GNU bug report logs - #34852
26.1; seq-intersection ignores nil as element

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Message #11 received at 34852 <at> debbugs.gnu.org (full text, mbox):

From: "Basil L. Contovounesios" <contovob <at> tcd.ie>
To: "Miguel V. S. Frasson" <mvsfrasson <at> gmail.com>
Cc: Nicolas Petton <nicolas <at> petton.fr>, 34852 <at> debbugs.gnu.org,
 Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#34852: 26.1; seq-intersection ignores nil as element
Date: Thu, 14 Mar 2019 12:22:40 +0000

[0001-Do-not-use-seq-contains-as-a-predicate-bug-34852.patch (text/x-diff, attachment)]

[0001-Return-boolean-instead-of-element-in-seq-contains.patch (text/x-diff, attachment)]

[Message part 3 (text/plain, inline)]

"Miguel V. S. Frasson" <mvsfrasson <at> gmail.com> writes:

> seq-intersection skips nil as common element, so returns wrong result.
>
> Reproducing from emacs -Q:
>
> In *scratch* buffer, eval the expressions
>
> (progn
>   (require 'seq)
>   (seq-intersection '(1 2 nil) '(1 nil) 'eq))
>
> -> (1)
>
> (seq-intersection '(1 2 nil) '(1 nil) 'equal)
>
> -> (1)
>
> Expected result in both cases: (1 nil)

This is actually due to seq-contains returning the element found, rather
than a boolean indicating whether the element was found:

(seq-contains '(nil) nil) ; => nil

The nature of seq-contains as a predicate-ish has been discussed in the
past[1], and Stefan recently fixed a similar problem with
map-contains-key[2].

[1]: https://lists.gnu.org/archive/html/emacs-devel/2016-07/msg01256.html
[2]: * lisp/emacs-lisp/map.el: Make the functions generic
  2018-12-11 17:54:13 -0500
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=1691a51094d35ac4b2c311fa407c6b77eea7a105

One solution is to leave seq-contains as it is, and switch to using
seq-position (or some new predicate) as a predicate instead.  Another is
to make seq-contains return a boolean instead of the needle found, which
would be a backward-incompatible change similar to that for
map-contains-key.  I attach a patch for each of these respective
solutions; WDYT?

-- 
Basil

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