GNU bug report logs - #14773
24.2; pcase

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: John Williams <jrw <at> google.com>
To: bug-gnu-emacs <at> gnu.org
Subject: 24.2; pcase
Date: 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!


In GNU Emacs 24.2.1 (x86_64-pc-linux-gnu, GTK+ Version 2.24.10)
 of 2013-03-05 on touchstone.was.corp.google.com, modified by Debian
Windowing system distributor `The X.Org Foundation', version 11.0.11103000
Configured using:
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Important settings:
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Major mode: Fundamental

Minor modes in effect:
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Recent input:
M-P C-e C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b 
C-b C-b C-b C-b C-b ) <return> q M-: M-P C-a C-e C-b 
C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b M-p 
M-p M-p M-p M-p <return> M-: M-P C-a C-f C-f C-f C-f 
C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f C-f 
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l e f t - b r a n c h - v a r M-f M-f M-f M-f M-f M-f 
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g h t - b r a n c h - v a r C-e <return> M-: M-P C-e 
C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b C-b 
C-b C-b C-b C-b C-b C-b <backspace> v a r C-e <return> 
<down-mouse-4> <mouse-4> <down-mouse-4> <mouse-4> <double-down-mouse-4> 
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<help-echo> <help-echo> <help-echo> <help-echo> <help-echo> 
<help-echo> <menu-bar> <help-menu> <send-emacs-bug
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Recent messages:
(b 2)
Mark set
let: End of file during parsing
Entering debugger...
Back to top level.
(let ((x 1)) (if (let* ((var nil)) var) (let ((var nil)) (list (quote a) var)) (let* ((x 2)) (if t (let ((var x)) (list (quote b) var)) nil))))
(let ((x 1)) (let* ((x (f))) (if (let* ((var x)) var) (let ((var x)) (list (quote a) var)) (let* ((x (g))) (if t (let ((var x)) (list (quote b) var)) nil)))))
(let ((x 1)) (let* ((x (left-branch-var))) (if (let* ((var x)) var) (let ((var x)) (list (quote a) var)) (let* ((x (right-branch-var))) (if t (let ((var x)) (list (quote b) var)) nil)))))
(let ((x 1)) (let* ((x (left-branch-var))) (if (let* ((var x)) var) (let ((var x)) (list (quote a) var)) nil)))
byte-code: Beginning of buffer [3 times]
byte-code: Beginning of buffer

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Message #8 received at 14773 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: John Williams <jrw <at> google.com>
Cc: 14773 <at> debbugs.gnu.org
Subject: Re: bug#14773: 24.2; pcase
Date: Wed, 03 Jul 2013 05:34:59 -0400

> (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

Message #11 received at 14773 <at> debbugs.gnu.org (full text, mbox):

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
Cc: John Williams <jrw <at> google.com>, 14773 <at> debbugs.gnu.org
Subject: Re: bug#14773: 24.2; pcase
Date: Wed, 03 Jul 2013 18:53:38 +0200

Stefan Monnier <monnier <at> IRO.UMontreal.CA> writes:

> > (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.

Why is it unreachable?

(defvar left nil)
(defvar right t)

(pcase 1
  ((and (let var left) (guard var)) 'left)
  (_ 'right))

==> 'right

But

(pcase 1
  ((and (let var left) (guard var)) 'left)
  ((and (let var right) (guard var)) 'right))

==> nil

It should also return right, no?


Michael.

Message #14 received at 14773 <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: John Williams <jrw <at> google.com>, 14773 <at> debbugs.gnu.org
Subject: Re: bug#14773: 24.2; pcase
Date: Wed, 03 Jul 2013 18:53:11 -0400

> Why is it unreachable?

Right, it's not, duh!

It's clearly too aggressive,


        Stefan

Message #19 received at 14773-done <at> debbugs.gnu.org (full text, mbox):

From: Stefan Monnier <monnier <at> IRO.UMontreal.CA>
To: Michael Heerdegen <michael_heerdegen <at> web.de>
Cc: 14773-done <at> debbugs.gnu.org, John Williams <jrw <at> google.com>
Subject: Re: bug#14773: 24.2; pcase
Date: Mon, 08 Jul 2013 17:56:41 -0400

> (pcase 1
>   ((and (let var left) (guard var)) 'left)
>   ((and (let var right) (guard var)) 'right))
> ==> nil
> It should also return right, no?

It should be fixed now.  Thank you,


        Stefan

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