[Haskell-cafe] Families of type classes

[Klaus Ostermann]

Daniel Fischer schrieb:
> I'd prefer (very strongly) something like
> 
> class Graph g n e | g -> n, g -> e where
>     isConnectedTo :: g -> n -> e -> Bool  -- or perhaps rather without "g"
>     startNode, endNode :: e -> n
>     . . .     -- other Methods of interest like nodes, edges, components . . .
> 
> with, e.g.
> 
> instance Graph (Map node [node]) node (node,node) where . . .

Thanks for the suggestion. This looks good, but
it seems as if the "g" needs to occur in every signature, otherwise
the interpreter throws a "No instance for ... arising from ..."
error if you want to apply the function. Hence startNode would need to be
startNode :: g -> e -> n rather than startNode :: e -> n

Is there any way to get rid of this dummy argument?

Klaus