, a function's bindings remain in effect as long as it continues execution, even during its calls to other functions. That is why we say the extent of the binding is dynamic. And any other function can refer to the bindings, if it uses the same variables while the bindings are in effect. That is why we say the scope is indefinite.
The actual implementation of variable scoping in GNU Emacs Lisp uses a technique called shallow binding. Each variable has a standard place in which its current value is always found--the value cell of the symbol.
In shallow binding, setting the variable works by storing a value in the value cell. When a new local binding is created, the local value is stored in the value cell, and the old value (belonging to a previous binding) is pushed on a stack. When a binding is eliminated, the old value is popped off the stack and stored in the value cell.
We use shallow binding because it has the same results as deep binding, but runs faster, since there is never a need to search for a binding.
Binding a variable in one function and using it in another is a powerful technique, but if used without restraint, it can make programs hard to understand. There are two clean ways to use this technique:
You should write comments to inform other programmers that they can see all uses of the variable before them, and to advise them not to add uses elsewhere.
case-fold-search is defined as "non-nil means ignore case
when searching"; various search and replace functions refer to it
directly or through their subroutines, but do not bind or set it.
Then you can bind the variable in other programs, knowing reliably what the effect will be.
Global and local variable bindings are found in most programming languages in one form or another. Emacs also supports another, unusual kind of variable binding: buffer-local bindings, which apply only to one buffer. Emacs Lisp is meant for programming editing commands, and having different values for a variable in different buffers is an important customization method.
A buffer-local variable has a buffer-local binding associated with a particular buffer. The binding is in effect when that buffer is current; otherwise, it is not in effect. If you set the variable while a buffer-local binding is in effect, the new value goes in that binding, so the global binding is unchanged; this means that the change is visible in that buffer alone.
A variable may have buffer-local bindings in some buffers but not in
others. The global binding is shared by all the buffers that don't have
their own bindings. Thus, if you set the variable in a buffer that does
not have a buffer-local binding for it, the new value is visible in all
buffers except those with buffer-local bindings. (Here we are assuming
that there are no let-style local bindings to complicate the issue.)
The most common use of buffer-local bindings is for major modes to change
variables that control the behavior of commands. For example, C mode and
Lisp mode both set the variable paragraph-start to specify that only
blank lines separate paragraphs. They do this by making the variable
buffer-local in the buffer that is being put into C mode or Lisp mode, and
then setting it to the new value for that mode.
The usual way to make a buffer-local binding is with
make-local-variable, which is what major mode commands use. This
affects just the current buffer; all other buffers (including those yet to
be created) continue to share the global value.
A more powerful operation is to mark the variable as
automatically buffer-local by calling
make-variable-buffer-local. You can think of this as making the
variable local in all buffers, even those yet to be created. More
precisely, the effect is that setting the variable automatically makes
the variable local to the current buffer if it is not already so. All
buffers start out by sharing the global value of the variable as usual,
but any setq creates a buffer-local binding for the current
buffer. The new value is stored in the buffer-local binding, leaving
the (default) global binding untouched. The global value can no longer
be changed with setq; you need to use setq-default to do
that.
Warning: when a variable has local values in one or more
buffers, you can get Emacs very confused by binding the variable with
let, changing to a different current buffer in which a different
binding is in effect, and then exiting the let. To preserve your
sanity, it is wise to avoid such situations. If you use
save-excursion around each piece of code that changes to a
different current buffer, you will not have this problem. Here is an
example of incorrect code:
(setq foo 'b)
(set-buffer "a")
(make-local-variable 'foo)
(setq foo 'a)
(let ((foo 'temp))
(set-buffer "b")
...)
foo => 'a ; The old buffer-local value from buffer `a'
; is now the default value.
(set-buffer "a")
foo => 'temp ; The local value that should be gone
; is now the buffer-local value in buffer `a'.
But save-excursion as shown here avoids the problem:
(let ((foo 'temp))
(save-excursion
(set-buffer "b")
...))
Local variables in a file you edit are also represented by buffer-local bindings for the buffer that holds the file within Emacs. See section How Emacs Chooses a Major Mode.
Command: make-local-variable variable
This function creates a buffer-local binding in the current buffer for variable (a symbol). Other buffers are not affected. The value returned is variable.
The buffer-local value of variable starts out as the same value variable previously had. If variable was void, it remains void.
;; In buffer `b1':
(setq foo 5) ; Affects all buffers.
=> 5
(make-local-variable 'foo) ; Now it is local in `b1'.
=> foo
foo ; That did not change
=> 5 ; the value.
(setq foo 6) ; Change the value
=> 6 ; in `b1'.
foo
=> 6
;; In buffer `b2', the value hasn't changed.
(save-excursion
(set-buffer "b2")
foo)
=> 5
Command: make-variable-buffer-local variable
This function marks variable (a symbol) automatically buffer-local, so that any attempt to set it will make it local to the current buffer at the time.
The value returned is variable.
Function: buffer-local-variables &optional buffer
This function tells you what the buffer-local variables are in buffer buffer. It returns an association list (see section Association Lists) in which each association contains one buffer-local variable and its value. When a buffer-local variable is void in buffer, then it appears directly in the resulting list. If buffer is omitted, the current buffer is used.
(make-local-variable 'foobar)
(makunbound 'foobar)
(make-local-variable 'bind-me)
(setq bind-me 69)
(setq lcl (buffer-local-variables))
;; First, built-in variables local in all buffers:
=> ((mark-active . nil)
(buffer-undo-list nil)
(mode-name . "Fundamental")
...
;; Next, non-built-in local variables.
;; This one is local and void:
foobar
;; This one is local and nonvoid:
(bind-me . 69))
Note that storing new values into the CDRs of cons cells in this list does not change the local values of the variables.
Command: kill-local-variable variable
This function deletes the buffer-local binding (if any) for variable (a symbol) in the current buffer. As a result, the global (default) binding of variable becomes visible in this buffer. Usually this results in a change in the value of variable, since the global value is usually different from the buffer-local value just eliminated.
It is possible to kill the local binding of a variable that automatically becomes local when set. This causes the variable to show its global value in the current buffer. However, if you set the variable again, this will once again create a local value.
kill-local-variable returns variable.
Function: kill-all-local-variables
This function eliminates all the buffer-local variable bindings of the current buffer except for variables marker as "permanent". As a result, the buffer will see the default values of most variables.
This function also resets certain other information pertaining to the
buffer: its local keymap is set to nil, its syntax table is set
to the value of standard-syntax-table, and its abbrev table is
set to the value of fundamental-mode-abbrev-table.
Every major mode command begins by calling this function, which has the effect of switching to Fundamental mode and erasing most of the effects of the previous major mode. To ensure that this does its job, the variables that major modes set should not be marked permanent.
kill-all-local-variables returns nil.
A local variable is permanent if the variable name (a symbol) has a
permanent-local property that is non-nil. Permanent
locals are appropriate for data pertaining to where the file came from
or how to save it, rather than with how to edit the contents.
The global value of a variable with buffer-local bindings is also called the default value, because it is the value that is in effect except when specifically overridden.
The functions default-value and setq-default allow you
to access and change the default value regardless of whether the current
buffer has a buffer-local binding. For example, you could use
setq-default to change the default setting of
paragraph-start for most buffers; and this would work even when
you are in a C or Lisp mode buffer which has a buffer-local value for
this variable.
The special forms defvar and defconst also set the
default value (if they set the variable at all), rather than any local
value.
Function: default-value symbol
This function returns symbol's default value. This is the value
that is seen in buffers that do not have their own values for this
variable. If symbol is not buffer-local, this is equivalent to
symbol-value (see section Accessing Variable Values).
Function: default-boundp variable
The function default-boundp tells you whether variable's
default value is nonvoid. If (default-boundp 'foo) returns
nil, then (default-value 'foo) would get an error.
default-boundp is to default-value as boundp is to
symbol-value.
Special Form: setq-default symbol value
This sets the default value of symbol to value.
symbol is not evaluated, but value is. The value of the
setq-default form is value.
If a symbol is not buffer-local for the current buffer, and is not
marked automatically buffer-local, this has the same effect as
setq. If symbol is buffer-local for the current buffer,
then this changes the value that other buffers will see (as long as they
don't have a buffer-local value), but not the value that the current
buffer sees.
;; In buffer `foo':
(make-local-variable 'local)
=> local
(setq local 'value-in-foo)
=> value-in-foo
(setq-default local 'new-default)
=> new-default
local
=> value-in-foo
(default-value 'local)
=> new-default
;; In (the new) buffer `bar':
local
=> new-default
(default-value 'local)
=> new-default
(setq local 'another-default)
=> another-default
(default-value 'local)
=> another-default
;; Back in buffer `foo':
local
=> value-in-foo
(default-value 'local)
=> another-default
Function: set-default symbol value
This function is like setq-default, except that symbol is
evaluated.
(set-default (car '(a b c)) 23)
=> 23
(default-value 'a)
=> 23
A Lisp program is composed mainly of Lisp functions. This chapter explains what functions are, how they accept arguments, and how to define them.
In a general sense, a function is a rule for carrying on a computation given several values called arguments. The result of the computation is called the value of the function. The computation can also have side effects: lasting changes in the values of variables or the contents of data structures.
Here are important terms for functions in Emacs Lisp and for other function-like objects.
car or append. These functions are also called
built-in functions or subrs. (Special forms are also
considered primitives.)
Usually the reason that a function is a primitives is because it is fundamental, or provides a low-level interface to operating system services, or because it needs to run fast. Primitives can be modified or added only by changing the C sources and recompiling the editor. See section Writing Emacs Primitives.
command-execute can invoke; it
is a possible definition for a key sequence. Some functions are
commands; a function written in Lisp is a command if it contains an
interactive declaration (see section Defining Commands). Such a function
can be called from Lisp expressions like other functions; in this case,
the fact that the function is a command makes no difference.
Strings are commands also, even though they are not functions. A symbol is a command if its function definition is a command; such symbols can be invoked with M-x. The symbol is a function as well if the definition is a function. See section Command Loop Overview.
This function returns t if object is a built-in function
(i.e. a Lisp primitive).
(subrp 'message) ; message is a symbol,
=> nil ; not a subr object.
(subrp (symbol-function 'message))
=> t
Function: byte-code-function-p object
This function returns t if object is a byte-code
function. For example:
(byte-code-function-p (symbol-function 'next-line))
=> t
A function written in Lisp is a list that looks like this:
(lambda (arg-variables...) [documentation-string] [interactive-declaration] body-forms...)
(Such a list is called a lambda expression for historical reasons, even though it is not really an expression at all--it is not a form that can be evaluated meaningfully.)
The first element of a lambda expression is always the symbol
lambda. This indicates that the list represents a function. The
reason functions are defined to start with lambda is so that
other lists, intended for other uses, will not accidentally be valid as
functions.
The second element is a list of argument variable names (symbols). This is called the lambda list. When a Lisp function is called, the argument values are matched up against the variables in the lambda list, which are given local bindings with the values provided. See section Local Variables.
The documentation string is an actual string that serves to describe the function for the Emacs help facilities. See section Documentation Strings of Functions.
The interactive declaration is a list of the form (interactive
code-string). This declares how to provide arguments if the
function is used interactively. Functions with this declaration are called
commands; they can be called using M-x or bound to a key.
Functions not intended to be called in this way should not have interactive
declarations. See section Defining Commands, for how to write an interactive
declaration.
The rest of the elements are the body of the function: the Lisp code to do the work of the function (or, as a Lisp programmer would say, "a list of Lisp forms to evaluate"). The value returned by the function is the value returned by the last element of the body.
Consider for example the following function:
(lambda (a b c) (+ a b c))
We can call this function by writing it as the CAR of an expression, like this:
((lambda (a b c) (+ a b c)) 1 2 3)
The body of this lambda expression is evaluated with the variable
a bound to 1, b bound to 2, and c bound to 3.
Evaluation of the body adds these three numbers, producing the result 6;
therefore, this call to the function returns the value 6.
Note that the arguments can be the results of other function calls, as in this example:
((lambda (a b c) (+ a b c)) 1 (* 2 3) (- 5 4))
Here all the arguments 1, (* 2 3), and (- 5 4) are
evaluated, left to right. Then the lambda expression is applied to the
argument values 1, 6 and 1 to produce the value 8.
It is not often useful to write a lambda expression as the CAR of
a form in this way. You can get the same result, of making local
variables and giving them values, using the special form let
(see section Local Variables). And let is clearer and easier to use.
In practice, lambda expressions are either stored as the function
definitions of symbols, to produce named functions, or passed as
arguments to other functions (see section Anonymous Functions).
However, calls to explicit lambda expressions were very useful in the
old days of Lisp, before the special form let was invented. At
that time, they were the only way to bind and initialize local
variables.
Our simple sample function, (lambda (a b c) (+ a b c)),
specifies three argument variables, so it must be called with three
arguments: if you try to call it with only two arguments or four
arguments, you get a wrong-number-of-arguments error.
It is often convenient to write a function that allows certain arguments
to be omitted. For example, the function substring accepts three
arguments--a string, the start index and the end index--but the third
argument defaults to the end of the string if you omit it. It is also
convenient for certain functions to accept an indefinite number of
arguments, as the functions and and + do.
To specify optional arguments that may be omitted when a function
is called, simply include the keyword &optional before the optional
arguments. To specify a list of zero or more extra arguments, include the
keyword &rest before one final argument.
Thus, the complete syntax for an argument list is as follows:
(required-vars... [&optional optional-vars...] [&rest rest-var])
The square brackets indicate that the &optional and &rest
clauses, and the variables that follow them, are optional.
A call to the function requires one actual argument for each of the
required-vars. There may be actual arguments for zero or more of the
optional-vars, and there cannot be any more actual arguments than
these unless &rest exists. In that case, there may be any number of
extra actual arguments.
If actual arguments for the optional and rest variables are omitted,
then they always default to nil. However, the body of the function
is free to consider nil an abbreviation for some other meaningful
value. This is what substring does; nil as the third argument
means to use the length of the string supplied. There is no way for the
function to distinguish between an explicit argument of nil and
an omitted argument.
Common Lisp note: Common Lisp allows the function to specify what
default value to use when an optional argument is omitted; GNU Emacs
Lisp always uses nil.
For example, an argument list that looks like this:
(a b &optional c d &rest e)
binds a and b to the first two actual arguments, which are
required. If one or two more arguments are provided, c and
d are bound to them respectively; any arguments after the first
four are collected into a list and e is bound to that list. If
there are only two arguments, c is nil; if two or three
arguments, d is nil; if four arguments or fewer, e
is nil.
There is no way to have required arguments following optional
ones--it would not make sense. To see why this must be so, suppose
that c in the example were optional and d were required.
If three actual arguments are given; then which variable would the third
argument be for? Similarly, it makes no sense to have any more
arguments (either required or optional) after a &rest argument.
Here are some examples of argument lists and proper calls:
((lambda (n) (1+ n)) ; One required:
1) ; requires exactly one argument.
=> 2
((lambda (n &optional n1) ; One required and one optional:
(if n1 (+ n n1) (1+ n))) ; 1 or 2 arguments.
1 2)
=> 3
((lambda (n &rest ns) ; One required and one rest:
(+ n (apply '+ ns))) ; 1 or more arguments.
1 2 3 4 5)
=> 15
A lambda expression may optionally have a documentation string just after the lambda list. This string does not affect execution of the function; it is a kind of comment, but a systematized comment which actually appears inside the Lisp world and can be used by the Emacs help facilities. See section Documentation, for how the documentation-string is accessed.
It is a good idea to provide documentation strings for all commands, and for all other functions in your program that users of your program should know about; internal functions might as well have only comments, since comments don't take up any room when your program is loaded.
The first line of the documentation string should stand on its own,
because apropos displays just this first line. It should consist
of one or two complete sentences that summarize the function's purpose.
The start of the documentation string is usually indented, but since these spaces come before the starting double-quote, they are not part of the string. Some people make a practice of indenting any additional lines of the string so that the text lines up. This is a mistake. The indentation of the following lines is inside the string; what looks nice in the source code will look ugly when displayed by the help commands.
You may wonder how the documentation string could be optional, since there are required components of the function that follow it (the body). Since evaluation of a string returns that string, without any side effects, it has no effect if it is not the last form in the body. Thus, in practice, there is no confusion between the first form of the body and the documentation string; if the only body form is a string then it serves both as the return value and as the documentation.
In most computer languages, every function has a name; the idea of a
function without a name is nonsensical. In Lisp, a function in the
strictest sense has no name. It is simply a list whose first element is
lambda, or a primitive subr-object.
However, a symbol can serve as the name of a function. This happens when you put the function in the symbol's function cell (see section Symbol Components). Then the symbol itself becomes a valid, callable function, equivalent to the list or subr-object that its function cell refers to. The contents of the function cell are also called the symbol's function definition. When the evaluator finds the function definition to use in place of the symbol, we call that symbol function indirection; see section Symbol Function Indirection.
In practice, nearly all functions are given names in this way and
referred to through their names. For example, the symbol car works
as a function and does what it does because the primitive subr-object
#<subr car> is stored in its function cell.
We give functions names because it is more convenient to refer to them
by their names in other functions. For primitive subr-objects such as
#<subr car>, names are the only way you can refer to them: there
is no read syntax for such objects. For functions written in Lisp, the
name is more convenient to use in a call than an explicit lambda
expression. Also, a function with a name can refer to itself--it can
be recursive. Writing the function's name in its own definition is much
more convenient than making the function definition point to itself
(something that is not impossible but that has various disadvantages in
practice).
Functions are often identified with the symbols used to name them. For
example, we often speak of "the function car", not distinguishing
between the symbol car and the primitive subr-object that is its
function definition. For most purposes, there is no need to distinguish.
Even so, keep in mind that a function need not have a unique name. While
a given function object usually appears in the function cell of only
one symbol, this is just a matter of convenience. It is easy to store
it in several symbols using fset; then each of the symbols is
equally well a name for the same function.
A symbol used as a function name may also be used as a variable; these two uses of a symbol are independent and do not conflict.
We usually give a name to a function when it is first created. This
is called defining a function, and it is done with the
defun special form.
Special Form: defun name argument-list body-forms
defun is the usual way to define new Lisp functions. It
defines the symbol name as a function that looks like this:
(lambda argument-list . body-forms)
This lambda expression is stored in the function cell of name.
The value returned by evaluating the defun form is name,
but usually we ignore this value.
As described previously (see section Lambda Expressions),
argument-list is a list of argument names and may include the
keywords &optional and &rest. Also, the first two forms
in body-forms may be a documentation string and an interactive
declaration.
Note that the same symbol name may also be used as a global variable, since the value cell is independent of the function cell.
Here are some examples:
(defun foo () 5)
=> foo
(foo)
=> 5
(defun bar (a &optional b &rest c)
(list a b c))
=> bar
(bar 1 2 3 4 5)
=> (1 2 (3 4 5))
(bar 1)
=> (1 nil nil)
(bar)
error--> Wrong number of arguments.
(defun capitalize-backwards ()
"Upcase the last letter of a word."
(interactive)
(backward-word 1)
(forward-word 1)
(backward-char 1)
(capitalize-word 1))
=> capitalize-backwards
Be careful not to redefine existing functions unintentionally.
defun redefines even primitive functions such as car
without any hesitation or notification. Redefining a function already
defined is often done deliberately, and there is no way to distinguish
deliberate redefinition from unintentional redefinition.
Defining functions is only half the battle. Functions don't do anything until you call them, i.e., tell them to run. This process is also known as invocation.
The most common way of invoking a function is by evaluating a list. For
example, evaluating the list (concat "a" "b") calls the function
concat. See section Evaluation, for a description of evaluation.
When you write a list as an expression in your program, the function
name is part of the program. This means that the choice of which
function to call is made when you write the program. Usually that's
just what you want. Occasionally you need to decide at run time which
function to call. Then you can use the functions funcall and
apply.
Function: funcall function &rest arguments
funcall calls function with arguments, and returns
whatever function returns.
Since funcall is a function, all of its arguments, including
function, are evaluated before funcall is called. This
means that you can use any expression to obtain the function to be
called. It also means that funcall does not see the expressions
you write for the arguments, only their values. These values are
not evaluated a second time in the act of calling function;
funcall enters the normal procedure for calling a function at the
place where the arguments have already been evaluated.
The argument function must be either a Lisp function or a
primitive function. Special forms and macros are not allowed, because
they make sense only when given the "unevaluated" argument
expressions. funcall cannot provide these because, as we saw
above, it never knows them in the first place.
(setq f 'list)
=> list
(funcall f 'x 'y 'z)
=> (x y z)
(funcall f 'x 'y '(z))
=> (x y (z))
(funcall 'and t nil)
error--> Invalid function: #<subr and>
Compare this example with that of apply.
Function: apply function &rest arguments
apply calls function with arguments, just like
funcall but with one difference: the last of arguments is a
list of arguments to give to function, rather than a single
argument. We also say that this list is appended to the other
arguments.
apply returns the result of calling function. As with
funcall, function must either be a Lisp function or a
primitive function; special forms and macros do not make sense in
apply.
(setq f 'list)
=> list
(apply f 'x 'y 'z)
error--> Wrong type argument: listp, z
(apply '+ 1 2 '(3 4))
=> 10
(apply '+ '(1 2 3 4))
=> 10
(apply 'append '((a b c) nil (x y z) nil))
=> (a b c x y z)
An interesting example of using apply is found in the description
of mapcar; see the following section.
It is common for Lisp functions to accept functions as arguments or
find them in data structures (especially in hook variables and property
lists) and call them using funcall or apply. Functions
that accept function arguments are often called functionals.
Sometimes, when you call such a function, it is useful to supply a no-op function as the argument. Here are two different kinds of no-op function:
This function returns arg and has no side effects.
This function ignores any arguments and returns nil.
A mapping function applies a given function to each element of a
list or other collection. Emacs Lisp has three such functions;
mapcar and mapconcat, which scan a list, are described
here. For the third mapping function, mapatoms, see
section Creating and Interning Symbols.
Function: mapcar function sequence
mapcar applies function to each element of sequence in
turn. The results are made into a nil-terminated list.
The argument sequence may be a list, a vector or a string. The result is always a list. The length of the result is the same as the length of sequence.
For example:
(mapcar 'car '((a b) (c d) (e f)))
=> (a c e)
(mapcar '1+ [1 2 3])
=> (2 3 4)
(mapcar 'char-to-string "abc")
=> ("a" "b" "c")
;; Call each function in my-hooks.
(mapcar 'funcall my-hooks)
(defun mapcar* (f &rest args)
"Apply FUNCTION to successive cars of all ARGS, until one
ends. Return the list of results."
;; If no list is exhausted,
(if (not (memq 'nil args))
;; Apply function to CARs.
(cons (apply f (mapcar 'car args))
(apply 'mapcar* f
;; Recurse for rest of elements.
(mapcar 'cdr args)))))
(mapcar* 'cons '(a b c) '(1 2 3 4))
=> ((a . 1) (b . 2) (c . 3))
Function: mapconcat function sequence separator
mapconcat applies function to each element of
sequence: the results, which must be strings, are concatenated.
Between each pair of result strings, mapconcat inserts the string
separator. Usually separator contains a space or comma or
other suitable punctuation.
The argument function must be a function that can take one argument and returns a string.
(mapconcat 'symbol-name
'(The cat in the hat)
" ")
=> "The cat in the hat"
(mapconcat (function (lambda (x) (format "%c" (1+ x))))
"HAL-8000"
"")
=> "IBM.9111"
In Lisp, a function is a list that starts with lambda (or
alternatively a primitive subr-object); names are "extra". Although
usually functions are defined with defun and given names at the
same time, it is occasionally more concise to use an explicit lambda
expression--an anonymous function. Such a list is valid wherever a
function name is.
Any method of creating such a list makes a valid function. Even this:
(setq silly (append '(lambda (x)) (list (list '+ (* 3 4) 'x))))
=> (lambda (x) (+ 12 x))
This computes a list that looks like (lambda (x) (+ 12 x)) and
makes it the value (not the function definition!) of
silly.
Here is how we might call this function:
(funcall silly 1)
=> 13
(It does not work to write (silly 1), because this function
is not the function definition of silly. We have not given
silly any function definition, just a value as a variable.)
Most of the time, anonymous functions are constants that appear in
your program. For example, you might want to pass one as an argument
to the function mapcar, which applies any given function to each
element of a list. Here we pass an anonymous function that multiplies
a number by two:
(defun double-each (list)
(mapcar '(lambda (x) (* 2 x)) list))
=> double-each
(double-each '(2 11))
=> (4 22)
In such cases, we usually use the special form function instead
of simple quotation to quote the anonymous function.
Special Form: function function-object
This special form returns function-object without evaluating it.
In this, it is equivalent to quote. However, it serves as a
note to the Emacs Lisp compiler that function-object is intended
to be used only as a function, and therefore can safely be compiled.
See section Quoting, for comparison.
Using function instead of quote makes a difference
inside a function or macro that you are going to compile. For example:
(defun double-each (list)
(mapcar (function (lambda (x) (* 2 x))) list))
=> double-each
(double-each '(2 11))
=> (4 22)
If this definition of double-each is compiled, the anonymous
function is compiled as well. By contrast, in the previous definition
where ordinary quote is used, the argument passed to
mapcar is the precise list shown:
(lambda (arg) (+ arg 5))
The Lisp compiler cannot assume this list is a function, even though it
looks like one, since it does not know what mapcar does with the
list. Perhaps mapcar will check that the CAR of the third
element is the symbol +! The advantage of function is
that it tells the compiler to go ahead and compile the constant
function.
We sometimes write function instead of quote when
quoting the name of a function, but this usage is just a sort of
comment.
(function symbol) == (quote symbol) == 'symbol
See documentation in section Access to Documentation Strings, for a
realistic example using function and an anonymous function.
The function definition of a symbol is the object stored in the function cell of the symbol. The functions described here access, test, and set the function cell of symbols.
Function: symbol-function symbol
This returns the object in the function cell of symbol. If the
symbol's function cell is void, a void-function error is
signaled.
This function does not check that the returned object is a legitimate function.
(defun bar (n) (+ n 2))
=> bar
(symbol-function 'bar)
=> (lambda (n) (+ n 2))
(fset 'baz 'bar)
=> bar
(symbol-function 'baz)
=> bar
If you have never given a symbol any function definition, we say that
that symbol's function cell is void. In other words, the function
cell does not have any Lisp object in it. If you try to call such a symbol
as a function, it signals a void-function error.
Note that void is not the same as nil or the symbol
void. The symbols nil and void are Lisp objects,
and can be stored into a function cell just as any other object can be
(and they can be valid functions if you define them in turn with
defun); but nil or void is an object. A
void function cell contains no object whatsoever.
You can test the voidness of a symbol's function definition with
fboundp. After you have given a symbol a function definition, you
can make it void once more using fmakunbound.
Returns t if the symbol has an object in its function cell,
nil otherwise. It does not check that the object is a legitimate
function.
This function makes symbol's function cell void, so that a
subsequent attempt to access this cell will cause a void-function
error. (See also makunbound, in section Local Variables.)
(defun foo (x) x)
=> x
(fmakunbound 'foo)
=> x
(foo 1)
error--> Symbol's function definition is void: foo
This function stores object in the function cell of symbol. The result is object. Normally object should be a function or the name of a function, but this is not checked.
There are three normal uses of this function:
defun. See section Classification of List Forms, for an
example of this usage.
defun
were not a primitive, it could be written in Lisp (as a macro) using
fset.
Here are examples of the first two uses:
;; Givefirstthe same definitioncarhas. (fset 'first (symbol-function 'car)) => #<subr car> (first '(1 2 3)) => 1 ;; Make the symbolcarthe function definition ofxfirst. (fset 'xfirst 'car) => car (xfirst '(1 2 3)) => 1 (symbol-function 'xfirst) => car (symbol-function (symbol-function 'xfirst)) => #<subr car> ;; Define a named keyboard macro. (fset 'kill-two-lines "\^u2\^k") => "\^u2\^k"
When writing a function that extends a previously defined function, the following idiom is often used:
(fset 'old-foo (symbol-function 'foo)) (defun foo () "Just like old-foo, except more so." (old-foo) (more-so))
This does not work properly if foo has been defined to autoload.
In such a case, when foo calls old-foo, Lisp attempts
to define old-foo by loading a file. Since this presumably
defines foo rather than old-foo, it does not produce the
proper results. The only way to avoid this problem is to make sure the
file is loaded before moving aside the old definition of foo.
See also the function indirect-function in section Symbol Function Indirection.
You can define an inline function by using defsubst instead
of defun. An inline function works just like an ordinary
function except for one thing: when you compile a call to the function,
the function's definition is open-coded into the caller.
Making a function inline makes explicit calls run faster. But it also has disadvantages. For one thing, it reduces flexibility; if you change the definition of the function, calls already inlined still use the old definition until you recompile them. Since the flexibility of redefining functions is an important features of Emacs, you should not make a function inline unless its speed is really crucial.
Another disadvantage is that making a large function inline can increase the size of compiled code both in files and in memory. Since the advantages of inline functions are greatest for small functions, you generally should not make large functions inline.
It's possible to define a macro to expand into the same code that an
inline function would execute. But the macro would have a limitation:
you can use it only explicitly--a macro cannot be called with
apply, mapcar and so on. Also, it takes some work to
convert an ordinary function into a macro. (See section Macros.) To convert
it into an inline function is very easy; simply replace defun
with defsubst.
Inline functions can be used and open coded later on in the same file, following the definition, just like macros.
Emacs versions prior to 19 did not have inline functions.
Here is a table of several functions that do things related to function calling and function definitions. They are documented elsewhere, but we provide cross references here.
apply
autoload
call-interactively
commandp
documentation
eval
funcall
ignore
indirect-function
interactive
interactive.
interactive-p
mapatoms
mapcar
mapconcat
undefined
Macros enable you to define new control constructs and other language features. A macro is defined much like a function, but instead of telling how to compute a value, it tells how to compute another Lisp expression which will in turn compute the value. We call this expression the expansion of the macro.
Macros can do this because they operate on the unevaluated expressions for the arguments, not on the argument values as functions do. They can therefore construct an expansion containing these argument expressions or parts of them.
If you are using a macro to do something an ordinary function could do, just for the sake of speed, consider using an inline function instead. See section Inline Functions.
Suppose we would like to define a Lisp construct to increment a
variable value, much like the ++ operator in C. We would like to
write (inc x) and have the effect of (setq x (1+ x)).
Here's a macro definition that does the job:
(defmacro inc (var) (list 'setq var (list '1+ var)))
When this is called with (inc x), the argument var has
the value x---not the value of x. The body
of the macro uses this to construct the expansion, which is (setq
x (1+ x)). Once the macro definition returns this expansion, Lisp
proceeds to evaluate it, thus incrementing x.
A macro call looks just like a function call in that it is a list which starts with the name of the macro. The rest of the elements of the list are the arguments of the macro.
Evaluation of the macro call begins like evaluation of a function call except for one crucial difference: the macro arguments are the actual expressions appearing in the macro call. They are not evaluated before they are given to the macro definition. By contrast, the arguments of a function are results of evaluating the elements of the function call list.
Having obtained the arguments, Lisp invokes the macro definition just
as a function is invoked. The argument variables of the macro are bound
to the argument values from the macro call, or to a list of them in the
case of a &rest argument. And the macro body executes and
returns its value just as a function body does.
The second crucial difference between macros and functions is that the value returned by the macro body is not the value of the macro call. Instead, it is an alternate expression for computing that value, also known as the expansion of the macro. The Lisp interpreter proceeds to evaluate the expansion as soon as it comes back from the macro.
Since the expansion is evaluated in the normal manner, it may contain calls to other macros. It may even be a call to the same macro, though this is unusual.
You can see the expansion of a given macro call by calling
macroexpand.
Function: macroexpand form &optional environment
This function expands form, if it is a macro call. If the result
is another macro call, it is expanded in turn, until something which is
not a macro call results. That is the value returned by
macroexpand. If form is not a macro call to begin with, it
is returned as given.
Note that macroexpand does not look at the subexpressions of
form (although some macro definitions may do so). Even if they
are macro calls themselves, macroexpand does not expand them.
The function macroexpand does not expand calls to inline functions.
Normally there is no need for that, since a call to an inline function is
no harder to understand than a call to an ordinary function.
If environment is provided, it specifies an alist of macro definitions that shadow the currently defined macros. This is used by byte compilation.
(defmacro inc (var)
(list 'setq var (list '1+ var)))
=> inc
(macroexpand '(inc r))
=> (setq r (1+ r))
(defmacro inc2 (var1 var2)
(list 'progn (list 'inc var1) (list 'inc var2)))
=> inc2
(macroexpand '(inc2 r s))
=> (progn (inc r) (inc s)) ; inc not expanded here.
You might ask why we take the trouble to compute an expansion for a macro and then evaluate the expansion. Why not have the macro body produce the desired results directly? The reason has to do with compilation.
When a macro call appears in a Lisp program being compiled, the Lisp compiler calls the macro definition just as the interpreter would, and receives an expansion. But instead of evaluating this expansion, it compiles the expansion as if it had appeared directly in the program. As a result, the compiled code produces the value and side effects intended for the macro, but executes at full compiled speed. This would not work if the macro body computed the value and side effects itself--they would be computed at compile time, which is not useful.
In order for compilation of macro calls to work, the macros must be
defined in Lisp when the calls to them are compiled. The compiler has a
special feature to help you do this: if a file being compiled contains a
defmacro form, the macro is defined temporarily for the rest of
the compilation of that file. To use this feature, you must define the
macro in the same file where it is used and before its first use.
While byte-compiling a file, any require calls at top-level are
executed. One way to ensure that necessary macro definitions are
available during compilation is to require the file that defines them.
See section Features.
A Lisp macro is a list whose CAR is macro. Its CDR should
be a function; expansion of the macro works by applying the function
(with apply) to the list of unevaluated argument-expressions
from the macro call.
It is possible to use an anonymous Lisp macro just like an anonymous
function, but this is never done, because it does not make sense to pass
an anonymous macro to mapping functions such as mapcar. In
practice, all Lisp macros have names, and they are usually defined with
the special form defmacro.
Special Form: defmacro name argument-list body-forms...
defmacro defines the symbol name as a macro that looks
like this:
(macro lambda argument-list . body-forms)
This macro object is stored in the function cell of name. The
value returned by evaluating the defmacro form is name, but
usually we ignore this value.
The shape and meaning of argument-list is the same as in a
function, and the keywords &rest and &optional may be used
(see section Advanced Features of Argument Lists). Macros may have a documentation string, but
any interactive declaration is ignored since macros cannot be
called interactively.
It could prove rather awkward to write macros of significant size,
simply due to the number of times the function list needs to be
called. To make writing these forms easier, a macro ``'
(often called backquote) exists.
Backquote allows you to quote a list, but selectively evaluate
elements of that list. In the simplest case, it is identical to the
special form quote (see section Quoting). For example, these
two forms yield identical results:
(` (a list of (+ 2 3) elements))
=> (a list of (+ 2 3) elements)
(quote (a list of (+ 2 3) elements))
=> (a list of (+ 2 3) elements)
By inserting a special marker, `,', inside of the argument to backquote, it is possible to evaluate desired portions of the argument:
(list 'a 'list 'of (+ 2 3) 'elements)
=> (a list of 5 elements)
(` (a list of (, (+ 2 3)) elements))
=> (a list of 5 elements)
It is also possible to have an evaluated list spliced into the
resulting list by using the special marker `,@'. The elements of
the spliced list become elements at the same level as the other elements
of the resulting list. The equivalent code without using ` is
often unreadable. Here are some examples:
(setq some-list '(2 3))
=> (2 3)
(cons 1 (append some-list '(4) some-list))
=> (1 2 3 4 2 3)
(` (1 (,@ some-list) 4 (,@ some-list)))
=> (1 2 3 4 2 3)
(setq list '(hack foo bar))
=> (hack foo bar)
(cons 'use
(cons 'the
(cons 'words (append (cdr list) '(as elements)))))
=> (use the words foo bar as elements)
(` (use the words (,@ (cdr list)) as elements (,@ nil)))
=> (use the words foo bar as elements)
The reason for (,@ nil) is to avoid a bug in Emacs version 18.
The bug occurs when a call to ,@ is followed only by constant
elements. Thus,
(` (use the words (,@ (cdr list)) as elements))
would not work, though it really ought to. (,@ nil) avoids the
problem by being a nonconstant element that does not affect the result.
This macro returns list as quote would, except that the
list is copied each time this expression is evaluated, and any sublist
of the form (, subexp) is replaced by the value of
subexp. Any sublist of the form (,@ listexp)
is replaced by evaluating listexp and splicing its elements
into the containing list in place of this sublist. (A single sublist
can in this way be replaced by any number of new elements in the
containing list.)
There are certain contexts in which `,' would not be recognized and should not be used:
;; Use of a `,' expression as the CDR of a list. (` (a . (, 1))) ; Not(a . 1)=> (a \, 1) ;; Use of `,' in a vector. (` [a (, 1) c]) ; Not[a 1 c]error--> Wrong type argument ;; Use of a `,' as the entire argument of ``'. (` (, 2)) ; Not 2 => (\, 2)
Common Lisp note: in Common Lisp, `,' and `,@' are implemented as reader macros, so they do not require parentheses. Emacs Lisp implements them as functions because reader macros are not supported (to save space).
The basic facts of macro expansion have all been described above, but there consequences are often counterintuitive. This section describes some important consequences that can lead to trouble, and rules to follow to avoid trouble.
When defining a macro you must pay attention to the number of times the arguments will be evaluated when the expansion is executed. The following macro (used to facilitate iteration) illustrates the problem. This macro allows us to write a simple "for" loop such as one might find in Pascal.
(defmacro for (var from init to final do &rest body)
"Execute a simple \"for\" loop, e.g.,
(for i from 1 to 10 do (print i))."
(list 'let (list (list var init))
(cons 'while (cons (list '<= var final)
(append body (list (list 'inc var)))))))
=> for
(for i from 1 to 3 do
(setq square (* i i))
(princ (format "\n%d %d" i square)))
==>
(let ((i 1))
(while (<= i 3)
(setq square (* i i))
(princ (format "%d %d" i square))
(inc i)))
-|1 1
-|2 4
-|3 9
=> nil
(The arguments from, to, and do in this macro are
"syntactic sugar"; they are entirely ignored. The idea is that you
will write noise words (such as from, to, and do)
in those positions in the macro call.)
This macro suffers from the defect that final is evaluated on
every iteration. If final is a constant, this is not a problem.
If it is a more complex form, say (long-complex-calculation x),
this can slow down the execution significantly. If final has side
effects, executing it more than once is probably incorrect.
A well-designed macro definition takes steps to avoid this problem by
producing an expansion that evaluates the argument expressions exactly
once unless repeated evaluation is part of the intended purpose of the
macro. Here is a correct expansion for the for macro:
(let ((i 1)
(max 3))
(while (<= i max)
(setq square (* i i))
(princ (format "%d %d" i square))
(inc i)))
Here is a macro definition that creates this expansion:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
(` (let (((, var) (, init))
(max (, final)))
(while (<= (, var) max)
(,@ body)
(inc (, var))))))
Unfortunately, this introduces another problem.
The new definition of for has a new problem: it introduces a
local variable named max which the user does not expect. This
causes trouble in examples such as the following:
(let ((max 0))
(for x from 0 to 10 do
(let ((this (frob x)))
(if (< max this)
(setq max this)))))
The references to max inside the body of the for, which
are supposed to refer to the user's binding of max, really access
the binding made by for.
The way to correct this is to use an uninterned symbol instead of
max (see section Creating and Interning Symbols). The uninterned symbol can be
bound and referred to just like any other symbol, but since it is created
by for, we know that it cannot appear in the user's program.
Since it is not interned, there is no way the user can put it into the
program later. It will never appear anywhere except where put by
for. Here is a definition of for which works this way:
(defmacro for (var from init to final do &rest body)
"Execute a simple for loop: (for i from 1 to 10 do (print i))."
(let ((tempvar (make-symbol "max")))
(` (let (((, var) (, init))
((, tempvar) (, final)))
(while (<= (, var) (, tempvar))
(,@ body)
(inc (, var)))))))
This creates an uninterned symbol named max and puts it in the
expansion instead of the usual interned symbol max that appears
in expressions ordinarily.
Another problem can happen if you evaluate any of the macro argument
expressions during the computation of the expansion, such as by calling
eval (see section Eval). If the argument is supposed to refer to the
user's variables, you may have trouble if the user happens to use a
variable with the same name as one of the macro arguments. Inside the
macro body, the macro argument binding is the most local binding of this
variable, so any references inside the form being evaluated do refer
to it. Here is an example:
(defmacro foo (a)
(list 'setq (eval a) t))
=> foo
(setq x 'b)
(foo x) ==> (setq b t)
=> t ; and b has been set.
;; but
(setq a 'b)
(foo a) ==> (setq 'b t) ; invalid!
error--> Symbol's value is void: b
It makes a difference whether the user types a or x,
because a conflicts with the macro argument variable a.
In general it is best to avoid calling eval in a macro
definition at all.
Occasionally problems result from the fact that a macro call is expanded each time it is evaluated in an interpreted function, but is expanded only once (during compilation) for a compiled function. If the macro definition has side effects, they will work differently depending on how many times the macro is expanded.
In particular, constructing objects is a kind of side effect. If the macro is called once, then the objects are constructed only once. In other words, the same structure of objects is used each time the macro call is executed. In interpreted operation, the macro is reexpanded each time, producing a fresh collection of objects each time. Usually this does not matter--the objects have the same contents whether they are shared or not. But if the surrounding program does side effects on the objects, it makes a difference whether they are shared. Here is an example:
(defmacro new-object ()
(list 'quote (cons nil nil)))
(defun initialize (condition)
(let ((object (new-object)))
(if condition
(setcar object condition))
object))
If initialize is interpreted, a new list (nil) is
constructed each time initialize is called. Thus, no side effect
survives between calls. If initialize is compiled, then the
macro new-object is expanded during compilation, producing a
single "constant" (nil) that is reused and altered each time
initialize is called.
Loading a file of Lisp code means bringing its contents into the Lisp environment in the form of Lisp objects. Emacs finds and opens the file, reads the text, evaluates each form, and then closes the file.
The load functions evaluate all the expressions in a file just
as the eval-current-buffer function evaluates all the
expressions in a buffer. The difference is that the load functions
read and evaluate the text in the file as found on disk, not the text
in an Emacs buffer.
The loaded file must contain Lisp expressions, either as source code or, optionally, as byte-compiled code. Each form in the file is called a top-level form. There is no special format for the forms in a loadable file; any form in a file may equally well be typed directly into a buffer and evaluated there. (Indeed, most code is tested this way.) Most often, the forms are function definitions and variable definitions.
A file containing Lisp code is often called a library. Thus, the "Rmail library" is a file containing code for Rmail mode. Similarly, a "Lisp library directory" is a directory of files containing Lisp code.
There are several interface functions for loading. For example, the
autoload function creates a Lisp object that loads a file when it
is evaluated (see section Autoload). require also causes files to be
loaded (see section Features). Ultimately, all these facilities call the
load function to do the work.
Function: load filename &optional missing-ok nomessage nosuffix
This function finds and opens a file of Lisp code, evaluates all the forms in it, and closes the file.
To find the file, load first looks for a file named
`filename.elc', that is, for a file whose name has
`.elc' appended. If such a file exists, it is loaded. But if
there is no file by that name, then load looks for a file whose
name has `.el' appended. If that file exists, it is loaded.
Finally, if there is no file by either name, load looks for a
file named filename with nothing appended, and loads it if it
exists. (The load function is not clever about looking at
filename. In the perverse case of a file named `foo.el.el',
evaluation of (load "foo.el") will indeed find it.)
If the optional argument nosuffix is non-nil, then the
suffixes `.elc' and `.el' are not tried. In this case, you
must specify the precise file name you want.
If filename is a relative file name, such as `foo' or
`baz/foo.bar', load searches for the file using the variable
load-path. It appends filename to each of the directories
listed in load-path, and loads the first file it finds whose
name matches. The current default directory is tried only if it is
specified in load-path, where it is represented as nil.
All three possible suffixes are tried in the first directory in
load-path, then all three in the second directory in
load-path, etc.
If you get a warning that `foo.elc' is older than `foo.el', it means you should consider recompiling `foo.el'. See section Byte Compilation.
Messages like `Loading foo...' and `Loading foo...done' appear
in the echo area during loading unless nomessage is
non-nil.
Any errors that are encountered while loading a file cause load
to abort. If the load was done for the sake of autoload, certain
kinds of top-level forms, those which define functions, are undone.
The error file-error is signaled (with `Cannot open load
file filename') if no file is found. No error is signaled if
missing-ok is non-nil---then load just returns
nil.
load returns t if the file loads successfully.
The value of this variable is a list of directories to search when
loading files with load. Each element is a string (which must be
a directory name) or nil (which stands for the current working
directory). The value of load-path is initialized from the
environment variable EMACSLOADPATH, if it exists; otherwise it is
set to the default specified in `emacs/src/paths.h' when Emacs is
built.
The syntax of EMACSLOADPATH is the same as that of PATH;
fields are separated by `:', and `.' is used for the current
default directory. Here is an example of how to set your
EMACSLOADPATH variable from a csh `.login' file:
setenv EMACSLOADPATH .:/user/bil/emacs:/usr/lib/emacs/lisp
Here is how to set it using sh:
export EMACSLOADPATH EMACSLOADPATH=.:/user/bil/emacs:/usr/local/lib/emacs/lisp
Here is an example of code you can place in a `.emacs' file to add
several directories to the front of your default load-path:
(setq load-path
(append
(list nil
"/user/bil/emacs"
"/usr/local/lisplib")
load-path))
In this example, the path searches the current working directory first, followed then by the `/user/bil/emacs' directory and then by the `/usr/local/lisplib' directory, which are then followed by the standard directories for Lisp code.
When Emacs version 18 processes command options `-l' or
`-load' which specify Lisp libraries to be loaded, it temporarily
adds the current directory to the front of load-path so that
files in the current directory can be specified easily. Newer Emacs
versions also find such files in the current directory, but without
altering load-path.
This variable is non-nil if Emacs is in the process of loading a
file, and it is nil otherwise. This is how defun and
provide determine whether a load is in progress, so that their
effect can be undone if the load fails.
To learn how load is used to build Emacs, see section Building Emacs.
The autoload facility allows you to make a function or macro available but put off loading its actual definition. An attempt to call a symbol whose definition is an autoload object automatically reads the file to install the real definition and its other associated code, and then calls the real definition.
To prepare a function or macro for autoloading, you must call
autoload, specifying the function name and the name of the file
to be loaded. A file such as `emacs/lisp/loaddefs.el' usually does
this when Emacs is first built.
The following example shows how doctor is prepared for
autoloading in `loaddefs.el':
(autoload 'doctor "doctor" "\ Switch to *doctor* buffer and start giving psychotherapy." t)
The backslash and newline immediately following the double-quote are a convention used only in the preloaded Lisp files such as `loaddefs.el'; they cause the documentation string to be put in the `etc/DOC' file. (See section Building Emacs.) In any other source file, you would write just this:
(autoload 'doctor "doctor" "Switch to *doctor* buffer and start giving psychotherapy." t)
Calling autoload creates an autoload object containing the name
of the file and some other information, and makes this the function
definition of the specified symbol. When you later try to call that
symbol as a function or macro, the file is loaded; the loading should
redefine that symbol with its proper definition. After the file
completes loading, the function or macro is called as if it had been
there originally.
If, at the end of loading the file, the desired Lisp function or macro
has not been defined, then the error error is signaled (with data
"Autoloading failed to define function function-name").
The autoloaded file may, of course, contain other definitions and may
require or provide one or more features. If the file is not completely
loaded (due to an error in the evaluation of the contents) any function
definitions or provide calls that occurred during the load are
undone. This is to ensure that the next attempt to call any function
autoloading from this file will try again to load the file. If not for
this, then some of the functions in the file might appear defined, but
they may fail to work properly for the lack of certain subroutines
defined later in the file and not loaded successfully.
Emacs as distributed comes with many autoloaded functions.
The calls to autoload are in the file `loaddefs.el'.
There is a convenient way of updating them automatically.
Write `;;;###autoload' on a line by itself before the real
definition of the function, in its autoloadable source file; then the
command M-x update-file-autoloads automatically puts the
autoload call into `loaddefs.el'. M-x
update-directory-autoloads is more powerful; it updates autoloads for
all files in the current directory.
You can also put other kinds of forms into `loaddefs.el', by writing `;;;###autoload' followed on the same line by the form. M-x update-file-autoloads copies the form from that line.
The commands for updating autoloads work by visiting and editing the file `loaddefs.el'. To make the result take effect, you must save that file's buffer.
Function: autoload symbol filename &optional docstring interactive type
This function defines the function (or macro) named symbol so as
to load automatically from filename. The string filename is
a file name which will be passed to load when the function is
called.
The argument docstring is the documentation string for the function. Normally, this is the same string that is in the function definition itself. This makes it possible to look at the documentation without loading the real definition.
If interactive is non-nil, then the function can be
called interactively. This lets completion in M-x work without
loading the function's real definition. The complete interactive
specification need not be given here. If type is macro,
then the function is really a macro. If type is keymap,
then the function is really a keymap.
If symbol already has a non-nil function definition that
is not an autoload object, autoload does nothing and returns
nil. If the function cell of symbol is void, or is already
an autoload object, then it is set to an autoload object that looks like
this:
(autoload filename docstring interactive type)
For example,
(symbol-function 'run-prolog)
=> (autoload "prolog" 169681 t nil)
In this case, "prolog" is the name of the file to load, 169681 refers
to the documentation string in the `emacs/etc/DOC'
file (see section Documentation Basics), t means the function is
interactive, and nil that it is not a macro.
You may load a file more than once in an Emacs session. For example, after you have rewritten and reinstalled a function definition by editing it in a buffer, you may wish to return to the original version; you can do this by reloading the file in which it is located.
When you load or reload files, bear in mind that the load and
load-library functions automatically load a byte-compiled file
rather than a non-compiled file of similar name. If you rewrite a file
that you intend to save and reinstall, remember to byte-compile it if
necessary; otherwise you may find yourself inadvertently reloading the
older, byte-compiled file instead of your newer, non-compiled file!
When writing the forms in a library, keep in mind that the library
might be loaded more than once. For example, the choice of
defvar vs. defconst for defining a variable depends on
whether it is desirable to reinitialize the variable if the library is
reloaded: defconst does so, and defvar does not.
(See section Defining Global Variables.)
The simplest way to add an element to an alist is like this:
(setq minor-mode-alist
(cons '(leif-mode " Leif") minor-mode-alist))
But this would add multiple elements if the library is reloaded. To avoid the problem, write this:
(or (assq 'leif-mode minor-mode-alist)
(setq minor-mode-alist
(cons '(leif-mode " Leif") minor-mode-alist)))
Occasionally you will want to test explicitly whether a library has already been loaded; you can do so as follows:
(if (not (boundp 'foo-was-loaded))
execute-first-time-only)
(setq foo-was-loaded t)
provide and require are an alternative to
autoload for loading files automatically. They work in terms of
named features. Autoloading is triggered by calling a specific
function, but a feature is loaded the first time another program asks
for it by name.
The use of named features simplifies the task of determining whether required definitions have been defined. A feature name is a symbol that stands for a collection of functions, variables, etc. A program that needs the collection may ensure that they are defined by requiring the feature. If the file that contains the feature has not yet been loaded, then it will be loaded (or an error will be signaled if it cannot be loaded). The file thus loaded must provide the required feature or an error will be signaled.
To require the presence of a feature, call require with the
feature name as argument. require looks in the global variable
features to see whether the desired feature has been provided
already. If not, it loads the feature from the appropriate file. This
file should call provide at the top-level to add the feature to
features.
Features are normally named after the files they are provided in
so that require need not be given the file name.
For example, in `emacs/lisp/prolog.el',
the definition for run-prolog includes the following code:
(defun run-prolog () "Run an inferior Prolog process,\ input and output via buffer *prolog*." (interactive) (require 'comint) (switch-to-buffer (make-comint "prolog" prolog-program-name)) (inferior-prolog-mode))
The expression (require 'shell) loads the file `shell.el' if
it has not yet been loaded. This ensures that make-shell is
defined.
The `shell.el' file contains the following top-level expression:
(provide 'shell)
This adds shell to the global features list when the
`shell' file is loaded, so that (require 'shell) will
henceforth know that nothing needs to be done.
When require is used at top-level in a file, it takes effect if
you byte-compile that file (see section Byte Compilation). This is in case
the required package contains macros that the byte compiler must know
about.
Although top-level calls to require are evaluated during
byte compilation, provide calls are not. Therefore, you can
ensure that a file of definitions is loaded before it is byte-compiled
by including a provide followed by a require for the same
feature, as in the following example.
(provide 'my-feature) ; Ignored by byte compiler,
; evaluated by load.
(require 'my-feature) ; Evaluated by byte compiler.
This function announces that feature is now loaded, or being loaded, into the current Emacs session. This means that the facilities associated with feature are or will be available for other Lisp programs.
The direct effect of calling provide is to add feature to
the front of the list features if it is not already in the list.
The argument feature must be a symbol. provide returns
feature.
features
=> (bar bish)
(provide 'foo)
=> foo
features
=> (foo bar bish)
During autoloading, if the file is not completely loaded (due to an
error in the evaluation of the contents) any function definitions or
provide calls that occurred during the load are undone.
See section Autoload.
Function: require feature &optional filename
This function checks whether feature is present in the current
Emacs session (using (featurep feature); see below). If it
is not, then require loads filename with load. If
filename is not supplied, then the name of the symbol
feature is used as the file name to load.
If feature is not provided after the file has been loaded, Emacs
will signal the error error (with data `Required feature
feature was not provided').
This function returns t if feature has been provided in the
current Emacs session (i.e., feature is a member of
features.)
The value of this variable is a list of symbols that are the features
loaded in the current Emacs session. Each symbol was put in this list
with a call to provide. The order of the elements in the
features list is not significant.
You can discard the functions and variables loaded by a library to
reclaim memory for other Lisp objects. To do this, use the function
unload-feature:
Command: unload-feature feature
This command unloads the library that provided feature feature.
It undefines all functions and variables defined with defvar,
defmacro, defconst, defsubst and
defalias by the library which provided feature
feature. It then restores any autoloads associated with those
symbols.
The unload-feature function is written in Lisp; its actions are
based on the variable load-history.
Variable: load-history feature association list
This variable's value is an alist connecting library names with the names of functions and variables they define, the features they provide, and the features they require.
Each element is a list and describes one library. The CAR of the list is the name of the library, as a string. The rest of the list is composed of these kinds of objects:
(require . feature) indicating the
features that are required.
(provide . feature) indicating the
features that are provided.
The value of load-history may have one element whose CAR is
nil. This element describes definitions made with
eval-buffer on a buffer that is not visiting a file.
The command eval-region updates load-history, but does so
by adding the symbols defined to the element for the file being visited,
rather than replacing that element.
You can ask for code to be executed if and when a particular library is
loaded, by calling eval-after-load.
Function: eval-after-load library form
This function arranges to evaluate form at the end of loading the library library, if and when library is loaded.
The library name library must exactly match the argument of
load. To get the proper results when an installed library is
found by searching load-path, you should not include any
directory names in library.
An error in form does not undo the load, but does prevent execution of the rest of form.
An alist of expressions to evaluate if and when particular libraries are loaded. Each element looks like this:
(filename forms...)
The function load checks after-load-alist in order to
implement eval-after-load.
GNU Emacs Lisp has a compiler that translates functions written in Lisp into a special representation called byte-code that can be executed more efficiently. The compiler replaces Lisp function definitions with byte-code. When a byte-code function is called, its definition is evaluated by the byte-code interpreter.
Because the byte-compiled code is evaluated by the byte-code interpreter, instead of being executed directly by the machine's hardware (as true compiled code is), byte-code is completely transportable from machine to machine without recompilation. It is not, however, as fast as true compiled code.
In general, any version of Emacs can run byte-compiled code produced by recent earlier versions of Emacs, but the reverse is not true. In particular, if you compile a program with Emacs 18, you can run the compiled code in Emacs 19, but not vice versa.
See section Debugging Problems in Compilation, for how to investigate errors occurring in byte compilation.
You can byte-compile an individual function or macro definition with
the byte-compile function. You can compile a whole file with
byte-compile-file, or several files with
byte-recompile-directory or batch-byte-compile.
When you run the byte compiler, you may get warnings in a buffer called `*Compile-Log*'. These report usage in your program that suggest a problem, but are not necessarily erroneous.
Be careful when byte-compiling code that uses macros. Macro calls are expanded when they are compiled, so the macros must already be defined for proper compilation. For more details, see section Macros and Byte Compilation.
While byte-compiling a file, any require calls at top-level are
executed. One way to ensure that necessary macro definitions are
available during compilation is to require the file that defines them.
See section Features.
A byte-compiled function is not as efficient as a primitive function written in C, but runs much faster than the version written in Lisp. For a rough comparison, consider the example below:
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
=> silly-loop
(silly-loop 100000)
=> ("Thu Jan 12 20:18:38 1989"
"Thu Jan 12 20:19:29 1989") ; 51 seconds
(byte-compile 'silly-loop)
=> [Compiled code not shown]
(silly-loop 100000)
=> ("Thu Jan 12 20:21:04 1989"
"Thu Jan 12 20:21:17 1989") ; 13 seconds
In this example, the interpreted code required 51 seconds to run, whereas the byte-compiled code required 13 seconds. These results are representative, but actual results will vary greatly.
This function byte-compiles the function definition of symbol,
replacing the previous definition with the compiled one. The function
definition of symbol must be the actual code for the function;
i.e., the compiler does not follow indirection to another symbol.
byte-compile does not compile macros. byte-compile
returns the new, compiled definition of symbol.
(defun factorial (integer)
"Compute factorial of INTEGER."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
=> factorial
(byte-compile 'factorial)
=>
#[(integer)
"^H\301U\203^H^@\301\207\302^H\303^HS!\"\207"
[integer 1 * factorial]
4 "Compute factorial of INTEGER."]
The result is a compiled function object. The string it contains is the actual byte-code; each character in it is an instruction. The vector contains all the constants, variable names and function names used by the function, except for certain primitives that are coded as special instructions.
This command reads the defun containing point, compiles it, and evaluates the result. If you use this on a defun that is actually a function definition, the effect is to install a compiled version of that function.
Command: byte-compile-file filename
This function compiles a file of Lisp code named filename into a file of byte-code. The output file's name is made by appending `c' to the end of filename.
Compilation works by reading the input file one form at a time. If it is a definition of a function or macro, the compiled function or macro definition is written out. Other forms are batched together, then each batch is compiled, and written so that its compiled code will be executed when the file is read. All comments are discarded when the input file is read.
This command returns t. When called interactively, it prompts
for the file name.
% ls -l push*
-rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el
(byte-compile-file "~/emacs/push.el")
=> t
% ls -l push*
-rw-r--r-- 1 lewis 791 Oct 5 20:31 push.el
-rw-rw-rw- 1 lewis 638 Oct 8 20:25 push.elc
Command: byte-recompile-directory directory flag
This function recompiles every `.el' file in directory that needs recompilation. A file needs recompilation if a `.elc' file exists but is older than the `.el' file.
If a `.el' file exists, but there is no corresponding `.elc'
file, then flag is examined. If it is nil, the file is
ignored. If it is non-nil, the user is asked whether the file
should be compiled.
The returned value of this command is unpredictable.
This function runs byte-compile-file on the files remaining on
the command line. This function must be used only in a batch execution
of Emacs, as it kills Emacs on completion. An error in one file does
not prevent processing of subsequent files. (The file which gets the
error will not, of course, produce any compiled code.)
% emacs -batch -f batch-byte-compile *.el
Function: byte-code code-string data-vector max-stack
This function actually interprets byte-code. A byte-compiled function
is actually defined with a body that calls byte-code. Don't call
this function yourself. Only the byte compiler knows how to generate
valid calls to this function.
In newer Emacs versions (19 and up), byte-code is usually executed as
part of a compiled function object, and only rarely as part of a call to
byte-code.
These features permit you to write code to be evaluated during compilation of a program.
Special Form: eval-and-compile body
This form marks body to be evaluated both when you compile the containing code and when you run it (whether compiled or not).
You can get a similar result by putting body in a separate file
and referring to that file with require. Using require is
preferable if there is a substantial amount of code to be executed in
this way.
Special Form: eval-when-compile body
This form marks body to be evaluated at compile time only. The result of evaluation by the compiler becomes a constant which appears in the compiled program. When the program is interpreted, not compiled at all, body is evaluated normally.
At top-level, this is analogous to the Common Lisp idiom
(eval-when (compile) ...). Elsewhere, the Common Lisp
`#.' reader macro (but not when interpreting) is closer to what
eval-when-compile does.
Byte-compiled functions have a special data type: they are byte-code function objects.
Internally, a byte-code function object is much like a vector; however, the evaluator handles this data type specially when it appears as a function to be called. The printed representation for a byte-code function object is like that for a vector, with an additional `#' before the opening `['.
In Emacs version 18, there was no byte-code function object data type;
compiled functions used the function byte-code to run the byte
code.
A byte-code function object must have at least four elements; there is no maximum number, but only the first six elements are actually used. They are:
nil. For functions
preloaded before Emacs is dumped, this is usually an integer which is an
index into the `DOC' file; use documentation to convert this
into a string (see section Access to Documentation Strings).
nil for a function that isn't interactive.
Here's an example of a byte-code function object, in printed
representation. It is the definition of the command
backward-sexp.
#[(&optional arg) "^H\204^F^@\301^P\302^H[!\207" [arg 1 forward-sexp] 2 254435 "p"]
The primitive way to create a byte-code object is with
make-byte-code:
Function: make-byte-code &rest elements
This function constructs and returns a byte-code function object with elements as its elements.
You should not try to come up with the elements for a byte-code function yourself, because if they are inconsistent, Emacs may crash when you call the function. Always leave it to the byte-compiler to create these objects; it, we hope, always makes the elements consistent.
You can access the elements of a byte-code object using aref;
you can also use vconcat to create a vector with the same
elements.
People do not write byte-code; that job is left to the byte compiler. But we provide a disassembler to satisfy a cat-like curiosity. The disassembler converts the byte-compiled code into humanly readable form.
The byte-code interpreter is implemented as a simple stack machine. Values get stored by being pushed onto the stack, and are popped off and manipulated, the results being pushed back onto the stack. When a function returns, the top of the stack is popped and returned as the value of the function.
In addition to the stack, values used during byte-code execution can be stored in ordinary Lisp variables. Variable values can be pushed onto the stack, and variables can be set by popping the stack.
Command: disassemble object &optional stream
This function prints the disassembled code for object. If
stream is supplied, then output goes there. Otherwise, the
disassembled code is printed to the stream standard-output. The
argument object can be a function name or a lambda expression.
As a special exception, if this function is used interactively, it outputs to a buffer named `*Disassemble*'.
Here are two examples of using the disassemble function. We
have added explanatory comments to help you relate the byte-code to the
Lisp source; these do not appear in the output of disassemble.
These examples show unoptimized byte-code. Nowadays byte-code is
usually optimized, but we did not want to rewrite these examples, since
they still serve their purpose.
(defun factorial (integer)
"Compute factorial of an integer."
(if (= 1 integer) 1
(* integer (factorial (1- integer)))))
=> factorial
(factorial 4)
=> 24
(disassemble 'factorial)
-| byte-code for factorial:
doc: Compute factorial of an integer.
args: (integer)
0 constant 1 ; Push 1 onto stack.
1 varref integer ; Get value of integer
; from the environment
; and push the value
; onto the stack.
2 eqlsign ; Pop top two values off stack,
; compare them,
; and push result onto stack.
3 goto-if-nil 10 ; Pop and test top of stack;
; if nil, go to 10,
; else continue.
6 constant 1 ; Push 1 onto top of stack.
7 goto 17 ; Go to 17 (in this case, 1 will be
; returned by the function).
10 constant * ; Push symbol * onto stack.
11 varref integer ; Push value of integer onto stack.
12 constant factorial ; Push factorial onto stack.
13 varref integer ; Push value of integer onto stack.
14 sub1 ; Pop integer, decrement value,
; push new value onto stack.
; Stack now contains:
; - decremented value of integer
; - factorial
; - value of integer
; - *
15 call 1 ; Call function factorial using
; the first (i.e., the top) element
; of the stack as the argument;
; push returned value onto stack.
; Stack now contains:
; - result of result of recursive
; call to factorial
; - value of integer
; - *
16 call 2 ; Using the first two
; (i.e., the top two)
; elements of the stack
; as arguments,
; call the function *,
; pushing the result onto the stack.
17 return ; Return the top element
; of the stack.
=> nil
The silly-loop function is somewhat more complex:
(defun silly-loop (n)
"Return time before and after N iterations of a loop."
(let ((t1 (current-time-string)))
(while (> (setq n (1- n))
0))
(list t1 (current-time-string))))
=> silly-loop
(disassemble 'silly-loop)
-| byte-code for silly-loop:
doc: Return time before and after N iterations of a loop.
args: (n)
0 constant current-time-string ; Push
; current-time-string
; onto top of stack.
1 call 0 ; Call current-time-string
; with no argument,
; pushing result onto stack.
2 varbind t1 ; Pop stack and bind t1
; to popped value.
3 varref n ; Get value of n from
; the environment and push
; the value onto the stack.
4 sub1 ; Subtract 1 from top of stack.
5 dup ; Duplicate the top of the stack;
; i.e. copy the top of
; the stack and push the
; copy onto the stack.
6 varset n ; Pop the top of the stack,
; and bind n to the value.
; In effect, the sequence dup varset
; copies the top of the stack
; into the value of n
; without popping it.
7 constant 0 ; Push 0 onto stack.
8 gtr ; Pop top two values off stack,
; test if n is greater than 0
; and push result onto stack.
9 goto-if-nil-else-pop 17 ; Goto 17 if n > 0
; else pop top of stack
; and continue
; (this exits the while loop).
12 constant nil ; Push nil onto stack
; (this is the body of the loop).
13 discard ; Discard result of the body
; of the loop (a while loop
; is always evaluated for
; its side effects).
14 goto 3 ; Jump back to beginning
; of while loop.
17 discard ; Discard result of while loop
; by popping top of stack.
18 varref t1 ; Push value of t1 onto stack.
19 constant current-time-string ; Push
; current-time-string
; onto top of stack.
20 call 0 ; Call current-time-string again.
21 list2 ; Pop top two elements off stack,
; create a list of them,
; and push list onto stack.
22 unbind 1 ; Unbind t1 in local environment.
23 return ; Return value of the top of stack.
=> nil
There are three ways to investigate a problem in an Emacs Lisp program, depending on what you are doing with the program when the problem appears.
Another useful debugging tool is a dribble file. When a dribble file is open, Emacs copies all keyboard input characters to that file. Afterward, you can examine the file to find out what input was used. See section Terminal Input.
For debugging problems in terminal descriptions, the
open-termscript function can be useful. See section Terminal Output.
The Lisp debugger provides you with the ability to suspend evaluation of a form. While evaluation is suspended (a state that is commonly known as a break), you may examine the run time stack, examine the values of local or global variables, or change those values. Since a break is a recursive edit, all the usual editing facilities of Emacs are available; you can even run programs that will enter the debugger recursively. See section Recursive Editing.
The most important time to enter the debugger is when a Lisp error happens. This allows you to investigate the immediate causes of the error.
However, entry to the debugger is not a normal consequence of an
error. Many commands frequently get Lisp errors when invoked in
inappropriate contexts (such as C-f at the end of the buffer) and
during ordinary editing it would be very unpleasant to enter the
debugger each time this happens. If you want errors to enter the
debugger, set the variable debug-on-error to non-nil.
This variable determines whether the debugger is called when a error is
signaled and not handled. If debug-on-error is t, all
errors call the debugger. If it is nil, none call the debugger.
The value can also be a list of error conditions that should call the
debugger. For example, if you set it to the list
(void-variable), then only errors about a variable that has no
value invoke the debugger.
To debug an error that happens during loading of the `.emacs'
file, use the option `-debug-init', which binds
debug-on-error to t while `.emacs' is loaded.
If your `.emacs' file sets debug-on-error, the effect
lasts only until the end of loading `.emacs'. (This is an
undesirable by-product of the `-debug-init' feature.) If you want
`.emacs' to set debug-on-error permanently, use
after-init-hook, like this:
(add-hook 'after-init-hook
'(lambda () (setq debug-on-error t)))
When a program loops infinitely and fails to return, your first problem is to stop the loop. On most operating systems, you can do this with C-g, which causes quit.
Ordinary quitting gives no information about why the program was
looping. To get more information, you can set the variable
debug-on-quit to non-nil. Quitting with C-g is not
considered an error, and debug-on-error has no effect on the
handling of C-g. Contrariwise, debug-on-quit has no effect
on errors.
Once you have the debugger running in the middle of the infinite loop, you can proceed from the debugger using the stepping commands. If you step through the entire loop, you will probably get enough information to solve the problem.
This variable determines whether the debugger is called when quit
is signaled and not handled. If debug-on-quit is non-nil,
then the debugger is called whenever you quit (that is, type C-g).
If debug-on-quit is nil, then the debugger is not called
when you quit. See section Quitting.
To investigate a problem that happens in the middle of a program, one useful technique is to cause the debugger to be entered when a certain function is called. You can do this to the function in which the problem occurs, and then step through the function, or you can do this to a function called shortly before the problem, step quickly over the call to that function, and then step through its caller.
Command: debug-on-entry function-name
This function requests function-name to invoke the debugger each time
it is called. It works by inserting the form (debug 'debug) into
the function definition as the first form.
Any function defined as Lisp code may be set to break on entry, regardless of whether it is interpreted code or compiled code. Even functions that are commands may be debugged--they will enter the debugger when called inside a function, or when called interactively (after the reading of the arguments). Primitive functions (i.e., those written in C) may not be debugged.
When debug-on-entry is called interactively, it prompts
for function-name in the minibuffer.
Caveat: if debug-on-entry is called more than once on the same
function, the second call does nothing. If you redefine a function
after using debug-on-entry on it, the code to enter the debugger
is lost.
debug-on-entry returns function-name.
(defun fact (n)
(if (zerop n) 1
(* n (fact (1- n)))))
=> fact
(debug-on-entry 'fact)
=> fact
(fact 3)
=> 6
------ Buffer: *Backtrace* ------
Entering:
* fact(3)
eval-region(4870 4878 t)
byte-code("...")
eval-last-sexp(nil)
(let ...)
eval-insert-last-sexp(nil)
* call-interactively(eval-insert-last-sexp)
------ Buffer: *Backtrace* ------
(symbol-function 'fact)
=> (lambda (n)
(debug (quote debug))
(if (zerop n) 1 (* n (fact (1- n)))))
Command: cancel-debug-on-entry function-name
This function undoes the effect of debug-on-entry on
function-name. When called interactively, it prompts for
function-name in the minibuffer.
If cancel-debug-on-entry is called more than once on the same
function, the second call does nothing. cancel-debug-on-entry
returns function-name.
You can cause the debugger to be called at a certain point in your
program by writing the expression (debug) at that point. To do
this, visit the source file, insert the text `(debug)' at the
proper place, and type C-M-x. Be sure to undo this insertion
before you save the file!
The place where you insert `(debug)' must be a place where an
additional form can be evaluated and its value ignored. (If the value
isn't ignored, it will alter the execution of the program!) Usually
this means inside a progn or an implicit progn
(see section Sequencing).
When the debugger is entered, it displays the previously selected buffer in one window and a buffer named `*Backtrace*' in another window. The backtrace buffer contains one line for each level of Lisp function execution currently going on. At the beginning of this buffer is a message describing the reason that the debugger was invoked (such as the error message and associated data, if it was invoked due to an error).
The backtrace buffer is read-only and uses a special major mode, Debugger mode, in which letters are defined as debugger commands. The usual Emacs editing commands are available; thus, you can switch windows to examine the buffer that was being edited at the time of the error, switch buffers, visit files, or do any other sort of editing. However, the debugger is a recursive editing level (see section Recursive Editing) and it is wise to go back to the backtrace buffer and exit the debugger (with the q command) when you are finished with it. Exiting the debugger gets out of the recursive edit and kills the backtrace buffer.
The contents of the backtrace buffer show you the functions that are executing and the arguments that were given to them. It also allows you to specify a stack frame by moving point to the line describing that frame. (A stack frame is the place where the Lisp interpreter records information about a particular invocation of a function. The frame whose line point is on is considered the current frame.) Some of the debugger commands operate on the current frame.
The debugger itself should always be run byte-compiled, since it makes assumptions about how many stack frames are used for the debugger itself. These assumptions are false if the debugger is running interpreted.
Inside the debugger (in Debugger mode), these special commands are available in addition to the usual cursor motion commands. (Keep in mind that all the usual facilities of Emacs, such as switching windows or buffers, are still available.)
The most important use of debugger commands is for stepping through code, so that you can see how control flows. The debugger can step through the control structures of an interpreted function, but cannot do so in a byte-compiled function. If you would like to step through a byte-compiled function, replace it with an interpreted definition of the same function. (To do this, visit the source file for the function and type C-M-x on its definition.)
Continuing is possible after entry to the debugger due to function entry or exit, explicit invocation, quitting or certain errors. Most errors cannot be continued; trying to continue an unsuitable error causes the same error to occur again.
The stack frame made for the function call which enters the debugger in this way will be flagged automatically so that the debugger will be called again when the frame is exited. You can use the u command to cancel this flag.
If the debugger was entered due to a C-g but you really want to quit, and not debug, use the q command.
The r command makes a difference when the debugger was invoked due to exit from a Lisp call frame (as requested with b); then the value specified in the r command is used as the value of that frame.
You can't use r when the debugger was entered due to an error.
Here we describe fully the function used to invoke the debugger.
Function: debug &rest debugger-args
This function enters the debugger. It switches buffers to a buffer named `*Backtrace*' (or `*Backtrace*<2>' if it is the second recursive entry to the debugger, etc.), and fills it with information about the stack of Lisp function calls. It then enters a recursive edit, leaving that buffer in Debugger mode and displayed in the selected window.
Debugger mode provides a c command which operates by exiting the
recursive edit, switching back to the previous buffer, and returning to
whatever called debug. The r command also returns from
debug. These are the only ways the function debug can
return to its caller.
If the first of the debugger-args passed to debug is
nil (or if it is not one of the following special values), then
the rest of the arguments to debug are printed at the top of the
`*Backtrace*' buffer. This mechanism is used to display a message
to the user.
However, if the first argument passed to debug is one of the
following special values, then it has special significance. Normally,
these values are passed to debug only by the internals of Emacs
and the debugger, and not by programmers calling debug.
The special values are:
lambda
lambda, the debugger displays
`Entering:' as a line of text at the top of the buffer. This means
that a function is being entered when debug-on-next-call is
non-nil.
debug
debug, the debugger displays
`Entering:' just as in the lambda case. However,
debug as the argument indicates that the reason for entering the
debugger is that a function set to debug on entry is being entered.
In addition, debug as the first argument directs the debugger
to mark the function that called debug so that it will invoke the
debugger when exited. (When lambda is the first argument, the
debugger does not do this, because it has already been done by the
interpreter.)
t
t, the debugger displays the following
as the top line in the buffer:
Beginning evaluation of function call form:
This indicates that it was entered due to the evaluation of a list form
at a time when debug-on-next-call is non-nil.
exit
exit, it indicates the exit of a
stack frame previously marked to invoke the debugger on exit. The
second argument given to debug in this case is the value being
returned from the frame. The debugger displays `Return value:' on
the top line of the buffer, followed by the value being returned.
error
error, the debugger indicates that
it is being entered because an error or quit was signaled and not
handled, by displaying `Signaling:' followed by the error signaled
and any arguments to signal. For example,
(let ((debug-on-error t))
(/ 1 0))
------ Buffer: *Backtrace* ------
Signaling: (arith-error)
/(1 0)
...
------ Buffer: *Backtrace* ------
If an error was signaled, presumably the variable
debug-on-error is non-nil. If quit was signaled,
then presumably the variable debug-on-quit is non-nil.
nil
nil as the first of the debugger-args when you want
to enter the debugger explicitly. The rest of the debugger-args
are printed on the top line of the buffer. You can use this feature to
display messages--for example, to remind yourself of the conditions
under which debug is called.
This section describes functions and variables used internally by the debugger.
The value of this variable is the function to call to invoke the
debugger. Its value must be a function of any number of arguments (or,
more typically, the name of a function). Presumably this function will
enter some kind of debugger. The default value of the variable is
debug.
The first argument that Lisp hands to the function indicates why it
was called. The convention for arguments is detailed in the description
of debug.
This function prints a trace of Lisp function calls currently active.
This is the function used by debug to fill up the
`*Backtrace*' buffer. It is written in C, since it must have access
to the stack to determine which function calls are active. The return
value is always nil.
In the following example, backtrace is called explicitly in a
Lisp expression. When the expression is evaluated, the backtrace is
printed to the stream standard-output: in this case, to the
buffer `backtrace-output'. Each line of the backtrace represents
one function call. If the arguments of the function call are all known,
they are displayed; if they are being computed, that fact is stated.
The arguments of special forms are elided.
(with-output-to-temp-buffer "backtrace-output"
(let ((var 1))
(save-excursion
(setq var (eval '(progn
(1+ var)
(list 'testing (backtrace))))))))
=> nil
----------- Buffer: backtrace-output ------------
backtrace()
(list ...computing arguments...)
(progn ...)
eval((progn (1+ var) (list (quote testing) (backtrace))))
(setq ...)
(save-excursion ...)
(let ...)
(with-output-to-temp-buffer ...)
eval-region(1973 2142 #<buffer *scratch*>)
byte-code("... for eval-print-last-sexp ...")
eval-print-last-sexp(nil)
* call-interactively(eval-print-last-sexp)
----------- Buffer: backtrace-output ------------
The character `*' indicates a frame whose debug-on-exit flag is set.
This variable determines whether the debugger is called before the
next eval, apply or funcall. It is automatically
reset to nil when the debugger is entered.
The d command in the debugger works by setting this variable.
Function: backtrace-debug level flag
This function sets the debug-on-exit flag of the eval frame
level levels down to flag. If flag is non-nil,
this will cause the debugger to be entered when that frame exits.
Even a nonlocal exit through that frame will enter the debugger.
The debug-on-exit flag is an entry in the stack frame of a function call. This flag is examined on every exit from a function.
Normally, this function is only called by the debugger.
Variable: command-debug-status
This variable records the debugging status of current interactive
command. Each time a command is called interactively, this variable is
bound to nil. The debugger can set this variable to leave
information for future debugger invocations during the same command.
The advantage of using this variable rather that defining another global variable is that the data will never carry over to a later other command invocation.
Function: backtrace-frame frame-number
The function backtrace-frame is intended for use in Lisp
debuggers. It returns information about what computation is happening
in the eval frame level levels down.
If that frame has not evaluated the arguments yet (or is a special
form), the value is (nil function arg-forms...).
If that frame has evaluated its arguments and called its function
already, the value is (t function
arg-values...).
In the return value, function is whatever was supplied as CAR
of evaluated list, or a lambda expression in the case of a macro
call. If the function has a &rest argument, that is represented
as the tail of the list arg-values.
If the argument is out of range, backtrace-frame returns
nil.
The Lisp reader reports invalid syntax, but cannot say where the real problem is. For example, the error "End of file during parsing" in evaluating an expression indicates an excess of open parentheses (or square brackets). The reader detects this imbalance at the end of the file, but it cannot figure out where the close parenthesis should have been. Likewise, "Invalid read syntax: ")"" indicates an excess close parenthesis or missing open parenthesis, but not where the missing parenthesis belongs. How, then, to find what to change?
If the problem is not simply an imbalance of parentheses, a useful technique is to try C-M-e at the beginning of each defun, and see if it goes to the place where that defun appears to end. If it does not, there is a problem in that defun.
However, unmatched parentheses are the most common syntax errors in Lisp, and we can give further advice for those cases.
The first step is to find the defun that is unbalanced. If there is
an excess open parenthesis, the way to do this is to insert a
close parenthesis at the end of the file and type C-M-b
(backward-sexp). This will move you to the beginning of the
defun that is unbalanced. (Then type C-SPC C-_ C-u
C-SPC to set the mark there, undo the insertion of the
close parenthesis, and finally return to the mark.)
The next step is to determine precisely what is wrong. There is no way to be sure of this except to study the program, but often the existing indentation is a clue to where the parentheses should have been. The easiest way to use this clue is to reindent with C-M-q and see what moves.
Before you do this, make sure the defun has enough close parentheses. Otherwise, C-M-q will get an error, or will reindent all the rest of the file until the end. So move to the end of the defun and insert a close parenthesis there. Don't use C-M-e to move there, since that too will fail to work until the defun is balanced.
Then go to the beginning of the defun and type C-M-q. Usually all the lines from a certain point to the end of the function will shift to the right. There is probably a missing close parenthesis, or a superfluous open parenthesis, near that point. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q, since the old indentation is probably appropriate to the intended parentheses.
After you think you have fixed the problem, use C-M-q again. It should not change anything, if the problem is really fixed.
To deal with an excess close parenthesis, first insert an open parenthesis at the beginning of the file and type C-M-f to find the end of the unbalanced defun. (Then type C-SPC C-_ C-u C-SPC to set the mark there, undo the insertion of the open parenthesis, and finally return to the mark.)
Then find the actual matching close parenthesis by typing C-M-f at the beginning of the defun. This will leave you somewhere short of the place where the defun ought to end. It is possible that you will find a spurious close parenthesis in that vicinity.
If you don't see a problem at that point, the next thing to do is to type C-M-q at the beginning of the defun. A range of lines will probably shift left; if so, the missing open parenthesis or spurious close parenthesis is probably near the first of those lines. (However, don't assume this is true; study the code to make sure.) Once you have found the discrepancy, undo the C-M-q, since the old indentation is probably appropriate to the intended parentheses.
When an error happens during byte compilation, it is normally due to invalid syntax in the program you are compiling. The compiler prints a suitable error message in the `*Compile-Log*' buffer, and then stops. The message may state a function name in which the error was found, or it may not. Regardless, here is how to find out where in the file the error occurred.
What you should do is switch to the buffer ` *Compiler Input*'. (Note that the buffer name starts with a space, so it does not show up in M-x list-buffers.) This buffer contains the program being compiled, and point shows how far the byte compiler was able to read.
If the error was due to invalid Lisp syntax, point shows exactly where the invalid syntax was detected. The cause of the error is not necessarily near by! Use the techniques in the previous section to find the error.
If the error was detected while compiling a form that had been read successfully, then point is located at the end of the form. In this case, it can't localize the error precisely, but can still show you which function to check.
Edebug is a source-level debugger for Emacs Lisp programs that provides the following features:
The first three sections of this chapter should tell you enough about Edebug to enable you to use it.
To debug a Lisp program with Edebug, you must first prepare the Lisp functions that you want to debug. See section Preparing Functions for Edebug.
Once a function is prepared, any call to the function activates Edebug. This involves entering a recursive edit which is a level of Edebug activation.
Activating Edebug may stop execution and let you step through the function, or it may continue execution while checking for debugging commands, depending on the selected Edebug execution mode. See section Edebug Modes.
Within Edebug, you normally view an Emacs buffer showing the source of the Lisp function you are debugging. We call this the Edebug buffer---but note that it is not always the same buffer, and it is not reserved for Edebug use.
An arrow at the left margin indicates the line where the function is executing. Point initially shows where within the line the function is executing, but this ceases to be true if you move point yourself.
If you prepare the definition of fac (shown below) for Edebug and
then execute (fac 3), here is what you normally see. Point is at
the open-parenthesis before if.
(defun fac (n)
=>-!-(if (< 0 n)
(* n (fac (1- n)))
1))
The places within a function where Edebug can stop execution are called
stop points. These occur both before and after each subexpression
that is a list, and also after each variable reference. Stop points
before variables are optional, under the control of the value of
edebug-stop-before-symbols. Here we show with periods the stop
points normally found in the function fac:
(defun fac (n)
.(if .(< 0 n.).
.(* n. .(fac (1- n.).).).
1).)
While a buffer is the Edebug buffer, the special commands of Edebug are available in it, instead of many usual editing commands. Type ? to display a list of Edebug commands. In particular, you can exit the innermost Edebug activation level with C-], and you can return all the way to top level with q.
For example, you can type the Edebug command SPC to execute until
the next stop point. If you type SPC once after entry to
fac, here is the state that you get:
(defun fac (n)
=>(if -!-(< 0 n)
(* n (fac (1- n)))
1))
When Edebug stops execution after an expression, it displays the expression's value in the echo area. Use the r command to display the value again later.
While Edebug is active, it catches all errors (if debug-on-error is
non-nil) and quits (if debug-on-quit is non-nil)
instead of the standard debugger. When this happens, Edebug displays the
last stop point that it knows about. This may be the location of a call
to a function which was not prepared for Edebug debugging, within which
the error actually occurred.
In order to use Edebug to debug a function, you must first prepare the function. Preparing a function inserts additional code into it which invokes Edebug at the proper places.
Any call to an Edebug-prepared function activates Edebug. This may or
may not stop execution, depending on the Edebug execution mode in use.
Some Edebug modes only update the display to indicate the progress of
the evaluation without stopping execution. The default initial Edebug
mode is step which does stop execution. See section Edebug Modes.
Once you have loaded Edebug, the command C-M-x is redefined so
that when used on a function or macro definition, it prepares the
function or macro if given a prefix argument. If the variable
edebug-all-defuns is non-nil, that inverts the meaning of
the prefix argument: then C-M-x prepares the function or macro
unless it has a prefix argument. The default value of
edebug-all-defuns is nil. The command M-x
edebug-all-defuns toggles the value of the variable
edebug-all-defuns.
If edebug-all-defuns is non-nil, then the commands
eval-region and eval-current-buffer also prepare any
functions and macros whose definitions they evaluate.
Loading a file does not prepare functions and macros for Edebug.
See section Evaluation for discussion of other evaluation functions available inside of Edebug.
Edebug supports several execution modes for running the program you are debugging. We call these alternatives Edebug modes; do not confuse them with major modes or minor modes. The current Edebug mode determines how Edebug displays the progress of the evaluation, whether it stops at each stop point, or continues to the next breakpoint, for example.
Normally, you specify the Edebug mode for execution by typing a command to continue the program in a certain mode. Here is a table of these commands. All except for S resume execution of the program, at least for a certain distance.
In general, the execution modes earlier in the above list run the program more slowly or stop sooner.
When you enter a new Edebug level, the mode comes from the value of the
variable edebug-initial-mode. By default, this specifies
step mode. If the mode thus specified is not stop mode, then the
Edebug level executes the program (or part of it).
While executing or tracing, you can interrupt the execution by typing any Edebug command. Edebug stops the program at the next stop point and then executes the command that you typed. For example, typing t during execution switches to trace mode at the next stop point.
You can use the S command to stop execution without doing anything else.
If your function happens to read input, a character you hit intending to interrupt execution may be read by the function instead. You can avoid such unintended results by paying attention to when your program wants input.
Keyboard macros containing the commands in this section do not completely work: exiting from Edebug, to resume the program, loses track of the keyboard macro. This is not easy to fix.
With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression.
Be careful that the position C-M-f finds is a place that the
program will really get to; this may not be true in a
condition-case, for example.
This command does forward-sexp starting at point rather than the
stop point, thus providing more flexibility. If you want to execute one
expression from the current stop point, type w first, to move
point there.
This command does not exit the currently executing function unless you are positioned after the last sexp of the function.
If the program does a non-local exit, it may fail to reach the temporary breakpoint that this command sets.
One undesirable side effect of using edebug-step-in is that the
next time the stepped-into function is called, Edebug will be called
there as well.
The f command runs the program forward over one expression. More precisely, set a temporary breakpoint at the position that C-M-f would reach, then execute in go mode so that the program will stop at breakpoints. See section Breakpoints for the details on breakpoints.
With a prefix argument n, the temporary breakpoint is placed n sexps beyond point. If the containing list ends before n more elements, then the place to stop is after the containing expression.
Be careful that the position C-M-f finds is a place that the
program will really get to; this may not be true in a
condition-case, for example.
The f command uses the existing value of point as the basis for setting the breakpoint, because that is more flexible. To execute one expression from the current stop point, type w and then f.
The o command continues "out of" an expression. It places a temporary breakpoints at the end of the containing sexp. If the containing sexp is the top level defun, it continues until just before the function returns. If that is where you are now, it returns from the function and then stops.
This command does not exit the currently executing function unless you are positioned after the last sexp of the function.
The i command steps into the function about to be called. Use this command before any of the arguments of the function call are evaluated, since otherwise it is too late.
One undesirable side effect of using i is that the next time the stepped-into function is called, Edebug will be called there as well.
The h command proceeds to the stop point near where point is, using a temporary breakpoint.
All the commands in this section may fail to work as expected in case of nonlocal exit, because a nonlocal exit can bypass the temporary breakpoint where you expected the program to stop.
Some miscellaneous commands are described here.
You cannot use debugger commands in the backtrace buffer in Edebug as you would in the standard debugger.
The backtrace buffer is killed automatically when you continue execution.
While using Edebug, you can specify breakpoints in the program you are testing: points where execution should stop. You can set a breakpoint at any stop point, as defined in section Using Edebug---even before a symbol. For setting and unsetting breakpoints, the stop point that is affected is the first one at or after point in the Edebug buffer. Here are the Edebug commands for breakpoints:
nil value. If you use a prefix argument, the
breakpoint is temporary (it turns off the first time it stops the
program).
While in Edebug, you can set a breakpoint with b
(edebug-set-breakpoint) and unset one with u
(edebug-unset-breakpoint). First you must move point to a
position at or before the desired Edebug stop point, then hit the key to
change the breakpoint. Unsetting a breakpoint that has not been set
does nothing.
Reevaluating the function with edebug-defun clears all
breakpoints in the function.
A conditional breakpoint tests a condition each time the program gets there, to decide whether to stop. To set a conditional breakpoint, use x, and specify the condition expression in the minibuffer.
You can make both conditional and unconditional breakpoints temporary by using a prefix arg to the command to set the breakpoint. After breaking at a temporary breakpoint, it is automatically cleared.
Edebug always stops or pauses at a breakpoint except when the Edebug mode is Go-nonstop. In that mode, it ignores breakpoints entirely.
To find out where your breakpoints are, use the B
(edebug-next-breakpoint) command, which moves point to the next
breakpoint in the function following point, or to the first breakpoint
if there are no following breakpoints. This command does not continue
execution--it just moves point in the buffer.
These Edebug commands let you view aspects of the buffer and window status that obtained before entry to Edebug.
edebug-save-windows
variable.
While within Edebug, you can evaluate expressions "as if" Edebug were not running. Edebug tries to be invisible to the expression's evaluation.
You can use the evaluation list buffer, called `*edebug*', to evaluate expressions interactively. You can also set up the evaluation list of expressions to be evaluated automatically each time Edebug is reentered.
In the `*edebug*' buffer you can use the commands of Lisp Interaction as well as these special commands:
You can evaluate expressions in the evaluation list window with LFD or C-x C-e, just as you would in `*scratch*'; but they are evaluated in the context outside of Edebug.
The expressions you enter interactively (and their results) are lost
when you continue execution of your function unless you add them to the
evaluation list with C-c C-u (edebug-update-eval-list).
This command builds a new list from the first expression of each
evaluation list group. Groups are separated by a line starting
with a comment.
When the evaluation list is redisplayed, each expression is displayed followed by the result of evaluating it, and a comment line. If an error occurs during an evaluation, the error message is displayed in a string as if it were the result. Therefore expressions that use variables not currently valid do not interrupt your debugging.
Here is an example of what the evaluation list window looks like after several expressions have been added to it:
(current-buffer) #<buffer *scratch*> ;--------------------------------------------------------------- (point-min) 1 ;--------------------------------------------------------------- (point-max) 2 ;--------------------------------------------------------------- edebug-outside-point-max "Symbol's value as variable is void: edebug-outside-point-max" ;--------------------------------------------------------------- (recursion-depth) 0 ;--------------------------------------------------------------- this-command eval-last-sexp ;---------------------------------------------------------------
To delete a group, move point into it and type C-c C-d
(edebug-delete-eval-item), or simply delete the text for it and
update the evaluation list with C-c C-u. When you add a new
group, be sure to add a comment at the beginning.
After selecting `*edebug*', you can return to the source code
buffer (the Edebug buffer) with C-c C-w. The *edebug*
buffer is killed when you continue execution of your function, and
recreated next time it is needed.
If the results of your expressions contain circular references to other parts of the same structure, you can print them more usefully with the `custom-print'.
To load the package and activate custom printing only for Edebug, simply
use the command edebug-install-custom-print-funcs. Then set the
variable print-circle to enable special handling of circular
structure. To restore the standard print functions, use
edebug-reset-print-funcs.
Edebug tries to be transparent to the program you are debugging, but it does not succeed completely. In addition, most evaluations you do within Edebug (see section Evaluation) occur in the same outside context which is temporarily restored for the evaluation. This section explains precisely how use Edebug fails to be completely transparent.
Whenever Edebug is entered just to think about whether to take some action, it needs to save and restore certain data.
max-lisp-eval-depth and max-specpdl-size are both
incremented for each edebug-enter call so that your code should
not be impacted by Edebug frames on the stack.
When Edebug needs to display something (e.g., in trace mode), it saves the current window configuration from "outside" Edebug (see section Window Configurations). When you exit Edebug (by continuing the program), it restores the previous window configuration.
Emacs redisplays only when it pauses. Usually, when you continue Edebug, the program comes back into Edebug at a breakpoint or after stepping, without pausing or reading input in between. In such cases, Emacs never gets a chance to redisplay the "outside" configuration. What you see is the window configuration for within Edebug, with no interruption.
The window configuration proper does not include which buffer is current or where point and mark are in the current buffer, but Edebug saves and restores these also.
Entry to Edebug for displaying something also saves and restores the following data. (Some of these variables are deliberately not restored if an error or quit signal occurs.)
edebug-save-windows is non-nil.
edebug-save-displayed-buffer-points is non-nil.
overlay-arrow-position and
overlay-arrow-string are saved and restored. This permits
recursive use of Edebug, and use of Edebug while using GUD.
cursor-in-echo-area is locally bound to nil so that
the cursor shows up in the window.
When Edebug is entered and actually reads commands from the user, it saves (and later restores) these additional data:
last-command, this-command, last-command-char, and
last-input-char. Commands used within Edebug do not affect these
variables outside of Edebug.
But note that it is not possible to preserve the status reported by
(this-command-keys) and the variable unread-command-char.
standard-output and standard-input.
Edebug operation unavoidably alters some data in Emacs, and this can interfere with debugging certain programs.
max-lisp-eval-depth and max-specpdl-size, are also
increased proportionally.
this-command-keys is changed by
executing commands within Edebug and there appears to be no way to reset
the key sequence from Lisp.
unread-command-char
or unread-command-events. Entering Edebug while these variables
have nontrivial values can interfere with execution of the program you
are debugging.
command-history. In rare cases this can alter execution.
When Edebug prepares for stepping through an expression that uses a Lisp
macro, it needs additional advice to do the job properly. This is
because there is no way to tell which parts of the macro call are forms
to be evaluated. You must explain the format of calls to each macro to
enable Edebug to handle it. To do this, use def-edebug-form-spec
to define the format of calls to a given macro.
Macro: def-edebug-form-spec macro argpattern
Specify which parts of a call to macro macro are subexpressions to be evaluated. The second argument, argpattern, details what the argument list looks like.
Here is a table of the possibilities for argpattern and its subexpressions:
t
0
sexp
form
symbolp
integerp
stringp
vectorp
atom
function
function
nil.
'object
(patterns)
[patterns]
&optional
[&optional pattern].
&rest
&rest may appear at the same level of a
specification list, and &rest must not be followed by
&optional.
To specify repetition of certain types of arguments, followed by
dissimilar arguments, use [&rest patterns...].
&or
[...]. Only one &or may appear in a list, and it may
not be followed by &optional or &rest. One of the
alternatives must match, unless the &or is preceded by
&optional or &rest.
If the actual arguments of a macro call fail to match the specification, taking account of alternatives, optional arguments and repeated arguments, Edebug reports a syntax error in use of the macro.
The combination of backtracking, &optional, &rest,
&or, and [...] for grouping provides the equivalent of
regular expressions. The (...) lists require balanced
parentheses, which is the only context free (finite state with stack)
construct supported.
Here are some examples of using def-edebug-form-spec. First, for
the let special form:
(def-edebug-form-spec let
'((&rest
&or symbolp (symbolp &optional form))
&rest form))
Here's the spec for the for loop macro (see section Common Problems Using Macros) and for the case and do macros in `cl.el':
(def-edebug-form-spec for
'(symbolp 'from form 'to form 'do &rest form))
(def-edebug-form-spec case
'(form &rest (sexp form)))
(def-edebug-form-spec do
'((&rest &or symbolp (symbolp &optional form form))
(form &rest form)
&rest body))
Finally, the functions mapcar, mapconcat, mapatoms,
apply, and funcall all take function arguments, and Edebug
defines specifications for them. Here's one example:
(def-edebug-form-spec apply '(function &rest form))
The backquote (`) macro results in an expression that is not necessarily evaluated. Edebug cannot step through code generated by use of backquote.
These options affect the behavior of Edebug:
User Option: edebug-all-defuns
If non-nil, normal evaluation of defun and defmacro
forms prepares the functions and macros for stepping with Edebug. This
applies to eval-defun, eval-region and
eval-current-buffer.
The default value is nil.
User Option: edebug-stop-before-symbols
If non-nil, Edebug places stop points before symbols as well as
after.
This option takes effect for a function when you prepare it for stepping with Edebug. Changing the option's value during execution of Edebug has no effect on the functions already set up for Edebug execution.
User Option: edebug-save-windows
If non-nil, save and restore window configuration on Edebug calls.
It takes some time to save and restore, so if your program does not care
what happens to the window configurations, it is better to set this
variable to nil.
The default value is t.
User Option: edebug-save-point
If non-nil, Edebug saves and restores point and the mark in
source code buffers. The default value is t.
User Option: edebug-save-displayed-buffer-points
If non-nil, save and restore point in all buffers when entering
Edebug mode.
Saving and restoring point in other buffers is necessary if you are debugging code that changes the point of a buffer which is displayed in a non-selected window. If Edebug or the user then selects the window, the buffer's point will be changed to the window's point.
Saving and restoring is an expensive operation since it visits each window and each displayed buffer twice for each Edebug call, so it is best to avoid it if you can.
The default value is nil.
User Option: edebug-initial-mode
If this variable is non-nil, it specifies an Edebug mode to start
in each time the program enters a new Edebug recursive-edit level.
Possible values are step, go, Go-nonstop,
trace, Trace-fast, continue, and
Continue-fast.
The default value is step.
Non-nil means display a trace of function entry and exit.
Tracing output is displayed in a buffer named `*edebug-trace*', one
function entry or exit per line, indented by the recursion level. You
can customize this display by replacing the functions
edebug-print-trace-entry and edebug-print-trace-exit.
The default value is nil.
Printing and reading are the operations of converting Lisp objects to textual form and vice versa. They use the printed representations and read syntax described in section Lisp Data Types.
This chapter describes the Lisp functions for reading and printing. It also describes streams, which specify where to get the text (if reading) or where to put it (if printing).
Reading a Lisp object means parsing a Lisp expression in textual
form and producing a corresponding Lisp object. This is how Lisp
programs get into Lisp from files of Lisp code. We call the text the
read syntax of the object. For example, reading the text `(a
. 5)' returns a cons cell whose CAR is a and whose
CDR is the number 5.
Printing a Lisp object means producing text that represents that object--converting the object to its printed representation. Printing the cons cell described above produces the text `(a . 5)'.
Reading and printing are more or less inverse operations: printing the
object that results from reading a given piece of text often produces
the same text, and reading the text that results from printing an object
usually produces a similar-looking object. For example, printing the
symbol foo produces the text `foo', and reading that text
returns the symbol foo. Printing a list whose elements are
a and b produces the text `(a b)', and reading that
text produces a list (but not the same list) with elements are a
and b.
However, these two operations are not precisely inverses. There are two kinds of exceptions:
Most of the Lisp functions for reading text take an input stream as an argument. The input stream specifies where or how to get the characters of the text to be read. Here are the possible types of input stream:
Occasionally function is called with one argument (always a character). When that happens, function should save the argument and arrange to return it on the next call. This is called unreading the character; it happens when the Lisp reader reads one character too many and want to "put it back where it came from".
t
t used as a stream means that the input is read from the
minibuffer. In fact, the minibuffer is invoked once and the text
given by the user is made into a string that is then used as the
input stream.
nil
nil used as a stream means that the value of
standard-input should be used instead; that value is the
default input stream, and must be a non-nil input stream.
Here is an example of reading from a stream which is a buffer, showing where point is located before and after:
---------- Buffer: foo ----------
This-!- is the contents of foo.
---------- Buffer: foo ----------
(read (get-buffer "foo"))
=> is
(read (get-buffer "foo"))
=> the
---------- Buffer: foo ----------
This is the-!- contents of foo.
---------- Buffer: foo ----------
Note that the first read skips a space at the beginning of the buffer. Reading skips any amount of whitespace preceding the significant text.
In Emacs 18, reading a symbol discarded the delimiter terminating the symbol. Thus, point would end up at the beginning of `contents' rather than after `the'. The Emacs 19 behavior is superior because it correctly handles input such as `bar(foo)' where the delimiter that ends one object is needed as the beginning of another object.
Here is an example of reading from a stream that is a marker,
initialized to point at the beginning of the buffer shown. The value
read is the symbol This.
---------- Buffer: foo ----------
This is the contents of foo.
---------- Buffer: foo ----------
(setq m (set-marker (make-marker) 1 (get-buffer "foo")))
=> #<marker at 1 in foo>
(read m)
=> This
m
=> #<marker at 6 in foo> ;; After the first space.
Here we read from the contents of a string:
(read "(When in) the course")
=> (When in)
The following example reads from the minibuffer. The
prompt is: `Lisp expression: '. (That is always the prompt
used when you read from the stream t.) The user's input is shown
following the prompt.
(read t)
=> 23
---------- Buffer: Minibuffer ----------
Lisp expression: 23 RET
---------- Buffer: Minibuffer ----------
Finally, here is an example of a stream that is a function, named
useless-stream. Before we use the stream, we initialize the
variable useless-list to a list of characters. Then each call to
the function useless-stream obtains the next characters in the list
or unreads a character by adding it to the front of the list.
(setq useless-list (append "XY()" nil))
=> (88 89 40 41)
(defun useless-stream (&optional unread)
(if unread
(setq useless-list (cons unread useless-list))
(prog1 (car useless-list)
(setq useless-list (cdr useless-list)))))
=> useless-stream
Now we read using the stream thus constructed:
(read 'useless-stream)
=> XY
useless-list
=> (41)
Note that the close parenthesis remains in the list. The reader has read it, discovered that it ended the input, and unread it. Another attempt to read from the stream at this point would get an error due to the unmatched close parenthesis.
This function is used internally as an input stream to read from the
input file opened by the function load. Don't use this function
yourself.
This section describes the Lisp functions and variables that pertain to reading.
In the functions below, stream stands for an input stream (see
the previous section). If stream is nil or omitted, it
defaults to the value of standard-input.
An end-of-file error results if an unterminated list or
vector is found.
Function: read &optional stream
This function reads one textual Lisp expression from stream, returning it as a Lisp object. This is the basic Lisp input function.
Function: read-from-string string &optional start end
This function reads the first textual Lisp expression from the text in string. It returns a cons cell whose CAR is that expression, and whose CDR is an integer giving the position of the next remaining character in the string (i.e., the first one not read).
If start is supplied, then reading begins at index start in the string (where the first character is at index 0). If end is also supplied, then reading stops at that index as if the rest of the string were not there.
For example:
(read-from-string "(setq x 55) (setq y 5)")
=> ((setq x 55) . 11)
(read-from-string "\"A short string\"")
=> ("A short string" . 16)
;; Read starting at the first character.
(read-from-string "(list 112)" 0)
=> ((list 112) . 10)
;; Read starting at the second character.
(read-from-string "(list 112)" 1)
=> (list . 6)
;; Read starting at the seventh character,
;; and stopping at the ninth.
(read-from-string "(list 112)" 6 8)
=> (11 . 8)
This variable holds the default input stream: the stream that
read uses when the stream argument is nil.
An output stream specifies what to do with the characters produced by printing. Most print functions accept an output stream as an optional argument. Here are the possible types of output stream:
t
nil
nil specified as an output stream means that the value of
standard-output should be used as the output stream; that value
is the default output stream, and must be a non-nil output
stream.
Here is an example of a buffer used as an output stream. Point is initially located as shown immediately before the `h' in `the'. At the end, point is located directly before that same `h'.
---------- Buffer: foo ----------
This is t-!-he contents of foo.
---------- Buffer: foo ----------
(print "This is the output" (get-buffer "foo"))
=> "This is the output"
---------- Buffer: foo ----------
This is t
"This is the output"
-!-he contents of foo.
---------- Buffer: foo ----------
Now we show a use of a marker as an output stream. Initially, the
marker points in buffer foo, between the `t' and the
`h' in the word `the'. At the end, the marker has been
advanced over the inserted text so that it still points before the same
`h'. Note that the location of point, shown in the usual fashion,
has no effect.
---------- Buffer: foo ----------
"This is the -!-output"
---------- Buffer: foo ----------
m
=> #<marker at 11 in foo>
(print "More output for foo." m)
=> "More output for foo."
---------- Buffer: foo ----------
"This is t
"More output for foo."
he -!-output"
---------- Buffer: foo ----------
m
=> #<marker at 35 in foo>
The following example shows output to the echo area:
(print "Echo Area output" t)
=> "Echo Area output"
---------- Echo Area ----------
"Echo Area output"
---------- Echo Area ----------
Finally, we show an output stream which is a function. The function
eat-output takes each character that it is given and conses it
onto the front of the list last-output (see section Building Cons Cells and Lists).
At the end, the list contains all the characters output, but in reverse
order.
(setq last-output nil)
=> nil
(defun eat-output (c)
(setq last-output (cons c last-output)))
=> eat-output
(print "This is the output" 'eat-output)
=> "This is the output"
last-output
=> (10 34 116 117 112 116 117 111 32 101 104
116 32 115 105 32 115 105 104 84 34 10)
Now we can put the output in the proper order by reversing the list:
(concat (nreverse last-output))
=> "
\"This is the output\"
"
This section describes the Lisp functions for printing Lisp objects.
Some of the Emacs printing functions add quoting characters to the output when necessary so that it can be read properly. The quoting characters used are `\' and `"'; they are used to distinguish strings from symbols, and to prevent punctuation characters in strings and symbols from being taken as delimiters. See section Printed Representation and Read Syntax, for full details. You specify quoting or no quoting by the choice of printing function.
If the text is to be read back into Lisp, then it is best to print with quoting characters to avoid ambiguity. Likewise, if the purpose is to describe a Lisp object clearly for a Lisp programmer. However, if the purpose of the output is to look nice for humans, then it is better to print without quoting.
Printing a self-referent Lisp object requires an infinite amount of text. In certain cases, trying to produce this text leads to a stack overflow. Emacs detects such recursion and prints `#level' instead of recursively printing an object already being printed. For example, here `#0' indicates a recursive reference to the object at level 0 of the current print operation:
(setq foo (list nil))
=> (nil)
(setcar foo foo)
=> (#0)
In the functions below, stream stands for an output stream.
(See the previous section for a description of output streams.) If
stream is nil or omitted, it defaults to the value of
standard-output.
Function: print object &optional stream
The print is a convenient way of printing. It outputs the
printed representation of object to stream, printing in
addition one newline before object and another after it. Quoting
characters are used. print returns object. For example:
(progn (print 'The\ cat\ in)
(print "the hat")
(print " came back"))
-|
-| The\ cat\ in
-|
-| "the hat"
-|
-| " came back"
-|
=> " came back"
Function: prin1 object &optional stream
This function outputs the printed representation of object to
stream. It does not print any spaces or newlines to separate
output as print does, but it does use quoting characters just
like print. It returns object.
(progn (prin1 'The\ cat\ in)
(prin1 "the hat")
(prin1 " came back"))
-| The\ cat\ in"the hat"" came back"
=> " came back"
Function: princ object &optional stream
This function outputs the printed representation of object to stream. It returns object.
This function is intended to produce output that is readable by people,
not by read, so quoting characters are not used and double-quotes
are not printed around the contents of strings. It does not add any
spacing between calls.
(progn
(princ 'The\ cat)
(princ " in the \"hat\""))
-| The cat in the "hat"
=> " in the \"hat\""
Function: terpri &optional stream
This function outputs a newline to stream. The name stands for "terminate print".
Function: write-char character &optional stream
This function outputs character to stream. It returns character.
Function: prin1-to-string object &optional noescape
This function returns a string containing the text that prin1
would have printed for the same argument.
(prin1-to-string 'foo)
=> "foo"
(prin1-to-string (mark-marker))
=> "#<marker at 2773 in strings.texi>"
If noescape is non-nil, that inhibits use of quoting
characters in the output. (This argument is supported in Emacs versions
19 and later.)
(prin1-to-string "foo")
=> "\"foo\""
(prin1-to-string "foo" t)
=> "foo"
See format, in section Conversion of Characters and Strings, for other ways to obtain
the printed representation of a Lisp object as a string.
The value of this variable is the default output stream, used when the
stream argument is omitted or nil.
Variable: print-escape-newlines
If this variable is non-nil, then newline characters in strings
are printed as `\n'. Normally they are printed as actual newlines.
This variable affects the print functions prin1 and print,
as well as everything that uses them. It does not affect princ.
Here is an example using prin1:
(prin1 "a\nb")
-| "a
-| b"
=> "a
=> b"
(let ((print-escape-newlines t))
(prin1 "a\nb"))
-| "a\nb"
=> "a
=> b"
In the second expression, the local binding of
print-escape-newlines is in effect during the call to
prin1, but not during the printing of the result.
The value of this variable is the maximum number of elements of a list that will be printed. If the list being printed has more than this many elements, then it is abbreviated with an ellipsis.
If the value is nil (the default), then there is no limit.
(setq print-length 2)
=> 2
(print '(1 2 3 4 5))
-| (1 2 ...)
=> (1 2 ...)
The value of this variable is the maximum depth of nesting of
parentheses that will be printed. Any list or vector at a depth
exceeding this limit is abbreviated with an ellipsis. A value of
nil (which is the default) means no limit.
This variable exists in version 19 and later versions.
A minibuffer is a special buffer that Emacs commands use to read arguments more complicated than the single numeric prefix argument. These arguments include file names, buffer names, and command names (as in M-x). The minibuffer is displayed on the bottom line of the screen, in the same place as the echo area, but only while it is in use for reading an argument.
In most ways, a minibuffer is a normal Emacs buffer. Most operations within a buffer, such as editing commands, work normally in a minibuffer. However, many operations for managing buffers do not apply to minibuffers. The name of a minibuffer always has the form ` *Minibuf-number', and it cannot be changed. Minibuffers are displayed only in special windows used only for minibuffers; these windows always appear at the bottom of a frame. (Sometime frames have no minibuffer window, and sometimes a special kind of frame contains nothing but a minibuffer window; see section Minibuffers and Frames.)
The minibuffers window is normally a single line; you can resize it temporarily with the window sizing commands, but reverts to its normal size when the minibuffer is exited.
A recursive minibuffer may be created when there is an active
minibuffer and a command is invoked that requires input from a
minibuffer. The first minibuffer is named ` *Minibuf-0*'.
Recursive minibuffers are named by incrementing the number at the end of
the name. (The names begin with a space so that they won't show up in
normal buffer lists.) Of several recursive minibuffers, the innermost
(or most recently entered) is the active minibuffer. We usually call
this "the" minibuffer. You can permit or forbid recursive minibuffers
by setting the variable enable-recursive-minibuffers or by
putting properties of that name on command symbols (see section Minibuffer Miscellany).
Like other buffers, a minibuffer may use any of several local keymaps (see section Keymaps); these contain various exit commands and in some cases completion commands. See section Completion.
minibuffer-local-map is for ordinary input (no completion).
minibuffer-local-ns-map is similar, except that SPC exits
just like RET. This is used mainly for Mocklisp compatibility.
minibuffer-local-completion-map is for permissive completion.
minibuffer-local-must-match-map is for strict completion and
for cautious completion.
The minibuffer is usually used to read text which is returned as a
string, but can also be used to read a Lisp object in textual form. The
most basic primitive for minibuffer input is
read-from-minibuffer.
Function: read-from-minibuffer prompt-string &optional initial keymap read hist
This function is the most general way to get input through the
minibuffer. By default, it accepts arbitrary text and returns it as a
string; however, if read is non-nil, then it uses
read to convert the text into a Lisp object (see section Input Functions).
The first thing this function does is to activate a minibuffer and display it with prompt-string as the prompt. This value must be a string.
Then, if initial is a string; its contents are inserted into the minibuffer as initial contents. The text thus inserted is treated as if the user had inserted it; the user can alter it with Emacs editing commands.
The value of initial may also be a cons cell of the form
(string . position). This means to insert
string in the minibuffer but put the cursor position
characters from the beginning, rather than at the end.
If keymap is non-nil, that keymap is the local keymap to
use while reading. If keymap is omitted or nil, the value
of minibuffer-local-map is used as the keymap. Specifying a
keymap is the most important way to customize minibuffer input for
various applications including completion.
The argument hist specifies which history list variable to use
for saving the input and for history commands used in the minibuffer.
It defaults to minibuffer-history. See section Minibuffer History.
When the user types a command to exit the minibuffer, the current
minibuffer contents are usually made into a string which becomes the
value of read-from-minibuffer. However, if read is
non-nil, read-from-