[Haskell-beginners] Class instance for a type without a parameter

[Peter Hall]

Can someone remind me why I can't do this:


    data Digit = D0 | D1 | D2 | D3 | D4 | D5 | D6 | D7 | D8 | D9
        deriving (Eq, Ord, Show)

    instance Num [Digit] where ...  -- This isn't allowed


I could make Num a => [a] an instance of Num, but I can't make Digit
also an instance of Num because D8 + D5 is not a digit. So then I
could introduce a ConvertableToNum class, but I feel like I'd be
heading down a path with a lot of unnecessary extra type classes.

Thanks,

Peter