Benja Fallenstein wrote:
However, if you'd be able to live with
data CGraph a b = CGraph [LNode a] (Node -> Node -> b)
then you should be able to write the instance like this--
instance Graph CGraph where empty = CGraph [] (const $ error "Node not in graph") isEmpty (CGraph xs _) = null xs labNodes (CGraph xs _) = xs mkGraph nodes edges = CGraph nodes f where f x y = fromMaybe (error "Edge not found") (lookup (x,y) edges') edges' = map (\(x,y,l) -> ((x,y),l)) edges match x (CGraph nodes f) = case lookup x nodes of Nothing -> (Nothing, CGraph nodes f) Just l -> let nodes' = filter ((/= x) . fst) nodes left = map (\(y,_) -> (f y x, y)) nodes' right = map (\(y,_) -> (f x y, y)) nodes' in (Just (left, x, l, right), CGraph nodes' f)
Thanks for pointing out this proposal. The actual problem is mkGraph that needs all the many edges created beforehand (that's what I wanted to avoid). Cheers Christian