On Thu, Feb 18, 2016 at 09:28:24PM -0800, Jeffrey Brown wrote:
I had not!
I'm not seeing how such a solution would work. The nodes in a graph all have to have the same type. If the phantom parameter distinguished two nodes, they could not be used together.
But maybe you see something there that I don't?
Argh, indeed you are correct. Maybe it can be worked around with existential quantification like this? {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE ExistentialQuantification #-} data Person = Person data Hamster = Hamster data GraphNode a = P String -- hide these | H String -- coll. of vertices data HumHamGraph = forall c . Test c => Gr [(c, c)] findCritters :: HumHamGraph -> GraphNode Person -> GraphNode Hamster findCritters = undefined class Test a where name :: a -> String instance Test (GraphNode Person) where name (P s) = s instance Test (GraphNode Hamster) where name (H s) = s toast = [(P "a,ga", H "beta"), (H "cas", P "cds")] For sure it looks ugly :s