I'm struggling with a problem related to generic programming, boiling down to this: ----snip---- {-# LANGUAGE StandaloneDeriving, KindSignatures, UndecidableInstances #-} data Foo (v :: * -> *) = Foo (v Bar) deriving instance (Show (v Bar)) => Show (Foo v) data Bar = Bar deriving Show ----snip---- So the Show instance for Foo v wants the rather specific Show instance for v Bar. But now imagine that v is always something like Maybe, itself done using something like this presumably: instance Show a => Show (Maybe a) where ... Can I somehow state as a constraint that Foo v has a Show instance if v is a type constructor like Maybe, i.e. once with a Show instances for any arguments that has a Show instance? (You can't see *why* I want to do this in this example - I actually have something like 100 nested datatypes which thread v through, and *every single one of them* adds to the context I need for every one of them.) I feel I want to write something like this: deriving instance (Show a => Show (v a)) => Show (Foo v) Is there a way to do this? Help would be much appreciated! -- Regards, Mike