GNU bug report logs - #67116
byte-compile-let: reversing the order of evaluation of the clauses CAN make a difference.

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Message #26 received at 67116 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> iro.umontreal.ca>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 67116 <at> debbugs.gnu.org, acm <at> muc.de
Subject: Re: bug#67116: byte-compile-let: reversing the order of evaluation
 of the clauses CAN make a difference.
Date: Sun, 12 Nov 2023 11:49:55 -0500

>> Sounds like a bug.
> It does?  I always thought that the order of evaluation in a let form
> is unspecified,

I'm not sure if we say so explicitly somewhere, but ELisp's order of
evaluation is very much always "left to right" and that carries over to
`let`.

> Why on Earth should we require any particular order of evaluation in
> a let form??

That's a popular bikeshedding subject, actually.

On one side you have the proponents of leaving the order unspecified
(like in C and Scheme) on the premise that it allows more choice for the
compiler, on the other you have the proponents of specifying the order
so as to remove an ugly corner case that bites programmers.

I am personally not swayed by the optimization argument (tho the OCaml
bytecode compiler had a good argument in favor of right-to-left
evaluation order) and in the case of ELisp, there's a lot of code out
there which already relies on the current evaluation order.


        Stefan

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