snowleopard
commented
5 years ago @michaelpj Could you let me know why you'd like to use Relation in particular? I was planning to remove Relation from the library, because it doesn't seem to provide anything on top of AdjacencyMap and is slower on most operations (although it may be worth double-checking using @nobrakal's benchmarks).
Answering your questions:
If you want a
Relation, you should construct something polymorphically using the (non-higher-kinded)Graphclass, instantiated toRelation.
Not necessarily: you can also use the Relation API directly, e.g. Algebra.Graph.Relation.path.
Relationis not an instance of the higher-kindedGraphclass, I assume due to theOrdconstraint onconnectand the fact thatSetisn'tTraversable.
Yes, that's right: both overlay and connect require the Ord constraint. Do you necessarily need the higher-kinded Graph type class?
The easiest way to fix this would just be to add a function that converts a
Graphinto aRelation(ToGraphexists, but it doesn't do the right thing, I think).
You are right, ToGraph does not allow you to convert a Graph into a Relation. But it does allow you to convert a Graph to an AdjacencyMap. For example:
import qualified Algebra.Graph as G
import qualified Algebra.Graph.AdjacencyMap as AM
import qualified Algebra.Graph.ToGraph as T
g :: G.Graph Int
g = G.path [1..5]
am :: AM.AdjacencyMap Int
am = AM.path [1..5]
check :: Bool
check = (T.toAdjacencyMap g == am)
main :: IO ()
main = print check -- TrueIt's also pretty useful if you happen to have been given a (non-polymorphic)
Graphand now you want to do construct the transitive graph or similar.
Aha! Are you using Relation only to gain access to relational operations like transitiveClosure? In this case, perhaps, adding these operations to AdjacencyMap or even ToGraph would be a better solution?
michaelpj
As I understand it, at the moment the following things are true:
Relation, you should construct something polymorphically using the (non-higher-kinded)Graphclass, instantiated toRelation.Relationis not an instance of the higher-kindedGraphclass, I assume due to theOrdconstraint onconnectand the fact thatSetisn'tTraversable.Consequently, if you want to use the higher-kinded
Graphclass to construct your graph, you can't use it to make aRelation.The easiest way to fix this would just be to add a function that converts a
Graphinto aRelation(ToGraphexists, but it doesn't do the right thing, I think). While somewhat wasteful, I think it would be handy. It's also pretty useful if you happen to have been given a (non-polymorphic)Graphand now you want to do construct the transitive graph or similar.