Control Structures

Although a Lisp program is a sequence of expressions, life would be quit boring if we could not, somehow, alter the flow of control to go ``non-linearly''. Lisp has almost all the usual control constructs one finds in other high level languages, like varieties of sequencing, conditional contructs, iteration, and some form of ``jumps''. The one missing contruct is the for loop. As we have already seen in an earlier section, all of these contructs are implemented as special forms. Table 4 shows some of the control constructs available and some examples.

 

Table 4: A representative sample of various types of control contructs available in Lisp.

progn	Sequential evaluation. Used when you	(progn (message ``1.'')
	really want multiple forms where only one	(message ``2.'')
	is allowed	(message ``3''))
if	The usual thing	(if condition
		then-form
		else-forms)
when	Just the 'if' portion of an if contruct but	(when condition A B C)
	allows multiple forms
unless	The else portion of the if construct	(unless condition A B C)
and	Combining conditionals to build	(and A B C ... )
or	complex conditional	(or A B C ...)
not		(not A)
while	Iteration	(while condition forms...)