From: <tomas@tuxteam.de>
To: help-gnu-emacs@gnu.org
Subject: Re: pcase and minus-sign
Date: Wed, 30 Nov 2016 14:39:05 +0100 [thread overview]
Message-ID: <20161130133905.GA21731@tuxteam.de> (raw)
In-Reply-To: <ae445475-96bc-fcd8-0a51-0b2236eb896a@easy-emacs.de>
-----BEGIN PGP SIGNED MESSAGE-----
Hash: SHA1
On Wed, Nov 30, 2016 at 02:30:11PM +0100, Andreas Röhler wrote:
>
>
> On 30.11.2016 13:38, Joost Kremers wrote:
> >
> >On Wed, Nov 30 2016, Andreas Röhler wrote:
> >>Hi,
> >>
> >>see code below. With numerical argument "1", first pattern is
> >>matched as expected.
> >>
> >>However without arg the minus is matched - the second pattern,
> >>not the default "_" at last.
> >>
> >>Any explanation?
> >>
> >>(defun foo (arg)
> >> (interactive "P")
> >> (pcase arg
> >> (1 (message "%s" "ARG was `1'"))
> >> (- (message "%s" "ARG was minus-sign"))
> >> (_ (message "%s" "ARG not minus-sign"))))
> >
> >Probably because - is a symbol and hence a variable. It works if
> >you quote it:
> >
> >(defun foo (arg)
> > (interactive "P")
> > (pcase arg
> > (1 (message "%s" "ARG was `1'"))
> > ('- (message "%s" "ARG was minus-sign"))
> > (_ (message "%s" "ARG not minus-sign"))))
> >
> >
> >
>
> Thanks, that helps. Seems it relates to the following in docstring:
>
> SYMBOL matches anything and binds it to SYMBOL.
>
> Now if I use some arbitrary char, like "a",
>
> (defun foo (arg)
> (interactive "P")
> (pcase arg
> (a (message "%s" "ARG was `a'"))
> (1 (message "%s" "ARG was `1'"))
> ('- (message "%s" "ARG was minus-sign"))
> (_ (message "%s" "ARG not minus-sign"))))
>
> It picks that a-branch at any case - as documented but strange.
On the contrary -- that's what makes pcase so enticing. You can
match *and* bind parts of the match at the same time As in
(pcase arg
...
`(,x . ,y) ; match a pair
(message "Matched a pair: the car is %S and the cdr is %S" x y))
...
Note how the symbols in the pattern function as placeholders to
pick up whatever is in the matched structure. In the case above,
if you feed it ('foo . 123) as "arg", x will be bound to 'foo,
123 to bar).
regards
- -- t
-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.12 (GNU/Linux)
iEYEARECAAYFAlg+1nkACgkQBcgs9XrR2kZUQwCfe0xA9UnxycPgTBUF+rAY7GA0
FucAn1EYDTDoNUbLepRs5it/m6+1xRpn
=7X8T
-----END PGP SIGNATURE-----
next prev parent reply other threads:[~2016-11-30 13:39 UTC|newest]
Thread overview: 14+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-11-30 12:12 pcase and minus-sign Andreas Röhler
2016-11-30 12:38 ` Joost Kremers
2016-11-30 13:30 ` Andreas Röhler
2016-11-30 13:31 ` Michael Heerdegen
2016-11-30 13:39 ` tomas [this message]
2016-11-30 13:56 ` Joost Kremers
2016-11-30 15:23 ` Andreas Röhler
2016-11-30 15:34 ` Michael Heerdegen
2016-11-30 15:48 ` Joost Kremers
2016-12-01 13:59 ` Andreas Röhler
2016-12-02 2:30 ` Stefan Monnier
2016-12-03 8:04 ` Andreas Röhler
2016-12-03 15:43 ` Stefan Monnier
2016-12-05 9:11 ` Andreas Röhler
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20161130133905.GA21731@tuxteam.de \
--to=tomas@tuxteam.de \
--cc=help-gnu-emacs@gnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
Code repositories for project(s) associated with this external index
https://git.savannah.gnu.org/cgit/emacs.git
https://git.savannah.gnu.org/cgit/emacs/org-mode.git
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.