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 #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Nikolaus Demmel <nikolaus <at> nikolaus-demmel.de>
To: bug-gnu-emacs <at> gnu.org
Subject: 23.4;
	Unclarity in documentation: flet replaced by cl-flet of cl-letf in
	24.3 and up.
Date: Sat, 27 Apr 2013 13:37:46 +0200

This is a documentation bug that is not about my emacs version, but
about the documentation for a change in 24.3. flet has been made
obsolete. This is a follow up to my question on [7], where Drew Adams
suggested I file a bug. From that discussion:

-----
I'm referring to the documentation [1,2,3,4]. Together with the
notification

    `flet' is an obsolete macro (as of 24.3); use either `cl-flet' or
    `cl-letf'.

as reported here [5], this documentation does not really tell me what to
do if I want to keep relying on the dynamic scoping of flet. A brief
google search [6] leads me to believe that

    (cl-letf (((symbol-function 'foo) #'(lambda (...) ...))) ...)

is the suggested solution, however I wanted to make sure that is the
case.
-----

Stefan Monnier has confirmed the suggested solution, but also hinted
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,
Niko



[1] http://www.gnu.org/software/emacs/manual/html_node/cl/Function-Bindings.html#index-cl_002dflet-24
[2] http://www.gnu.org/software/emacs/manual/html_node/cl/Obsolete-Macros.html#index-flet-173
[3] http://www.gnu.org/software/emacs/manual/html_node/cl/Obsolete-Macros.html#index-letf-175
[4] http://www.gnu.org/software/emacs/manual/html_node/cl/Modify-Macros.html#index-cl_002dletf-19
[5] https://github.com/moesenle/rosemacs-debs/issues/2
[6] https://groups.google.com/forum/#!msg/gnu.emacs.help/eaopyFH5hmY/emHfn-DYZZIJ 
[7] https://groups.google.com/forum/?fromgroups=#!topic/gnu.emacs.help/eXjBLqncchs



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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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