The blog article http://www.randomhacks.net/articles/2007/03/15/data-set-monad-haskell-macros describes a variant of the Monad class which allows to restrict the type of the monadic result, in order to be able to make Data.Set an instance of Monad (requiring Ord constraint for the monadic result). The same problem arises for container data structures with restricted element types, where the element type restriction depends on the implementation of the container structure (such as UArray). It would be cumbersome to have several class parts, even more, type constraints in type signatures may reveal implementation details. E.g. constraint (Container c x y z) might be needed for a 'zipWith' function, whereas (Container c y x z) is needed if you use 'zipWith' with swapped arguments. Here is another approach that looks tempting, but unfortunately does not work, and I wonder whether this can be made working. module RestrictedMonad where import Data.Set(Set) import qualified Data.Set as Set class AssociatedMonad m a where class RestrictedMonad m where return :: AssociatedMonad m a => a -> m a (>>=) :: (AssociatedMonad m a, AssociatedMonad m b) => m a -> (a -> m b) -> m b instance (Ord a) => AssociatedMonad Set a where instance RestrictedMonad Set where return = Set.singleton x >>= f = Set.unions (map f (Set.toList x)) GHC says: RestrictedMonad.hs:21:13: Could not deduce (Ord b) from the context (RestrictedMonad Set, AssociatedMonad Set a, AssociatedMonad Set b) arising from use of `Data.Set.unions' at RestrictedMonad.hs:21:13-22 Probable fix: add (Ord b) to the class or instance method `RestrictedMonad.>>=' In the definition of `>>=': >>= x f = Data.Set.unions (map f (Data.Set.toList x)) In the definition for method `RestrictedMonad.>>=' In the instance declaration for `RestrictedMonad Set'