Here is something else that I don't quite understand... Original version compiles: push :: Show b => State b -> Dispatcher b a -> (ScriptState a b) () push state dispatcher = do w <- get trace 95 $ "push: Pushing " ++ show state ++ " onto the stack" let s = stack w putStrict $ w { stack = (state, dispatcher):s } data State a = Start | Stop | (Show a, Eq a) => State a instance Eq a => Eq (State a) where (State a) == (State b) = a == b Start == Start = True Stop == Stop = True _ == _ = False instance Show a => Show (State a) where show (State a) = show a show Start = "Start" show Stop = "Stop" This version does not. Why does it require Eq in the ++ context? And why doesn't the other version? data (Show a, Eq a) => State a = Start | Stop | State a deriving (Eq, Show) Could not deduce (Eq b) from the context (Show b) arising from use of `show' at ./Script/Engine.hs:86:38-41 Probable fix: add (Eq b) to the type signature(s) for `push' In the first argument of `(++)', namely `show state' In the second argument of `(++)', namely `(show state) ++ " onto the stack"' -- http://wagerlabs.com/