Re: Is there already an alist function which gets all matching elements, not just the first?

[Alan Mackenzie <acm@muc.de>]

Hello, Philip.

On Wed, Oct 05, 2022 at 18:16:43 +0000, Philip Kaludercic wrote:
> Alan Mackenzie <acm@muc.de> writes:

> > Hello, Emacs.

> > I want to be able to get all matching elements from an alist (thinking
> > about extending imenu).  Something like

> >     (assoc-all key list)

> > which would return a list of matches.  After all, we have functions
> > which _delete_ all matches from an alist.

> > Does such a function already exist, perhaps in cl-*.el?

> This appears to do the right thing

> (map-filter (lambda (k _v) (eq k '3))
> 	    '((3 . 4)
> 	      (2 . 4)
> 	      (1 . 2)
> 	      (3 . 1)))
> ;; => ((3 . 4) (3 . 1))

The doc string for map-filter doesn't actually describe the function,
sadly.  It talks about key/value pairs whilst giving no clue where the
key and the value come from.  I suppose one might guess, but really
there's no alternative to studying the source code for map-filter.

> Perhaps a `map-member' could be implemented that would do something like
> what you are looking for?  Or does this already exist by some other name
> (I don't really use map.el). I've certainly wanted something like this
> more than a few times.

It seems like one of these things which quite a lot of people want, but
nobody wants it enough to be bothered to implement it.  ;-)

> > Thanks!

-- 
Alan Mackenzie (Nuremberg, Germany).