Re: Does the eval-when example work?

[Jean Abou Samra]

Le 19/10/2022 à 22:32, Vivien Kraus a écrit :
> Dear guile users,
>
> The manual, section 6.8.8, presents the eval-when form with an example:
>
> (use-modules (srfi srfi-19))
> (eval-when (expand load eval)
>    (define (date) (date->string (current-date))))
> (define-syntax %date (identifier-syntax (date)))
> (define *compilation-date* %date)
>
> I take the liberty to add:
>
> (display *compilation-date*)
> (newline)
>
> Now, when I save to test.scm and run guile -s test.scm, it gets
> compiled and it displays the current date. However, if I run this
> command a second time, the file is not recompiled, but the compilation
> date changes. Is it intended?
>
> If so, this is not exactly what I am looking for. I am looking for a
> way to run the (date) form during the compilation phase, and save the
> date to the compilation unit so that it does not change. Is this
> possible?



This code is behaving as expected. The manual's description of the example
and the naming "*compilation-date*" look buggy, however. It claims that the
code works in the REPL but not in a file, whereas the code does work in 
a file.
identifier-syntax just makes a macro that replaces an identifier with
some form. The code you quote just expands to the same as

(use-modules (srfi srfi-19))
(define (date)
  (date->string (current-date))
(define *compilation-date* (date))
(display *compilation-date*)
(newline)

On the other hand, if you define a macro that doesn't expand to the
function call "(date)" but to the result of this function call at
the time of compilation, it does work as a compilation timestamp
(and eval-when becomes really necessary).

(use-modules (srfi srfi-19))
(eval-when (expand load eval)
  (define compilation-start-date (date->string (current-date))))
(define-syntax *compilation-date*
  (lambda (sintax)
    (datum->syntax sintax compilation-start-date)))

(display *compilation-date*)
(newline)


Best,
Jean