GNU bug report logs - #43100
28.0.50; pcase not binding variables conditionally

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From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Pip Cet <pipcet <at> gmail.com>
Subject: bug#43100: closed (Re: bug#43100: 28.0.50; pcase not binding
 variables conditionally)
Date: Tue, 02 Mar 2021 13:39:02 +0000

[Message part 1 (text/plain, inline)]

Your bug report

#43100: 28.0.50; pcase not binding variables conditionally

which was filed against the emacs package, has been closed.

The explanation is attached below, along with your original report.
If you require more details, please reply to 43100 <at> debbugs.gnu.org.

-- 
43100: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=43100
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

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From: Pip Cet <pipcet <at> gmail.com>
To: 43100-done <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: Re: bug#43100: 28.0.50; pcase not binding variables conditionally
Date: Tue, 2 Mar 2021 13:38:01 +0000

On Sat, Aug 29, 2020 at 9:42 AM Pip Cet <pipcet <at> gmail.com> wrote:
>
> I'm having trouble with pcase's behavior.
>
> (pcase "a"
>   ((or (pred symbolp) name)
>    (let ((foo 'bar)) name)))

Stefan fixed this in 165353674e5fe7109ba9cbf526de0333902b7851, so I'm
closing this bug.

Pip

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From: Pip Cet <pipcet <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>
Subject: 28.0.50; pcase not binding variables conditionally
Date: Sat, 29 Aug 2020 09:41:10 +0000

[Message part 4 (text/plain, inline)]

I'm having trouble with pcase's behavior.

(pcase "a"
  ((or (pred symbolp) name)
   (let ((foo 'bar)) name)))

throws an error. It shouldn't. (Note that the dummy "let" is necessary
to force the pcase code generation to use a function call).

I believe the culprit is the code around this comment in pcase.el

;; If some of `vars' were not found in `prevvars', that's
;; OK it just means those vars aren't present in all
;; branches, so they can be used within the pattern
;; (e.g. by a `guard/let/pred') but not in the branch.

I believe that's incorrect: using the variable in a condition-case
should work, as should conditional shadowing of an existing binding,
as in this case:

(let ((name "default"))
  (pcase "a"
    ((or (pred symbolp) name)
     name)))

(which works), or this case:

(let ((name "default"))
  (pcase "a"
    ((or (pred symbolp) name)
     (let ((foo 'bar)) name))))

(which doesn't).

I believe the right fix is not to share code for the same branch if it
uses different variables, as in the attached patch. It's possible this
increases codegen complexity in some construed cases, but in practice
that shouldn't be a problem.

[0001-Allow-variable-bindings-to-differ-across-pcase-alter.patch (text/x-patch, attachment)]

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