"Miguel V. S. Frasson" <mvsfrasson@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