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From: Stefan Monnier <monnier@IRO.UMontreal.CA>
To: John Williams <jrw@google.com>
Cc: 14773@debbugs.gnu.org
Subject: bug#14773: 24.2; pcase
Date: Wed, 03 Jul 2013 05:34:59 -0400	[thread overview]
Message-ID: <jwv1u7gyoq9.fsf-monnier+emacs@gnu.org> (raw)
In-Reply-To: <wec8wqp8k6d5.fsf@tartarus.kir.corp.google.com> (John Williams's message of "Tue, 02 Jul 2013 14:19:02 -0700")

> (require 'pcase)
> (macroexpand
>    '(pcase 1
>       ((and (let var left) (guard var)) 'left)
>       ((and (let var right) (guard var)) 'right)))

> When I run this, the result I get is (let ((x 1)) (if (let* ((var left))
> var) (let ((var left)) (quote left)) nil)).  Note that "right" doesn't
> appear anywhere in the macro expansion!

Since the second branch of your pcase is unreachable, I think it's OK
for pcase to eliminate it.

Note that it output the message "Redundant pcase pattern: (and (let var
right) (guard var))" which explains the behavior.

This said, I can't remember making this optimization so aggressive (it
seems correct here, but it seems difficult to avoid similar incorrect
ones), so I wouldn't be surprised if there are other cases where it gets
it wrong.

Could it be that you had another (real) problem and that the above
example is a (too) simplified test case you extracted from it?


        Stefan





  reply	other threads:[~2013-07-03  9:34 UTC|newest]

Thread overview: 5+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2013-07-02 21:19 bug#14773: 24.2; pcase John Williams
2013-07-03  9:34 ` Stefan Monnier [this message]
2013-07-03 16:53   ` Michael Heerdegen
2013-07-03 22:53     ` Stefan Monnier
2013-07-08 21:56     ` Stefan Monnier

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