GNU bug report logs - #78656
[PATCH] Add new reduction primitives `fold-left' and `fold-right'

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Message #23 received at 78656 <at> debbugs.gnu.org (full text, mbox):

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 78656 <at> debbugs.gnu.org, Zach Shaftel <zach <at> shaf.tel>
Subject: Re: bug#78656: [PATCH] Add new reduction primitives `fold-left' and
 `fold-right'
Date: Tue, 03 Jun 2025 01:09:32 +0200

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

> On balance, I'd prefer to have a new function in seq.el rather than
> another primitive in C.  Since we didn't need this function until now,
> it is reasonable to assume it is not going to be a hot spot in any
> program, so an implementation in C is not really required.

Since this would just add an implicit `seq-reverse' call, this doesn't
sound convincing either.  `cl-reduce' is able to reduce :from-end.  If
at all, then adding a new argument to `seq-reduce' that causes reducing
from the end sounds more sensible to me.

@Zach: I really can feel your desire to have a primitive but part of the
story is also that Emacs Lisp is not that purely functional language
that some of us wish, since it's a lot more iteration oriented than
other Lisps - recursion depth is horribly limited, for example.  Trying
to use a purely functional style in Elisp doesn't work well.

So, in Elisp we are having most of these things in seq.el instead of the
core, and I think it's appropriate.  Especially folding is an operation
that is not used that often in Elisp.


Michael.

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