[Haskell-beginners] what's a proper way to make a Set typeclass? and why is it not done already?

[Markus Läll]

I started like this:

{-# LANGUAGE
   MultiParamTypeClasses,
   FlexibleInstances #-}

import qualified Data.List as L

class (Eq e) => Set s e where
   empty :: s e
   fromList :: [e] -> s e
   ...

..and started to implement it on a list:

instance (Eq e) => Set [] e where
   empty = []
   fromList xs = L.nub xs

But I can't understand, why there has to be a (Eq a) context to the
Set instance of a list, when the class definition seems to already say
it? It won't compile without it..

Secondly, why do I need FlexibleInstances -- it gives me an eror which
I don't understand ("all instance types must be of the form T a1 ...
an" and so on, and recommends to add the flexible instances.)

Also I couldn't find other elaborate Set typeclasses -- there seems to
be only the "Set a" type. Why is that(?), because you could use other
datastructures, better and worse in various ways, than the balanced
binary tree..


Markus