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: [EMAIL PROTECTED]
Homepage: http://www.joachim-breitner.de
ICQ#: 74513189
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