[Haskell-cafe] MonadPlus instance for IO

Daniel Fischer daniel.is.fischer at web.de
Wed Feb 2 08:41:42 EST 2005

Am Mittwoch, 2. Februar 2005 14:17 schrieb David Roundy:
> I'm sure I'm doing something stupid, but somehow ghc isn't recognizing the
> existance of a MonadPlus instance for IO:
> DarcsIO.lhs:48:
>     No instance for (MonadPlus IO)
>       arising from use of `mplus' at DarcsIO.lhs:48
>     In the definition of `foo':
>         foo = (fail "aaack") `mplus` (fail "foobar")
> Any idea what I'm doing wrong? I'm using
> $ ghc --version
> The Glorious Glasgow Haskell Compilation System, version 6.2.2
> packaged for debian.

Probably you haven't imported 'Control.Monad.Error', where the instance is 
defined. I did and all went well.


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