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Subject: bug#51412: 29.0.50; cl--mapcar-many fails inconsistently in the presence of circular lists; other shortcomings
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cl--mapcar-many is used as a backend for implementing several cl-
functions in cl-extra.  To deal with arbitrary number of arguments
(passed to cl--mapcar-many as list cl-seq), it goes through them all
whenever there are more than 2, in advance, and attempts to compute the
minimal length.

This fails if any of the arguments is a circular list.

To reproduce, just evaluate (cl-map 'list #'identity '#1=3D(a . #1#) nil ni=
l).

Note that (cl-map 'list #'identity '#1=3D(a . #1#) nil) doesn't fail so
this is at the very least inconsistent.  N.B.: CL spec says =E2=80=9CShould=
 be
prepared to signal an error=E2=80=9D for improper lists for such functions.
Long story short, this means circular lists can be supported by CL
implementations according to CL spec.

Other shortcomings: going through all sequences in advance is very
inefficient.  mapcar-many also checks sequence types at runtime even
though they are known in advance which is also wasteful.

The mechanism used in recent suggested implementation of cl-map-into
https://lists.gnu.org/archive/html/emacs-devel/2021-10/msg00636.html
could be used to reimplement other sequence-oriented cl-extra functions
and resolve these issues, if it's merged.

I wrote some supporting macros there with a view towards this use; this
bug report is partially an apology for them.

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To: akater <nuclearspace@HIDDEN>
Subject: bug#51412: Acknowledgement (29.0.50; cl--mapcar-many fails
 inconsistently in the presence of circular lists; other shortcomings)
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Subject: bug#51412: 29.0.50; cl--mapcar-many fails inconsistently in the presence of circular lists; other shortcomings
Resent-From: Lars Ingebrigtsen <larsi@HIDDEN>
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To: akater <nuclearspace@HIDDEN>
Cc: 51412 <at> debbugs.gnu.org, Stefan Monnier <monnier@HIDDEN>
From: Lars Ingebrigtsen <larsi@HIDDEN>
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Date: Wed, 14 Sep 2022 20:38:45 +0200
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akater <nuclearspace@HIDDEN> writes:

> cl--mapcar-many is used as a backend for implementing several cl-
> functions in cl-extra.  To deal with arbitrary number of arguments
> (passed to cl--mapcar-many as list cl-seq), it goes through them all
> whenever there are more than 2, in advance, and attempts to compute the
> minimal length.
>
> This fails if any of the arguments is a circular list.
>
> To reproduce, just evaluate (cl-map 'list #'identity '#1=3D(a . #1#) nil =
nil).

Debugger entered--Lisp error: (circular-list (a . #2))

> Note that (cl-map 'list #'identity '#1=3D(a . #1#) nil) doesn't fail so
> this is at the very least inconsistent.  N.B.: CL spec says =E2=80=9CShou=
ld be
> prepared to signal an error=E2=80=9D for improper lists for such function=
s.
> Long story short, this means circular lists can be supported by CL
> implementations according to CL spec.

It's inconsistent, so that should probably be fixed.  But isn't
signalling an error here the right thing to do?  But I guess as long as
not all the lists are circular, then it's fine to return something
here...

Perhaps Stafan has some comments; added to the CCs.

Last modified: Wed, 14 Sep 2022 18:45:01 UTC

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