PS In regards to the original http://okmij.org/ftp/Haskell/deepest-functor.lhs Am I right that the following code from the sample: class IsCollection t coll | t -> coll instance IsCollection (m a) (m ()) instance TypeCast Atom coll => IsCollection t coll class TypeCast a b | a -> b, b->a where typeCast :: a -> b class TypeCast' t a b | t a -> b, t b -> a where typeCast' :: t->a->b class TypeCast'' t a b | t a -> b, t b -> a where typeCast'' :: t->a->b instance TypeCast' () a b => TypeCast a b where typeCast x = typeCast' () x instance TypeCast'' t a b => TypeCast' t a b where typeCast' = typeCast'' instance TypeCast'' () a a where typeCast'' _ x = x may now be reduced to: class IsCollection t coll | t -> coll instance IsCollection (m a) (m ()) instance (Atom ~ coll) => IsCollection t coll ? -- View this message in context: http://www.nabble.com/Deepest-polymorphic-functor-tp24709303p24748240.html Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.