bug#59328: 29.0.50; `seq-keep' implementation only valid for lists

[Eli Zaretskii <eliz@gnu.org>]

> From: Michael Heerdegen <michael_heerdegen@web.de>
> Cc: 59328@debbugs.gnu.org,  larsi@gnus.org,  jonas@bernoul.li
> Date: Thu, 24 Nov 2022 16:25:44 +0100
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > > Without that change `seq-keep' would error for sequence types like
> > > streams.  Try for example
> > >
> > > #+begin_src emacs-lisp
> > > (require 'stream)
> > > (seq-keep
> > >  (lambda (x) (and (<= 0 x) x))
> > >  (stream (list -1 2 -3 4)))
> > > #+end_src
> >
> > Didn't you just say that 'stream' is not in Emacs?  If I try the above, the
> > debugger kicks in right on the 'require' line.
> 
> Yes.
> 
> But seq-keep is not a generic function, so it would be broken for
> sequence types defined elsewhere, and there is no way for those other
> sequence types to fix this.  Defining a generic interface for sequences
> would not make much sense if it then only supports lists (and maybe
> vectors).
> 
> So we want to support cases like streams.

Can tests for this be written in a way that they are only run if the
relevant packages are available on the user's system?  If so, I'd prefer to
have that than no tests at all.

Your call.

Thanks.