I've managed to crack something that always annoyed me when I used to do network programming. However, Hugs and GHC behave differently. I'd be interested in views on this approach and also which implementation behaves correctly. Suppose I want to send an ICMP packet. The first byte is the type and the second byte is the code. Furthermore, the code depends on the type. Now you know at compile time that you can't use codes for one type with a different type. However, in Pascal (which is what I used to use) you only seemed to be able to carry out run time checks. Here's a way I came up with for checking at compile time in Haskell. module Main(main) where -- ICMP has many more values for the type byte but we only need two for the example. data Redirect = Redirect data TimeExceeded = TimeExceeded data ICMPType = MkRedirect Redirect | MkTimeExceeded TimeExceeded -- 5 and 11 are the values that get sent. instance Enum ICMPType where fromEnum (MkRedirect Redirect) = 5 fromEnum (MkTimeExceeded TimeExceeded) = 11 data ICMPCodeRedirect = RedirNet | RedirHost | RedirNetToS | RedirHostToS deriving Enum data ICMPCodeTimeExceeded = ExcTTL | ExcFragTime deriving Enum class Encode a b | a -> b where encode :: a -> b instance Encode ICMPType String where encode = show . fromEnum instance Encode Redirect (ICMPCodeRedirect -> String) where encode a y = encode (MkRedirect a) ++ (show (fromEnum y)) instance Encode TimeExceeded (ICMPCodeTimeExceeded -> String) where encode b z = encode (MkTimeExceeded b) ++ (show (fromEnum z)) Now I can say things like encode Redirect Redir and get the value "50". But if I say encode Redirect ExcTTL then I get a type error. GHC gives Couldn't match `ICMPCodeRedirect' against `ICMPCodeTimeExceeded' Expected type: ICMPCodeRedirect -> String Inferred type: ICMPCodeTimeExceeded -> t and Hugs gives ERROR: Constraints are not consistent with functional dependency *** Constraint : Encode Redirect (ICMPCodeTimeExceeded -> [Char]) *** And constraint : Encode Redirect (ICMPCodeRedirect -> String) *** For class : Encode a b *** Break dependency : a -> b This is just what you want as it picks up errors at compile time not at run time. However, if I now comment out the functional dependency class Encode a b {- | a -> b -} where encode :: a -> b and include the expressions x = encode TimeExceeded ExcTTL main = putStrLn x then Hugs complains ERROR "codes.hs" (line 37): Unresolved top-level overloading *** Binding : x *** Outstanding context : Encode TimeExceeded (ICMPCodeTimeExceeded -> b) whereas GHC doesn't complain. Which is right? Dominic.