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

[Okamsn via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Stefan Monnier wrote:
>> The attached patch adds the pattern `cl-lambda` for Pcase, which works
>> like `cl-destructuring-bind`. There are two differences with the lambda
>> lists:
> 
> Hmm... I'm not sure mixing the CL destructuring patterns with the Pcase
> patterns (both of which are rather featureful and complex) will help
> their popularity.
> 
> Beside that problem (which means I'm not very favorable to the
> addition), the name should be changed because "lambda" is misleading.
> It suggests this has to do with a function (I had to read the code to
> understand what this is doing).

I agree that the name is not ideal. When I searched for what the 
destructuring pattern was called, the website Common Lisp HyperSpec 
called it a "destructuring lambda list" 
(https://www.lispworks.com/documentation/HyperSpec/Body/03_de.htm). I 
would have suggested "cl", as it is the kind of destructuring used by 
the macros of cl-lib, but I thought that would not work well with the 
existence of the "cl-type" pattern. I also thought about something like 
"cl-arglist", if that is better.

>> The pattern is useful when one wants to combine the features of `pcase`
>> and `cl-destructuring-bind`, such combining the optional values with the
>> `pred` or `guard` patterns.
> 
> Do you have examples uses?

Not of that idea, no.  I maintain a library that implements a 
destructuring pattern like cl-lib 
(https://github.com/okamsn/loopy/blob/master/doc/loopy-doc.org#basic-destructuring) 
and I have been thinking about how I could use Pcase to simplify the 
implementation of the destructuring and to stop using a re-invented 
wheel. While doing that, it occurred to me that cl-lib itself might be a 
better place for such a Pcase pattern. To be clear, the patch only 
implements the cl-lib destructuring, not the other destructuring ideas 
from my library.

For me, I am interested in using such a destructuring pattern with 
`pcase-let` and `pcase-setq`, but not so much with `pcase` itself.

> Maybe we could introduce a different Pcase pattern which covers those
> needs but stays closer to the Pcase pattern syntax?

As far as I understand Pcase, one thing that I think cl-lib does better 
is specifying default values for multiple optional variables. For 
example, for `(a &optional (b 2) (c 3))` in Pcase, I would write

     (or `(,a ,b ,c)
         (and `(,a ,b)
              (let c 3))
         (and `(,a)
              (let c 3)
              (let b 2)))

or

     `(,a . ,(or `(,b . ,(or `(,c)
                             (let c 3)))
                 (and (let b 2)
                      (let c 3))))

in which there is repetition in the default values. Is there a better 
way to specify default values for optional elements?

Thank you.