From: Stefan Monnier <monnier@iro.umontreal.ca>
To: help-gnu-emacs@gnu.org
Subject: Re: How to extract bindings from `pcase-let*`?
Date: Tue, 16 Mar 2021 22:59:32 -0400 [thread overview]
Message-ID: <jwvim5qlcz0.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: a9c76e97-7d8f-80b3-c380-2d98d4783574@protonmail.com
> ;; => ((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>
prev parent reply other threads:[~2021-03-17 2:59 UTC|newest]
Thread overview: 6+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 1:43 How to extract bindings from `pcase-let*`? Okam
2021-03-15 15:38 ` Stefan Monnier
2021-03-16 1:50 ` Okam
2021-03-16 2:45 ` Stefan Monnier
2021-03-17 1:33 ` Okam
2021-03-17 2:59 ` Stefan Monnier [this message]
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