Hello Frank, Saturday, December 24, 2005, 12:02:28 AM, you wrote: F> class P a where F> outl :: a b c -> b F> outr :: a b c -> c F> make :: b -> c -> a b c reusing my answer to David Roundy :))) are you seen in Hugs manual chapter about multi-parameter type classes? they use the following class Collects e ce | ce -> e where empty :: ce insert :: e -> ce -> ce member :: e -> ce -> Bool instance Eq e => Collects e [e] where ... instance Eq e => Collects e (e -> Bool) where ... instance Collects Char BitSet where ... instance (Hashable e, Collects a ce) => Collects e (Array Int ce) where ... may be you can solve your problem by using {-# OPTIONS_GHC -fglasgow-exts #-} class P b c abc | abc -> b, abc -> b where outl :: abc -> b outr :: abc -> c make :: b -> c -> abc instance P b c (P1 b c) where outl (P1 p q) = p outr (P1 p q) = q make = P1 instance P b c (P2 b c) where outl = fst outr = snd make = (,) data P1 p q = P1 p q deriving (Show, Eq, Ord) type P2 = (,) instance (P b c abc) => P [b] [c] [abc] where outl = undefined outr = undefined make = zipWith make although it is a bit too verbose... and have problems with defining outl/outr F> Andrew U. Frank F> Professor, Head of Department i helped Professor? wow! :) -- Best regards, Bulat mailto:bulatz@HotPOP.com