* bug#58646: [PATCH] doc: Fix eval-when example
@ 2022-10-19 21:04 Jean Abou Samra
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From: Jean Abou Samra @ 2022-10-19 21:04 UTC (permalink / raw)
To: 58646
From 0e35d0494b7c6f05819e3386d404776a64a2e5c9 Mon Sep 17 00:00:00 2001
From: Jean Abou Samra <jean@abou-samra.fr>
Date: Wed, 19 Oct 2022 23:01:06 +0200
Subject: [PATCH] doc: Fix eval-when example
* 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.
---
doc/ref/api-macros.texi | 25 ++++++++++++++++---------
1 file changed, 16 insertions(+), 9 deletions(-)
diff --git a/doc/ref/api-macros.texi b/doc/ref/api-macros.texi
index cdb33df31..ed7bbe763 100644
--- a/doc/ref/api-macros.texi
+++ b/doc/ref/api-macros.texi
@@ -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
+string literal. It will work at a REPL, but in a file, 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...
--
2.37.3
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2022-10-19 21:04 bug#58646: [PATCH] doc: Fix eval-when example Jean Abou Samra
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