I start some ip networks related work in haskell and wrote two basic classes Location and Packet. Before writing IpLocation and IpPacket instances i have written simple TestLocation and TestPacket instances just to compile this and check for errors in class definitions. But looks like i misunderstand some haskell principles... ====================================================== class Location a where point :: a -> String class Packet a where source, destination :: Location b => a -> b size :: Num b => a -> b ------------------------------------------------------ data TestLocation = TestSource | TestDestination data TestPacket = TestPacket ------------------------------------------------------ instance Location TestLocation where point a = "location" instance Packet TestPacket where source p = TestSource destination p = TestDestination size p = 99 $ hugs Test.hs ERROR Test.hs:20 - Inferred type is not general enough *** Expression : source *** Expected type : (Packet TestPacket, Location a) => TestPacket -> a *** Inferred type : (Packet TestPacket, Location TestLocation) => TestPacket -> TestLocation ====================================================== But if i remove source and destination from class and instance definitions alone "size" compiles well. How write this in a haskell way?