Stefan,
I tried lexically scoped type variables, but to no avail:
instance forall a b. (C a, C b) => C (a, b) where type T (a, b) = (T a, T b) val = (val :: T a, val :: T b)
The problem is ambiguity. The type checker can't determine which val function to use, i.e. which dictionary to pass to val. Assume: instance C Int where type T Int = Int val = 0 instance C Bool where type T Bool = Int val = 1 Now, if you want some val :: Int, which one do you get? The one of C Int of C Bool? Depending on the choice you may get a different result. We can't have that in a deterministic functional language. Hence the error. Adding a type signature doesn't change the matter. Providing an additional argument, as you propose, resolves the ambiguity. I hope this helps. Cheers, Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers@cs.kuleuven.be url: http://www.cs.kuleuven.be/~toms/