[Haskell-cafe] Re: Guards with do notation?

[Joachim Breitner]

Hi,

Am Dienstag, den 24.10.2006, 12:48 +0200 schrieb Benjamin Franksen:
> > 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)

Then why is the “guard” function, which can be used in a way to
implement what Misha wants, only available in MonadPlus, and not in
Monad?

Greetings,
Joachim
-- 
Joachim Breitner
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