Hi, I have been doing an experiment where I use type classes to express static (compile-time) computations in Haskell. With multi-parameter classes and functional dependencies, instance declarations can be used to define function by pattern matching (on the type level). I enclose an example where I define booleans, natural numbers, a less-than-or-equal-to function and a maximum function. In some simple examples (one_lte_two and two_lte_one), it works fine, but in the last example, where I try to compute the maximum of 1 and 2 in Hugs (hugs -98, February 2000), I get the following error message: Type checking ERROR "FuncDeps4.hs" (line 27): Unresolved top-level overloading *** Binding : max_one_two *** Outstanding context : (Lte Z (S Z) b, If b (S (S Z)) (S Z) c) Here I expected that the functional dependencies would allow b to be instantiated to T, and hence c to be instantiated to S (S Z), which would then be the type of max_one_two. Why doesn't this work? Regards, Thomas Hallgren module FuncDeps4 where data T data F class If b t e r | b t e -> r instance If T t e t instance If F t e e data Z data S n type One = S Z type Two = S One class Lte a b c | a b -> c where lte :: a -> b -> c instance Lte Z b T instance Lte (S n) Z F instance Lte a b c => Lte (S a) (S b) c one_lte_two = lte (u::One) (u::Two) -- :: T two_lte_one = lte (u::Two) (u::One) -- :: F class Max a b c | a b -> c where max' :: a -> b -> c instance (Lte a b l,If l b a c) => Max a b c -- This doesn't work without the type signature: max_one_two = max' (u::One) (u::Two) -- :: Two u=undefined -- Just a convenient abbreviation