mirror/guile
mirror/guile
1
Fork 0
mirror of https://git.savannah.gnu.org/git/guile.git synced 2025-05-20 03:30:27 +02:00
Code Activity

doc: Fix eval-when example

Browse source 
* doc/ref/api-macros.texi: make the macro expand to the literal
  date, not to a call to the date function.  The example previously
  did not actually need eval-when and did not show the intended
  effect.
This commit is contained in:
Jean Abou Samra 2022-12-11 12:26:18 +01:00 • committed by Arne Babenhauserheide
parent f8938f517e
commit 7d5ab8fa40
1 changed files with 18 additions and 11 deletions
Download patch file Download diff file Expand all files Collapse all files

29
doc/ref/api-macros.texi
View file

@ -63,7 +63,7 @@ transformers, consider the following example macro definition:
(begin exp ...)))))
(when #t
(display "hey ho\n")
(display "hey ho\n")
(display "let's go\n"))
@print{} hey ho
@print{} let's go
@ -87,7 +87,7 @@ One can also establish local syntactic bindings with @code{let-syntax}.
Bind each @var{keyword} to its corresponding @var{transformer} while
expanding @var{exp1} @var{exp2} @enddots{}.
A @code{let-syntax} binding only exists at expansion-time.
A @code{let-syntax} binding only exists at expansion-time.
@example
(let-syntax ((unless
@ -1236,14 +1236,19 @@ 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:
For example, the following code tries to embed a compilation
timestamp in the compiled bytecode using a macro that expands
to the date as a string literal. It will work at a REPL, but
not in a file, as it cannot be byte-compiled:
@example
;; incorrect
(use-modules (srfi srfi-19))
(define (date) (date->string (current-date)))
(define-syntax %date (identifier-syntax (date)))
(define *compilation-date* %date)
(define start-date (date->string (current-date)))
(define-syntax *compilation-date*
(lambda (sintax)
start-date))
(display *compilation-date*)
(newline)
@end example
It works at a REPL because the expressions are evaluated one-by-one, in order,
@ -1253,12 +1258,14 @@ evaluated until the compiled file is loaded.
The fix is to use @code{eval-when}.
@example
;; correct: using eval-when
(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)
(define start-date (date->string (current-date))))
(define-syntax *compilation-date*
(lambda (sintax)
start-date))
(display *compilation-date*)
(newline)
@end example
@deffn {Syntax} eval-when conditions exp...