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

[Michael Heerdegen <michael_heerdegen@web.de>]

Hello,

  [FWIW I tried to reopen 58278 but it seemed to...complicated for me]

The current implementation of the (non-generic!) function `seq-keep':

#+begin_src emacs-lisp
(defun seq-keep (function sequence)
  (delq nil (seq-map function sequence)))
;; ^^^^
#+end_src

obviously only works when `seq-map' returns a list.  This is the case
for the default implementation of the generic function `seq-map' but not
necessarily for other implementations of `seq-map'.

We need to filter out the `nil' elements with a way appropriate for any
sequence type supported by "seq.el" (i.e. with a generic function
defined in this lib), e.g.

#+begin_src emacs-lisp
(defun seq-keep (function sequence)
  (seq-filter #'identity (seq-map function sequence)))
#+end_src


BTW, is the name a good one?  Why "keep"?  It returns a sequence of
potentially all completely different elements.  And is the function that
useful and a good abstraction at all (I don't have thought about it
too long...)?


TIA,

Michael.