[Haskell-cafe] GHC 7.4: Expected behavior or bug?

27 Dec 2011

      Thanks to Mark Wright for pointing this out[1].

We have the equivalent of the following code in persistent:

{-# LANGUAGE MultiParamTypeClasses #-}
data Key backend entity = Key

class Monad (b m) => Foo b m where
    func :: b m (Key b m)

This code works fine with GHC 7.0, but I get the following message from GHC 7.4:

    Expecting two more arguments to `b'
    In the type `b m (Key b m)'
    In the class declaration for `Foo'

Is this expected behavior, or a bug? If the former, what would be a
possible workaround?

Thanks,
Michael

[1] https://github.com/yesodweb/persistent/issues/31

Michael Snoyman