Subject: Re: bug#7086: `booleanp' return value is multi-valued list

From: Lawrence Mitchell <[email-masked]>
Date: Fri, 24 Sep 2010 10:28:41 +0100

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 <[email-masked]> 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 <[email-masked]>