Subject: bug#67611: [PATCH] Add a Pcase pattern `cl-lambda` equivalent to `cl-destructuring-bind`

From: João Távora <[email-masked]>
Date: Fri, 12 Jan 2024 10:46:43 +0000

On Fri, Jan 12, 2024 at 3:05 AM Stefan Monnier <[email-masked]>
wrote:

> I don't think there's a clear enough need for it right now in Emacs core
> to motivate its integration in Emacs proper.  Also, because several
> people have expressed an opinion that leans towards recommending that
> Emacs's own code should probably better avoid using this functionality.

Those people are trying to come up with alternative pattern-matching
libraries which we start to see are not simpler or as powerful
as pcase.

But sure, other pattern-matching libraries exist out there.  See
CL's optima/trivia [1/2] for some very good ones.  Emacs just happens
to have pcase readily available.

> I'd be quite happy to include such a think in GNU ELPA, OTOH.

Could be, but why really?  Why are CL things which rest on such
high-quality well-known designs, older than many things Emacs,
relegated to a place where they're less discoverable?

Here, I think at least the new fresh implementation of
lambda-list parsing (which is the meat-and-bones of this patch)
is worth taking a look at.

It's the function 'cl--pcase-cl-lambda-var-groups'.

I would definitely consider grooming this function, renaming
it, and keeping it in core, exposing to general Elisp usage
in cl-macs.el (or cl-lambdalist.el, cl-ll.el), even if a new
pcase pattern that uses it goes to ELPA (but why, really?)

That util could help us write other lambda-list destructuring
alternatives that understand &optional, &rest.  Because with or
without it, we already use a lot of the &optional, &rest explicitly
in Emacs.  And we also use &key and &allow-other-keys in many other
places, though sometimes in ad-hoc lets-not-call-this-bull-by-its-name
fashion.

IOW there is no general purpose-util like Alexandria's
'parse-ordinary-lambda-list', and this could be it.  It could
not only be used to simplify and add coherence to many of these
existing compile-time structures, for example helping simplify
things and address FIXMEs in cl-lib.el.

João

[1] https://github.com/m2ym/optima
[2] https://github.com/guicho271828/trivia