On 4 October 2010 03:40, Michael Vanier
newtype MyMonad a = MyMonad ((StateT (MyData a) (Either SomeError) a)) deriving (Monad, MonadState (MyData a), MonadError SomeError, Typeable)
I think it's the `a'. I think it needs to be a concrete type. E.g. the following is OK: newtype MyMonad a = MyMonad ((StateT (MyData ()) (Either SomeError) a)) deriving (Monad, MonadState (MyData ()), MonadError SomeError, Typeable) But newtype MyMonad a = MyMonad ((StateT (MyData ()) (Either SomeError) [a])) deriving (Monad, MonadState (MyData ()), MonadError SomeError, Typeable) is not. This reminds me of the restriction that impredicative types remove, but I don't think it's related.
These error messages mean nothing to me. What's going on? Can the more specific code be made to work? This is with ghc 6.12.3.
It seems like eta-reducing `X' or `x' is "enough", but Foo x,, i.e. a parametrized type with a type variable isn't "enough". I think that's what's going on, but I don't know why.