The following module should elicit a type error: type T a = Int class C a where f :: a -> T a g :: T a -> a instance C Char main = print (g (f 'a')) but nhc98 compiles it without complaint. GHC & Hugs both emit an unresolved overloading error: test187.hs:9: Ambiguous type variable(s) `a' in the constraint `C a' arising from use of `g' at test187.hs:9 In the first argument of `print', namely `(g (f 'a'))' In the definition of `main': print (g (f 'a')) test187.hs:9: Ambiguous type variable(s) `a' in the constraint `Show a' arising from use of `print' at test187.hs:9 In the definition of `main': print (g (f 'a')) I believe nhc98's Binary library makes use of the same trick in its definition of the Bin type. The type synonym in the example should really be a newtype. Cheers, Simon