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bug#67611: [PATCH] Add a Pcase pattern `cl-lambda` equivalent to `cl-des
From: |
Okamsn |
Subject: |
bug#67611: [PATCH] Add a Pcase pattern `cl-lambda` equivalent to `cl-destructuring-bind` |
Date: |
Tue, 05 Dec 2023 02:42:12 +0000 |
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.