Modules often contain just function and structure-type definitions, in which case the module itself behaves in a purely functional way, and the time when the functions are created is not observable. If a module’s top-level expressions include side effects, however, then the timing of the effects can matter. The distinction between module declaration and instantiation provides some control over that timing. The concept of module visits further explains the interaction of effects with macro implementations.
Declaring a module does not immediately evaluate expressions in the module’s body. For example, evaluating
declares the module number-n, but it doesn’t immediately pick a random number for n or display the number. A require of number-n causes the module to be instantiated (i.e., it triggers an instantiation), which implies that the expressions in the body of the module are evaluated:
> (require 'number-n)
> (require 'number-n) > n
> (module use-n racket/base (require 'number-n) (printf "still ~a\n" n)) > (require 'use-n)
The dynamic-require function, like require, triggers instantiation of a module if it is not already instantiated, so dynamic-require with #f as a second argument is useful to just trigger the instantiation effects of a module:
> (module use-n-again racket/base (require 'number-n) (printf "also still ~a\n" n)) > (dynamic-require ''use-n-again #f)
also still 5
> (module number-m racket/base (provide m) (define m (random 10)) (printf "picked ~a\n" m))
> (module use-m racket/base (require 'number-m) (printf "still ~a\n" m)) > (require 'use-m)
In the same way that declaring a module does not by itself instantiate a module, declaring a module that requires another module does not by itself instantiate the required module, as illustrated in the preceding example. However, declaring a module does expand and compile the module. If a module imports another with (require (for-syntax ....)), then module that is imported for-syntax must be instantiated during expansion:
> (module number-p racket/base (provide p) (define p (random 10)) (printf "picked ~a\n" p))
> (module use-p-at-compile-time racket/base (require (for-syntax racket/base 'number-p)) (define-syntax (pm stx) #`#,p) (printf "was ~a at compile time\n" (pm)))
Unlike run-time instantiation in a namespace, when a module is used for-syntax for another module expansion in the same namespace, the for-syntaxed module is instantiated separately for each expansion. Continuing the previous example, if number-p is used a second time for-syntax, then a second random number is selected for a new p:
> (module use-p-again-at-compile-time racket/base (require (for-syntax racket/base 'number-p)) (define-syntax (pm stx) #`#,p) (printf "was ~a at second compile time\n" (pm)))
Separate compile-time instantiations of number-p helps prevent accidental propagation of effects from one module’s compilation to another module’s compilation. Preventing those effects make compilation reliably separate and more deterministic.
The expanded forms of use-p-at-compile-time and use-p-again-at-compile-time record the number that was selected each time, so those two different numbers are printed when the modules are instantiated:
> (dynamic-require ''use-p-at-compile-time #f)
was 1 at compile time
> (dynamic-require ''use-p-again-at-compile-time #f)
was 3 at second compile time
A namespace’s top level behaves like a separate module, where multiple interactions in the top level conceptually extend a single expansion of the module. So, when using (require (for-syntax ....)) twice in the top level, the second use does not trigger a new compile-time instance:
> (begin (require (for-syntax 'number-p)) 'done)
> (begin (require (for-syntax 'number-p)) 'done-again)
However, a run-time instance of a module is kept separate from all compile-time instances, including at the top level, so a non-for-syntax use of number-p will pick another random number:
> (require 'number-p)
When a module provides a macro for use by other modules, the
other modules use the macro by directly requireing the macro
The module implementing a macro, meanwhile, might require another module for-syntax to implement the macro. The for-syntax module needs a compile-time instantiation during any module expansion that might use the macro. That requirement sets up a kind of transitivity through require that is similar to instantiation transitivity, but “off by one” at the point where the for-syntax shift occurs in the chain.
Here’s an example to make that scenario concrete:
> (module number-q racket/base (provide q) (define q (random 10)) (printf "picked ~a\n" q))
> (module use-q-at-compile-time racket/base (require (for-syntax racket/base 'number-q)) (provide qm) (define-syntax (qm stx) #`#,q) (printf "was ~a at compile time\n" (qm)))
> (module use-qm racket/base (require 'use-q-at-compile-time) (printf "was ~a at second compile time\n" (qm)))
> (dynamic-require ''use-qm #f)
was 7 at compile time
was 4 at second compile time
In this example, when use-q-at-compile-time is expanded and compiled, number-q is instantiated once. In this case, that instantiation is needed to expand the (qm) macro, but the module system would proactively create a compile-time instantiation of number-q even if the qm macro turned out not to be used.
Then, as use-qm is expanded and compiled, a second compile-time instantiation of number-q is created. That compile-time instantiation is needed to expand the (qm) form within use-qm.
Instantiating use-qm correctly reports the number that was picked during that second module’s compilation. First, though, the require of use-q-at-compile-time in use-qm triggers a transitive instantiation of use-q-at-compile-time, which correctly reports the number that was picked in its compilation.
Overall, the example illustrates a transitive effect of require that we had already seen:
When a module is instantiated, the run-time expressions in its body are evaluated.
This rule does not explain the compile-time instantiations of number-q, however. To explain that, we need a new word, visit, for the concept that we saw in Compile-Time Instantiation:
When a module is visited, the compile-time expressions (such as macro definition) in its body are evaluated.
As a module is expanded, it is visited.
Note that when visiting one module causes a compile-time instantiation of another module, the transitiveness of instantiation through regular requires can trigger more compile-time instantiations. Instantiation itself won’t trigger further visits, however, because any instantiated module has already been expanded and compiled.
The compile-time expressions of a module that are evaluated by visiting include both the right-hand sides of define-syntax forms and the body of begin-for-syntax forms. That’s why a randomly selected number is printed immediately in the following example:
> (module compile-time-number racket/base (require (for-syntax racket/base)) (begin-for-syntax (printf "picked ~a\n" (random))) (printf "running\n"))
Instantiating the module evaluates only the run-time expressions, which prints “running” but not a new random number:
> (dynamic-require ''compile-time-number #f)
The description of instantiates and visit above is phrased in terms of normal requires and for-syntax requires, but a more precise specification is in terms of module phases. For example, if module A has (require (for-syntax B)) and module B has (require (for-template C)), then module C is instantiated when module A is instantiated, because the for-syntax and for-template shifts cancel. We have not yet specified what happens with for-meta 2 for when for-syntaxes combine; we leave that to the next section, Lazy Visits via Available Modules.
If you think of the top-level as a kind of module that is continuously expanded, the above rules imply that require of another module at the top level both instantiates and visits the other module (if it is not already instantiated and visited). That’s roughly true, but the visit is made lazy in a way that is also explained in the next section, Lazy Visits via Available Modules.
Meanwhile, dynamic-require only instantiates a module; it does not visit the module. That simplification is why some of the preceding examples use dynamic-require instead of require. The extra visits of a top-level require would make the earlier examples less clear.
A top-level require of a module does not actually visit the module. Instead, it makes the module available. An available module will be visited when a future expression needs to be expanded in the same context. The next expression may or may not involve some imported macro that needs its compile-time helpers evaluated by visiting, but the module system proactively visits the module, just in case.
In the following example, a random number is picked as a result of
visiting a module’s own body while that module is being expanded. A
require of the module instantiates it, printing “running”,
and also makes the module available. Evaluating any other
expression implies expanding the expression, and that expansion
triggers a visit of the available module—
> (module another-compile-time-number racket/base (require (for-syntax racket/base)) (begin-for-syntax (printf "picked ~a\n" (random))) (printf "running\n"))
> (require 'another-compile-time-number)
The final evaluation of 'another also visits any available modules, but no modules were made newly available by simply evaluating 'next.
When a module requires another module using for-meta n for some n greater than 1, the required module is made available at phase n. A module that is available at phase n is visited when some expression at phase n-1 is expanded.
> (module show-phase racket/base (printf "running at ~a\n" (variable-reference->module-base-phase (#%variable-reference)))) > (require 'show-phase)
running at 0
> (module use-at-phase-1 racket/base (require (for-syntax 'show-phase)))
running at 1
> (module unused-at-phase-2 racket/base (require (for-meta 2 'show-phase)))
For the last module above, show-phase is made available at phase 2, but no expressions within the module are ever expanded at phase 1, so there’s no phase-2 printout. The following module includes a phase-1 expression after the phase-2 require, so there’s a printout:
> (module use-at-phase-2 racket/base (require (for-meta 2 'show-phase) (for-syntax racket/base)) (define-syntax x 'ok))
running at 2
If we require the module use-at-phase-1 at the top level, then show-phase is made available at phase 1. Evaluating another expression causes use-at-phase-1 to be visited, which in turn instantiates show-phase:
> (require 'use-at-phase-1) > 'next
running at 1
> (require 'use-at-phase-2) > 'next
> (require (for-syntax racket/base)) > (begin-for-syntax 'compile-time-next)
running at 2