Failure compiling ghc-mtl with ghc-7.8.{2,3}

[Edward Z. Yang]

The last time I saw this error, it was because the package database
was messed up (there was an instance of MonadIO in scope, but it
was for the wrong package.)  However, I don't know what the source
of the problem is here.

Edward

Excerpts from i hamsa's message of 2014-07-20 08:26:52 +0100:
> I was trying to upgrade to ghc-7.8 the other day, and got this
> compilation failure when building ghc-mtl-1.2.1.0 (see the end of the
> message).
> 
> I'm using the haskell overlay on Gentoo Linux straight out of the box,
> no local cabal installations of anything.
> 
> Now I was told that other people can compile ghc-mtl with 7.8 just
> fine, so there must be something broken in my specific configuration.
> What would be an effective way to approach the situation?
> 
> In the sources I see that an instance of MonadIO GHC.Ghc does exist. I
> don't understand these errors. Are there multiple different MonadIO
> classes in different modules?
> 
> Thank you and happy hacking.
> 
> Now the errors:
> 
> Control/Monad/Ghc.hs:42:15:
>     No instance for (GHC.MonadIO Ghc)
>       arising from the 'deriving' clause of a data type declaration
>     Possible fix:
>       use a standalone 'deriving instance' declaration,
>         so you can specify the instance context yourself
>     When deriving the instance for (GHC.ExceptionMonad Ghc)
> 
> Control/Monad/Ghc.hs:46:15:
>     No instance for (MonadIO GHC.Ghc)
>       arising from the 'deriving' clause of a data type declaration
>     Possible fix:
>       use a standalone 'deriving instance' declaration,
>         so you can specify the instance context yourself
>     When deriving the instance for (MonadIO Ghc)
> 
> Control/Monad/Ghc.hs:49:15:
>     No instance for (GHC.MonadIO Ghc)
>       arising from the 'deriving' clause of a data type declaration
>     Possible fix:
>       use a standalone 'deriving instance' declaration,
>         so you can specify the instance context yourself
>     When deriving the instance for (GHC.GhcMonad Ghc)
>