John Meacham wrote:
however, (Set (a -> a)) is malformed. since a _requirement_ is that Set can only be applied to a type with an Eq constraint so the instance you try to do something like
returnid :: Set (a -> a) -- ^ static error! you need
returnid :: Eq (a -> a) => Set (a -> a)
the instant you introduce 'Set' you introduce the 'Eq' constraint. as long as you are just working on generic monads then there is no need for the Eq constraint.
OK, try this: foo :: (Monad m) => m Int foo = return id >>= (\i -> i 7) fooSet :: Set Int fooSet = foo Since we have (Eq Int), your type-checker should allow this. But your instance implementation of return and (>>=) made assumptions about their arguments that foo does not stick to. -- Ashley Yakeley