GNU bug report logs - #78704
[PATCH] Use `seq-do' instead of `seq-map' for side-effects

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From: Pip Cet <pipcet <at> protonmail.com>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: Nicolas Petton <nicolas <at> petton.fr>, 78704 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>, Zach Shaftel <zach <at> shaf.tel>
Subject: bug#78704: [PATCH] Use `seq-do' instead of `seq-map' for side-effects
Date: Sat, 07 Jun 2025 14:29:41 +0000

"Eli Zaretskii" <eliz <at> gnu.org> writes:

>> Date: Thu, 05 Jun 2025 23:18:26 -0400
>> From:  Zach Shaftel via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs <at> gnu.org>
>>
>> self explanatory i think.
>
> I'm not sure it is, but I added a few people who might have an
> opinion.

First of all, this is a good catch!  The situation is similar for map-do
/ map-apply, including a usage in radix-tree.el which may not be
reported by the byte compiler.

But I'm not sure adding important-return-value to functions which aren't
usually side-effect-free is a good idea given the warnings it currently
produces.

The main reason is that it's not a helpful warning unless we tell the
user which function to use instead: mapc is a special case which is
handled by an explicit message, but without a "use `seq-do' instead"
in the message, fixing the warning requires looking up docstrings to
find the right alternative, which might not exist.

Some other languages have chosen a different approach and provide a way
for functions to know, at compile time or run time, whether their return
value is used in a particular call.  Maybe we should do that instead?
As an analogy, the byte compiler won't complain about (equal x 3), but
will silently replace it by (eq x 3), so it's not like we always warn
the user about code which can be optimized in a similar fashion.

However, I did notice that we warn about fewer important-return-value
functions than I thought, and opened bug#78716 to discuss whether we
should add more warnings.

Pip

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