Hi, I've been getting into Haskell over the past few months and have just come up against a bit of a brick wall when trying to encapsulate some of the data structures in my code nicely. Basically what I want to have, is a type class where one of the intermediate values is opaque with respect to code using its implementations. This is a simplified version of what I'm trying to accomplish: class Foo t where encode :: String -> t decode :: t -> String instance Foo Int where encode = read decode = show test = decode . encode This currently fails, because the type checker insists on trying to figure out what its type should be - even though it shouldn't be needed. Any suggestions on how to encode this sort of thing? And if it is possible, can it be extended to multiple type parameters? as this is really what I want to use it for. About the only way I can think of "fixing" it, is by turning the code inside out - sort of like the way an AST drives the compiler, but without knowing how it represents things internally. Thanks, Sam