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

[Daniel Fischer]

On Sunday 11 March 2012, 16:57:30, Peter Hall wrote:
> 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
> 

It's because the language standard says that the types in the instance head 
must be of the form

T a1 ... ak

with `T' a type constructor and the ai distinct type variables. Why that 
is, I don't know.

You can make an

instance Num [Digit] where ...

if you enable

{-# LANGUAGE FlexibleInstances #-}

That's a harmless extension allowing instances of that form.