GNU bug report logs - #14293
23.4; Unclarity in documentation: flet replaced by cl-flet of cl-letf in 24.3 and up.

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Message #10 received at 14293-done <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Nikolaus Demmel <nikolaus <at> nikolaus-demmel.de>
Cc: 14293-done <at> debbugs.gnu.org
Subject: Re: bug#14293: 23.4;
	Unclarity in documentation: flet replaced by cl-flet of cl-letf in
	24.3 and up.
Date: Sat, 04 May 2013 15:50:24 -0400

> that using defadvice might be more appropriate. Maybe the information
> about how to precisely replace flet (i.e. same call if using cl-flet,
> different syntax if dynamic binding is desired using cl-letf, with an
> hint that defadvice might be better) could be included somewhere in the
> docs, maybe in [2]?

Thanks.   I installed the patch below.


        Stefan


=== modified file 'doc/misc/cl.texi'
--- doc/misc/cl.texi	2013-02-20 02:07:07 +0000
+++ doc/misc/cl.texi	2013-05-04 19:46:20 +0000
@@ -4850,10 +4850,27 @@
 direct C-language calls to the message routines rather than going
 through the Lisp @code{message} function.
 
+For those cases where the dynamic scoping of @code{flet} is desired,
+@code{cl-flet} is clearly not a substitute.  The most direct replacement would
+be instead to use @code{cl-letf} to temporarily rebind @code{(symbol-function
+'@var{fun})}.  But in most cases, a better substitute is to use an advice, such
+as:
+
+@example
+(defvar my-fun-advice-enable nil)
+(add-advice '@var{fun} :around
+            (lambda (orig &rest args)
+              (if my-fun-advice-enable (do-something)
+                (apply orig args))))
+@end example
+
+so that you can then replace the @code{flet} with a simple dynamically scoped
+binding of @code{my-fun-advice-enable}.
+
 @c Bug#411.
 Note that many primitives (e.g., @code{+}) have special byte-compile
-handling.  Attempts to redefine such functions using @code{flet} will
-fail if byte-compiled.
+handling.  Attempts to redefine such functions using @code{flet}, @code{letf},
+or an advice will fail when byte-compiled.
 @c Or cl-flet.
 @c In such cases, use @code{labels} instead.
 @end defmac

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