Subject: bug#47368: 28.0.50; map-elt returns nil without "deprecated" TESTFN

From: Michael Heerdegen <[email-masked]>
Date: Fri, 26 Mar 2021 04:59:29 +0100

Hi Stefan,

we are discussing here the limitation for `map-elt' calls with alists
caused by deprecating the TESTFN argument (done by you a while ago).

What's a good way to solve this?  Obviously the map abstraction doesn't
fit so super well for alists because unlike the other map type alists
don't know "their" test function.  But disallowing alists that don't
test with `eq' seems an unnecessary restriction.  Can we say that the
argument is allowed only for alists?

Regards,

Michael.

I <[email-masked]> wrote:

> > > The docstring of the map-elt function from the map.el package (version
> > > 3.0) mentions that TESTFN is deprecated because "its default depends on
> > > the MAP argument". However when I try e.g.
> > >
> > > (map-elt '(("A1" . 3)) "A1")
> > >
> > > it returns nil.
> >
> > This is expected, as alist keys are tested with eq by default.
> >
> > That's what the docstring is trying to warn about: alists default to
> > testing with eq, but can also use eql, equal, or anything else.
>
> Is it that obvious?  We have `assoc' and `assq' built-in - to me it's
> not obvious that "alist keys are tested with eq by default".  It's the
> default for `alist-get', ok, which is used by the implementation, but
> not everybody will know that.  I would add a sentence about that.