GNU bug report logs - #65459
completing-read INITIAL-VALUE unaware of COLLECTION and REQUIRE-MATCH

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From: Gregory Heytings <gregory <at> heytings.org>
To: Stefan Monnier <monnier <at> iro.umontreal.ca>
Cc: 65459 <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>, Heime <heimeborgia <at> protonmail.com>
Subject: bug#65459: completing-read INITIAL-VALUE unaware of COLLECTION and REQUIRE-MATCH
Date: Sat, 26 Aug 2023 08:06:39 +0000

>
> The issue is not "don't use `(n)advice.el`", but "don't modify functions 
> on the sly".  And by "functions" this refers to those things stored in 
> the `symbol-function` slot of symbols in the global obarray.  This is 
> because code that does `(my-foo ...)` usually expects to execute the 
> code found in `(defun my-foo ...)` and not something else, and readers 
> of that code often make the same assumption, so it can make debugging 
> really nasty.
>
> Changing a function with `fset` (or `cl-letf` or `defalias`) is worse 
> than using `advice-add`, for that reason: at least `advice-add` sets up 
> the help system such that `C-h f` will (hopefully) warn you about the 
> presence of an advice.
>
> For variables containing functions (such as 
> `minibuffer-default-add-function`), there is no such expectation that 
> calling this function will run some known piece of code, on the 
> contrary: the whole point of the variable is to run difference pieces of 
> code in different contexts, so modifying the function is perfectly OK, 
> regardless whether you do it with `setq` or `add-function`.  Here I'd 
> use `add-function` because it's simpler.
>

Thanks, that clarifies that question!  Perhaps a paragraph along those 
lines could be added to "(elisp)Advising Functions"?

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