Hello there, I've been experimenting with the use of type dependencies in type classes and have come across something i find surprising. Could it in fact be a bug in the implementation? I'm using Hugs Feb 2001, with switches +o and -98:

class Bug f a r | f a -> r where

bug::f->a->r

instance Bug (Int->r) Int r --instance ... instance (Bug f a r) => Bug f (c a) (c r)

f:: Bug(Int->Int) a r => a->r f = bug(id::Int->Int)

The above compiles fine and at the prompt .. Main> f (f [0::Int]) ...runs with an expected program error that member 'bug' has not been defined. Fine. But Main> f (f (f [0::Int])) -- ...fails to compile with an unresolved overloading: *** ERROR - Unresolved overloading *** Type : (Bug (Int->Int) Int a, Bug(Int->Int) a v => [b] *** Expression : f (f (f [0])) which is a surprise. it appears as though the compiler is failing to exploit the dependency '|f a->r' from which it could infer that 'a' in the above message must in fact be 'Int', etc... Many thanks for investigating this... ________________________________ Jonathon Bell jbell@mathsoft.com MathSoft, Inc. www.mathsoft.com 101 Main St, Cambridge, MA 02142 (617) 577-1017 x745