Without going too deep into the details of my type classes, I have written the following code (focusing on compile1): {-# OPTIONS -fglasgow-exts #-} compile1 :: (Builder b box) => t -> Name -> Ir.ANF -> b t compile1 f x body = do env <- compile body empty wire (Arg W) (env x) return f compile :: (Builder b box) => Ir.ANF -> Env box -> b (Env box) class (Monad b) => Builder b box where wire :: Source box -> Sink box -> b () ... type Env box = Name -> Sink box This program is rejected by GHC with the following message: Ccomp.hs:54:23: Could not deduce (Builder b box1) from the context (Builder b box) arising from use of `wire' at Ccomp.hs:54:23-42 Possible fix: add (Builder b box1) to the type signature(s) for `compile1' In the expression: wire (Arg W) (env x) In a 'do' expression: wire (Arg W) (env x) In the expression: do env <- compile body empty wire (Arg W) (env x) return f Note that compile1 has an explicit type signature much along the lines suggested by GHC. If I *remove* this type signature, the function compiles successfully, and ghci reporets this type for compile1: compile1 :: (Builder t box) => t1 -> Name -> Ir.ANF -> t t1 I believe this signature is isomorphic to the explicit signature I had attempted to use. Am I misusing the type-class system in some way, or should I be reporting a bug in GHC? Norman