Type families and functional dependencies play very poorly together. The internal GHC mechanisms to implement them are basically entirely separate, so I'm not surprised that this fails. I do think that what you want is type-safe, but just not supported right now.
Richard
On Oct 1, 2014, at 6:14 PM, Brandon Moore
I was trying to make a bridge like this
class MonadStateTF m where type StateOf m :: * instance (MonadState s m) => MonadStateTF m where type StateOF m = s
to allow writing constraints on state monads without a dangling type parameter type NumMonad m = (MonadStateTF m, Num (StateOf m))
Unfortunately, the declaration type StateOF m = s complains that "s" isn't mentioned on the left hand side rather than somehow making use of the fundep.
Brandon _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe