ah - of course... so if I have an instance instance A m m where f = id all works well! I still have a problem though - perhaps it is also possible to avoid... I am trying to define a parametric continuation monad transformer (from a paper by Ralf Hinze...) however the functions as given have type errors... in particular the following (which is similar to the simple case I just gave) class (Monad tm,MonadT (t tm)) => MonadT t tm where up ::tm ta -> t tm ta down :: t tm a -> tm ta we define the type for our parametric continuation: data ContT ans m a = CT ((a -> m ans) -> m ans) instance Monad m => MonadT (ContT ans) m where up m = CT $ \kappa -> m >>= kappa down (CT m) = m return however this gives a unification error (less polymorphic than expected) for the definition of down (that `a' is unified with `ans')?? Regards, Keean Schupke. Martin Norbäck wrote:
fre 2002-09-06 klockan 15.01 skrev MR K P SCHUPKE:
If I have a class:
class A m n where f :: m a -> n a
I can declare instances
instance A Maybe Maybe where f = id
instance A [] [] where f = id
but if I try and put a default method: f = id
I get a unification error (less polymorphic than expected).This seems a little odd - is there a work around for this?
Because that default method only works when m=n, which it's not in the general case.