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Re: canonical name ending "-p"
From: |
Pascal J. Bourguignon |
Subject: |
Re: canonical name ending "-p" |
Date: |
Fri, 18 Mar 2011 18:56:37 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/23.2 (gnu/linux) |
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 {}.