6.10.8 Eval-when

As syntax-case macros have the whole power of Scheme available to them, they present a problem regarding time: when a macro runs, what parts of the program are available for the macro to use?

The default answer to this question is that when you import a module (via define-module or use-modules), that module will be loaded up at expansion-time, as well as at run-time. Additionally, top-level syntactic definitions within one compilation unit made by define-syntax are also evaluated at expansion time, in the order that they appear in the compilation unit (file).

But if a syntactic definition needs to call out to a normal procedure at expansion-time, it might well need need special declarations to indicate that the procedure should be made available at expansion-time.

For example, the following code will work at a REPL, but not in a file:

;; incorrect
(use-modules (srfi srfi-19))
(define (date) (date->string (current-date)))
(define-syntax %date (identifier-syntax (date)))
(define *compilation-date* %date)

It works at a REPL because the expressions are evaluated one-by-one, in order, but if placed in a file, the expressions are expanded one-by-one, but not evaluated until the compiled file is loaded.

The fix is to use eval-when.

;; correct: using eval-when
(use-modules (srfi srfi-19))
(eval-when (compile load eval)
  (define (date) (date->string (current-date))))
(define-syntax %date (identifier-syntax (date)))
(define *compilation-date* %date)

Syntax: eval-when conditions exp...

Evaluate exp... under the given conditions. Valid conditions include eval, load, and compile. If you need to use eval-when, use it with all three conditions, as in the above example. Other uses of eval-when may void your warranty or poison your cat.

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