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)