From: Barry Margolin <barmar@alum.mit.edu>
To: help-gnu-emacs@gnu.org
Subject: Re: Why is booleanp defined this way?
Date: Fri, 17 Apr 2015 19:20:44 -0400 [thread overview]
Message-ID: <barmar-43C938.19204417042015@88-209-239-213.giganet.hu> (raw)
In-Reply-To: mailman.946.1429302909.904.help-gnu-emacs@gnu.org
In article <mailman.946.1429302909.904.help-gnu-emacs@gnu.org>,
Marcin Borkowski <mbork@mbork.pl> wrote:
> Hi all,
>
> this is what I found in subr.el:
>
> ,----
> | (defun booleanp (object)
> | "Return t if OBJECT is one of the two canonical boolean values: t or nil.
> | Otherwise, return nil."
> | (and (memq object '(nil t)) t))
> `----
>
> Seemingly, it doesn't make much sense: what is the purpose of saying
>
> (and (whatever) t)
>
> instead of just
>
> (whatever)
>
> for a predicate? Of course, this "normalizes" any "truthy" value to
> "t", but is it really needed for anything (except perhaps being
> elegant)?
I guess they felt that the result of booleanp should *be* booleanp. :)
While it probably doesn't matter when you're using it in a program,
since you'll usually be using it as part of a conditional operator (if,
cond, when), they might have felt it would be confusing when people used
it interactively:
(booleanp nil) => (nil t)
(booleanp t) => (t)
(booleanp something-else) => nil
As a general convention, predicates usually just return t or nil, unless
there's a useful non-nil value to return when it's true. In the case of
memq, returning the tail of the list starting with the match was felt to
be useful. But it's hard to see how returning those little lists instead
of t would be helpful to anyone calling booleanp.
--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
next prev parent reply other threads:[~2015-04-17 23:20 UTC|newest]
Thread overview: 33+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <mailman.946.1429302909.904.help-gnu-emacs@gnu.org>
2015-04-17 20:55 ` Why is booleanp defined this way? Pascal J. Bourguignon
2015-04-17 23:20 ` Barry Margolin [this message]
2015-04-17 23:30 ` Emanuel Berg
2015-04-18 0:43 ` Pascal J. Bourguignon
2015-04-18 3:13 ` Barry Margolin
2015-04-18 3:12 ` Barry Margolin
2015-04-18 2:01 ` Rusi
2015-04-18 2:23 ` Emanuel Berg
2015-04-18 2:33 ` Rusi
2015-04-18 2:55 ` Emanuel Berg
2015-04-18 3:11 ` Barry Margolin
2015-04-18 3:35 ` Rusi
2015-04-18 4:56 ` Barry Margolin
2015-04-19 23:08 ` Emanuel Berg
2015-04-19 23:00 ` Emanuel Berg
2015-04-18 21:24 ` Emanuel Berg
2015-04-18 7:52 ` Marcin Borkowski
[not found] ` <mailman.997.1429343558.904.help-gnu-emacs@gnu.org>
2015-04-18 12:43 ` Rusi
2015-04-18 4:09 ` Pascal J. Bourguignon
2015-04-18 5:00 ` Rusi
2015-04-18 3:50 ` Pascal J. Bourguignon
2015-04-18 5:03 ` Stefan Monnier
2015-04-17 20:34 Marcin Borkowski
2015-04-17 20:49 ` Jorge A. Alfaro-Murillo
[not found] ` <mailman.962.1429303822.904.help-gnu-emacs@gnu.org>
2015-04-17 23:06 ` Emanuel Berg
2015-04-18 0:41 ` Pascal J. Bourguignon
2015-04-18 1:06 ` Emanuel Berg
2015-04-18 1:23 ` Pascal J. Bourguignon
2015-04-18 7:44 ` Marcin Borkowski
2015-04-18 8:37 ` Stefan Nobis
2015-04-19 23:15 ` Emanuel Berg
2015-04-18 2:37 ` Drew Adams
2015-04-18 6:13 ` Tassilo Horn
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