Hi, I have a Haskell problem that keeps cropping up and I wondered if there was any solution/work-around/dirty-hack that could help. I keep wanting to define class instances for things like Functor or Monad, but with restrictions on the inner type. I'll explain with an example, because I find explaining this in words a bit difficult. Let's say I want to create a Monad instance for Set akin to that for lists: == import Data.Set import Prelude hiding (map) instance Monad Set where return = singleton m >>= f = fold union empty (map f m) -- Error: Could not deduce (Ord a, Ord b) from the context (Monad Set) == Everything fits (I think) -- except the type-class constraints. Obviously my Monad instance won't work if you have things inside the set that aren't Ord, but I can't work out how to define a restricted instance that only exists for types that have Ord instances. I can't express the constraint on the instance because the a and b types of return and >>= aren't visible in the class header. Shifting the constraint to be present in the type doesn't seem to help either (e.g. newtype Ord a => MySet a = MySet (Set a)...). Is there any way to get such instances as the one for Set working? I cannot carry around a compare function myself in a data type that wraps Set, because return cannot create such functions without the original type-class instance. I don't actually need a Monad for Set, but it neatly demonstrates my problem of wanting constraints on the type inside a Monad (or a Functor, or an Applicative, etc). I worked around a similar problem with Functor by opting for a new Functor-like type-class with the constraints, but doing that with Monad rules out using all the monad helper functions (liftM, mapM, etc), and the do notation, which would be a step too far. All suggestions are welcome, no matter how hacky, or how many GHC extensions are required :-) (provided they don't break all the other monads, e.g. redefining the signature of Monad). Thanks, Neil.