Joachim Breitner wrote:
> Hi,
>
> 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.
No, fail is indeed a method of class Monad, and it is there exactly for this
reason, i.e. because pattern matching may fail (even without guards, think
of
do
Just a <- ...
) The restriction is there because guards are not allowed in lambda
expressions, for which do-notation is merely syntactic sugar. (Some people
have argued for lifting this restriction in Haskell', see
http://thread.gmane.org/gmane.comp.lang.haskell.prime/1750/focus=1750)
HTH
Ben
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