[Haskell-beginners] Trouble using MultiParamTypeClasses

[Daniel Fischer]

On Tuesday 22 November 2011, 13:41:11, Amy de Buitléir wrote:
> I would be very grateful if someone could tell me what I'm doing wrong.
> Here's my code:
> 
> -----
> {-# LANGUAGE MultiParamTypeClasses #-}
> 
> class Eq a => Graph g a where
>   nodes :: g a -> [a]
>   neighbours :: g a -> a -> [a]

You apply `g' to the type `a', so `g' must be a type constructor taking one 
argument (because `g a' is a type).
That means `g' must have the kind `* -> *' (the kind of type constructors 
taking one type as argument and producing a type).

> 
> data WeightedGraph a w = WeightedGraph [(a, a, w)]

WeightedGraph takes two type arguments (`a' and `w' must be types, since 
they're put in tuples) and constructs a type from them, so it has the kind
* -> * -> *

> 
> instance Eq a => Graph a (WeightedGraph w a) where
>   nodes = [] --stub
>   neighbours = [] --stub

Since a is used as an argument to WeightedGraph, it must have kind *, but 
it is also used as the first parameter to the Graph class, which demands it 
has kind * -> *. Thus you have a kind mismatch.

You have probably confused the order of parameters, so the WeightedGraph 
thing should be the first parameter and a the second. However, you mustn't 
provide WeightedGraph with all type arguments it takes, since the type 
expression you pass as first parameter to Graph must still take a type 
argument to produce a type.

What you probably want is

instance Eq a => Graph (WeightedGraph w) a where
    nodes _ = []
    neighbours _ _ = []