From: "Pascal J. Bourguignon" <pjb@informatimago.com>
To: help-gnu-emacs@gnu.org
Subject: Re: canonical name ending "-p"
Date: Fri, 18 Mar 2011 18:56:37 +0100 [thread overview]
Message-ID: <878vwcia7e.fsf@kuiper.lan.informatimago.com> (raw)
In-Reply-To: mailman.8.1300470004.7441.help-gnu-emacs@gnu.org
ken <gebser@mousecar.com> writes:
> Lots of things in elisp end in "-p"... is there some particular meaning
> in this?
They are predicates. Predicates are functions that usually take a
single argument, and return a boolean attribute of this argument.
In some cases, it may be a generalized boolean, with an interesting
value for true.
For example, (digit-char-p ?9) --> 9 which is true, but more
interesting, since it's also the numeric value of the digit character.
Notice also, as this example shows, that some classical predicates don't
have the p or -p suffix. For example: atom
(atom x) == (not (consp x))
A single word predicate would end with p, a multiword one with -p:
integerp
upper-case-p
--
__Pascal Bourguignon__ http://www.informatimago.com/
A bad day in () is better than a good day in {}.
next parent reply other threads:[~2011-03-18 17:56 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.8.1300470004.7441.help-gnu-emacs@gnu.org>
2011-03-18 17:56 ` Pascal J. Bourguignon [this message]
2011-03-18 20:36 ` canonical name ending "-p" Evans Winner
2011-03-19 4:32 ` rusi
2011-03-18 17:39 ken
2011-03-18 18:11 ` Allan Gottlieb
2011-03-18 18:18 ` Perry Smith
2011-03-18 18:31 ` Drew Adams
2011-03-18 18:20 ` Tom Rauchenwald
2011-03-18 18:31 ` Drew Adams
[not found] ` <mailman.12.1300472466.7441.help-gnu-emacs@gnu.org>
2011-03-19 1:52 ` Stefan Monnier
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