Daniel Brown wrote:
class Baz a b | a -> b instance Baz (a -> b) (a -> [b]) instance Baz a a ...but Baz fails with this error...
When confronted with overlapping instances, the compiler chooses the most specific one (if it is unique), e.g. `Baz (a -> b) (a -> [b])` is more specific than `Baz a a`.
But it seems that the combination of the two features is broken: if the most specific instance is chosen before checking the functional dependency, then the fundep is satisfied; if the fundep is checked before choosing the most specific instance, then it isn't.
There is a way to write your example in Haskell as it is. The key idea is that functional dependencies can be given *per instance* rather than per class. To assert such dependencies, you need the `TypeCast' constraint, which is throughly discussed in the HList technical report. http://homepages.cwi.nl/~ralf/HList/ The following is the complete code for the example, which runs on GHC 6.4. We see that the functional dependencies work indeed: the compiler figures out the types of test1 and test2 and test3 (and thus resolved overloading) without any type signatures or other intervention on our part. {-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-} module Foo where {- class Baz a b | a -> b instance Baz (a -> b) (a -> [b]) instance Baz a a -} -- No functional dependencies here! class Baz a b where baz :: a -> b -- Rather, dependencies are here instance TypeCast a r => Baz a r where baz a = typeCast a instance TypeCast (a -> [b]) r => Baz (a -> b) r where baz f = let r = \a -> [f a] in typeCast r -- Chooses the instance Baz a a test1 = baz True -- True -- Chooses the instance Baz (a -> b) (a -> [b]) test2 = (baz show) (1::Int) -- ["1"] test3 x = (baz show) x test3' = test3 (Just True) -- ["Just True"] -- copied verbatim from the HList library 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