Re: predicates

["Pascal J. Bourguignon" <pjb@informatimago.com>]

Emanuel Berg <embe8573@student.uu.se> writes:

> Alex Bennée <alex.bennee@linaro.org> writes:
>
>> It's a predicate, giving a true/false response.
>
> This is how I understand it:
>
> In practice predicates can return anything.
>
> nil, and the empty list, should be interpreted as *false*.

They return a generalized boolean, which can indeed be anything.

> Computers are deterministic but that doesn't mean they
> are consistent.

One notable predicate that returns something useful not obtainable
otherwise, is:
              
   (digit-char-p ?4) --> 4

You could use: 
 
    (ignore-errors (parse-integer (string ?4))) --> 4

but parse-integer uses digit-char-p which is the more fundamental
operation.

-- 
__Pascal Bourguignon__                 http://www.informatimago.com/
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