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Re: How to extract bindings from `pcase-let*`?
From: |
Stefan Monnier |
Subject: |
Re: How to extract bindings from `pcase-let*`? |
Date: |
Tue, 16 Mar 2021 22:59:32 -0400 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/28.0.50 (gnu/linux) |
> ;; => ((1 4) (3 6))
> (loopy (flag pcase)
> (list elem '((1 (2 3)) (4 (5 6))))
> (collect `(,a (,_ ,b)) elem))
>
> in which the `collect` expression expands into something like
>
> (setq a (append a (list some-value-from-elem)))
> (setq b (append b (list some-other-value-from-elem)))
>
> where `some-value-from-elem` and `some-other-value-from-elem` are
> determined by Pcase, Dash, or other destructuring systems.
>
> Do you think that this is doable using Pcase?
You could do something like:
(pcase--u `((,(pcase--match INCOMING (pcase--macroexpand PATTERN))
,(lambda (vars)
`(progn
. ,(mapcar (lambda (v) `(push ,(cadr v) ,(car v)))
vars))))))
where INCOMING is the incoming data (`elem` in your above example).
As the `--` in there suggest, this is digging into pcase's internals.
I think this code will work with Emacs-28 but the `cadr` would need to
be replaced with `cdr` in earlier versions.
Also, this does a `pcase` rather than a `pcase-let` so it will just
silently do nothing if the pattern doesn't match. You can force the
`pcase-let` semantics with something like:
(pcase--u `((,(pcase--match INCOMING
(pcase--macroexpand
`(or ,PATTERN pcase--dontcare)))
,(lambda (vars)
`(progn
. ,(mapcar (lambda (v) `(push ,(cadr v) ,(car v)))
vars))))))
See IELM session below. I suggest you `M-x report-emacs-bug` to request
that this functionality be made accessible without having to rely on
internals (and put me in the `X-Debbugs-Cc` when you do that).
Stefan
PS: I used `push` rather than your `setq+append+list` since it is
algorithmically much better behaved (linear instead of quadratic
complexity).
ELISP> (pcase--u `((,(pcase--match 'elem
(pcase--macroexpand
'(or `(,a (,_ ,b)) pcase--dontcare)))
,(lambda (vars)
`(progn
. ,(mapcar (lambda (v) `(push ,(cadr v) ,(car v)))
vars))))))
(progn
(ignore (consp elem))
(let* ((x1507 (car-safe elem))
(x1508 (cdr-safe elem)))
(progn (ignore (consp x1508))
(let* ((x1509 (car-safe x1508)))
(progn (ignore (consp x1509))
(let* ((x1511 (cdr-safe x1509)))
(progn (ignore (consp x1511))
(let* ((x1512 (car-safe x1511))
(x1513 (cdr-safe x1511)))
(progn (ignore (null x1513))
(let* ((x1514 (cdr-safe x1508)))
(progn (ignore (null x1514))
(progn (push x1512 b)
(push x1507 a)))))))))))))
ELISP>