Ivan, I don't yet know how to explain this formally, but I know how to fix your problem.. You need to add a parameter to the Packet class so the compiler knows what type to use for the Location. .. The following code works for me.. You'll need to use a compiler/interpreter which supports multi-parameter type classes.. like ghci with the glasgow extensions turned on ----------------------------------------- class Location a where point :: a -> String class Packet a loc where source :: Location loc => a -> loc destination :: Location loc => a -> loc size :: Num b => a -> b ------------------------------------------------------ data TestLocation = TestSource | TestDestination data TestPacket = TestPacket ------------------------------------------------------ instance Location TestLocation where point a = "location" instance Packet TestPacket TestLocation where source p = TestSource destination p = TestDestination size p = 99