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bug#7086: `booleanp' return value is multi-valued list
From: |
Lawrence Mitchell |
Subject: |
bug#7086: `booleanp' return value is multi-valued list |
Date: |
Fri, 24 Sep 2010 10:28:41 +0100 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.0.50 (usg-unix-v) |
MON KEY wrote:
> This one matters. It is the penultimate Emacs lisp predicate.
> It tests for a boolean value yet it returns a list.
Which you can happily dispatch on:
(if (booleanp foo)
(if (null foo)
(do-something-if-foo-is-nil)
(do-something-if-foo-is-t))
(do-something-if-foo-is-neither-nil-nor-t))
It's hard to see what else is required
[...]
> On Thu, Sep 23, 2010 at 4:12 AM, Eli Zaretskii <eliz@gnu.org> wrote:
[...] MON KEY claims that (funcall (lambda (x) (booleanp x)))
giving an error is somehow problematic.
>> Exactly! See the above node in the manual.
>> But this has nothing to do with booleanp.
> It has everything to do with it. These are just the types of
> situations where interrogating a boolean is what is wanted.
What?
(funcall (lambda (x) (1+ x)))
=> (wrong-number-of-arguments (lambda (x) (1+ x)) 0)
(funcall (lambda (x) (booleanp x)))
=> (wrong-number-of-arguments (lambda (x) (booleanp x)) 0)
Calling a function expecting an argument without one is an error,
irrespective of the function involved.
Do you in fact want:
(require 'cl)
(defun* foo (&optional (x nil got-x-p))
(if got-x-p
(do-something-if-x-was-an-argument x)
(do-something-different)))
[...]
--
Lawrence Mitchell <wence@gmx.li>