Hi Christian,
On Jan 18, 2008 1:55 PM, Christian Maeder
data CGraph a b = CGraph [a] (a -> a -> b)
Can I define an instance for the fgl Graph class?
I had no idea how to define empty (except using undefined).
Well, presumably the function does not need to be defined on values not in the list, so returning an error is fair enough-- empty = CGraph [] (const $ error "Node not in graph") I suppose you want to use the index in the list as the Node (= Int), which should be fine, but you'll run into problems with mkGraph, because you don't have an Eq constraint on a, so you can't implement the function to pass to CGraph. Since mkGraph is required to have the type mkGraph :: [LNode a] -> [LEdge b] -> CGraph a b for *all* a and b, you don't have a way to add an Eq constraint anywhere, either. So, no dice... 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) - Benja