I have the following three programs: class Foo a b instance Foo (a -> b) (a -> [b]) instance Foo a a class Bar a b | a -> b instance Bar (a -> b) (a -> b) instance Bar a a class Baz a b | a -> b instance Baz (a -> b) (a -> [b]) instance Baz a a When compiled in ghc 6.4 (with -fglasgow-exts -fallow-overlapping-instances -fallow-undecidable-instances) Foo and Bar compile fine, but Baz fails with this error: Baz.hs:2:0: Functional dependencies conflict between instance declarations: Baz.hs:2:0: instance Baz (a -> b) (a -> [b]) Baz.hs:3:0: instance Baz a a This is how I interpret the error: The fundep says "a uniquely determines b", but if you have `Baz (Int -> Int) b`, b is `Int -> [Int]` according to the first instance and `Int -> Int` according to the second instance. b isn't uniquely determined by a, so the functional dependency isn't functional -- thus the conflict. 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. Is this a bug, or am I confused? Dan