[Haskell-cafe] Re: Guards with do notation?
Joachim Breitner
mail at joachim-breitner.de
Tue Oct 24 07:20:40 EDT 2006
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
e-Mail: mail at joachim-breitner.de
Homepage: http://www.joachim-breitner.de
ICQ#: 74513189
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