Is there a way of counting edges?

[tomjaguarpaw]

Is there really no way of counting edges in a graph? Graph has the noNodes method but there seems to be nothing corresponding to the number of edges. I hacked together this

noEdges :: FGL.Graph gr => gr a b -> Int
noEdges = FGL.ufold (\(a, _, _, a') i -> i + length a + length a') 0

but surely this functionality should be exposed from the library?

[ivan-m]

Well, my go-to solution would have been length . labEdges or something along those lines. Especially as your solution might double-count loops (I don't recall off the top of my head how ufold handles loops).

I suspect the reason there currently isn't such a function (which should really be called size) whereas labNodes (the proper name of which is order) is two-fold:

1. noNodes is easily defined - and more efficiently so - using the underlying implementation, especially the Map-based Tree implementation (I think this is the one way that Tree outperforms PatriciaTree).
2. Do you count inverse edges? How about loops?

I may end up adding something along these lines in soon.

[tomjaguarpaw]

You're right, length . labEdges is much better. But my confusion on this is strong evidence that there really needs to be a single function which does this.