On 24/10/06, Joachim Breitner <[EMAIL PROTECTED]> wrote:
Am Dienstag, den 24.10.2006, 00:44 +0300 schrieb Misha Aizatulin:
> hello all,
>
> why is it not possible to use guards in do-expressions like
>
> do
> (a, b) | a == b <- getPair
> return "a and b are equal"
Probably because it is not well-defined for all Monad what a failure is,
i.e. what to do in the other case. or something. Just my guess.
Exactly the same thing that guards do elsewhere? Count the falsity of
the guard as a pattern match failure. It's precisely the same as
doing, e.g.:
do Just (a, b) <- Nothing
return "a and b were Just"
Pattern-match failure in a do-block invokes fail in that monad.
--
-David House, [EMAIL PROTECTED]
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