I think it would be convenient to allow adding variables and functions, which are not members of the class, to a class instance so that they are visible only in the instance scope. It will help if the same functions are used by several class functions. Example: When implementing Num class for my datatype, I found that I routinely do unwrapping in each operator definition. I extracted it into functions, but as they are used only in instance definition, I want to put them there and restrict them to that scope. It would be neater than leaving them in the global scope or copypasting into each operator.
newtype Wrapped = Wrapped Integer deriving (Show, Eq)
instance Num Wrapped where (+) = lift2 (+) (-) = lift2 (-) (*) = lift2 (*) abs = lift abs signum = lift signum fromInteger = Wrapped lift2 f (Wrapped a) (Wrapped b) = Wrapped (f a b) lift f (Wrapped a) = Wrapped (f a)
The extension implementation should be very simple. -- Regards, Boris