GNU bug report logs - #7086
`booleanp' return value is multi-valued list

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From: Kevin Rodgers <kevin.d.rodgers <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: bug#7086: `booleanp' return value is multi-valued list
Date: Wed, 22 Sep 2010 20:06:02 -0600

On 9/22/10 4:35 PM, Juanma Barranquero wrote:
> On Wed, Sep 22, 2010 at 23:58, MON KEY<monkey <at> sandpframing.com>  wrote:
>
>> "Return non-`nil' if OBJECT is one of the two canonical boolean
>>   values: `t' or `nil'."
>>
>> It should be documented both in the manual and the docstring of `booleanp' that
>> its return value is a list.
>
> Why? It is a type predicate. If anything, it should be changed to
> return t or nil, as "(elisp)2.6 Type Predicates" says:
>
>       A type predicate function takes one argument; it returns `t' if the
>    argument belongs to the appropriate type, and `nil' otherwise.
>
> The fact that it returns nil or a list is an implementation detail.
>
>> Indeed, while I can appreciate why (booleanp nil) returns a two element list
>> I'm unable to reason a rationale for why (booleanp t) doesn't?
>
> Why do you expect any kind of rationale, other than the simple fact
> that it is implemented as
>
>    (memq object '(nil t)))

(and (memq object '(nil t))
     t)

would be better.

-- 
Kevin Rodgers
Denver, Colorado, USA

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