[Haskell-cafe] Families of type classes
daniel.is.fischer at web.de
Sun Nov 6 12:49:31 EST 2005
Am Sonntag, 6. November 2005 17:30 schrieb 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?
Two I see,
1. retain class Edge,
class Edge e n | e -> n, n -> e where
isConnectedTo :: n -> e -> Bool
startNode, endNode :: e -> n
class (Edge e n) => Graph g n e | g -> n, g -> e where
-- other methods if wanted
2. add more FunDeps,
class Graph g n e | g -> n, g -> e, e -> n, n -> e, n e -> g where . . .
Probably the second isn't wanted, because it introduces too much rigidity and
an ADT might be better.
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