GNU bug report logs - #31311
27.0; doc of `pcase'

Previous Next

Full log

View this message in rfc822 format

From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: 31311 <at> debbugs.gnu.org
Subject: bug#31311: 27.0; doc of `pcase'
Date: Sat, 12 May 2018 15:54:18 +0200

Thien-Thi Nguyen <ttn <at> gnu.org> writes:

>   a.  (pcase t   ('t   'true))  => true
>   b.  (pcase nil ('nil 'false)) => false
>   c.  (pcase t   (t    'true))  => true
>   d.  (pcase nil (nil  'false)) |= ERROR: Unknown pattern ‘nil’
>
> and wonder if/how to approach this corner case in the docs.  I
> suppose before anything, i'd like to confirm that c. and d. are
> correct and intended (i believe a. and b. to be correct and
> intended, but maybe i'm missing something there, too!).

That's all correct and intended - see 3ef31167.  t was originally
intended as catchall pattern equivalent to _ but has been deprecated
(since it was confusing).  Try e.g.

  (pcase nil
   (t t))

You get a message when you use it, but when evaluating and not when
compiling (so you must look in the *Messages* buffer - I don't think
this is intended...).  A catch-nothing pattern is not useful, so nil is
just invalid.

I once asked Stefan if t and nil could be handled like the ATOM case,
but he didn't want to do this (backward compatibility, don't recall).


Michael.

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.