Digraph edges notation

When you use the :digraph module and want to get a list of all edges, one would expect to see a list of vertex pairs, ideally with the “emanate from” / “incident to” order consistent. So something like [{:a, :b}, {:b, :c}, {:c, :b}] for a graph with vertices [:a, :b, :c] where :a has an edge incident to :b and :b and :c have a bidirectional edge. Instead you might see something inscrutable like [[:"$e" | 0], [:"$e" | 1], [:"$e" | 2]]. How are you supposed to interpret that list?

Definitely an odd design choice - you could get something closer to what you’re looking for by mapping :digraph.edge over the output of :digraph.edges, but that’s a lot of round-trips to ETS to get what could have been one call to :ets.select :thinking:


Alternatively, you could specify you our own edge names when creating the edges. As long as there is only a single edge in each direction between each pair of vertices, the following should work:

:digraph.add_edge(g, {from, to}, from, to, [])
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So that’s brilliant. I’m curious why that’s not the default behavior of add_edge/3 though? Implicitly naming an edge with {from, to} has to be more intuitive than whatever [:"$e" | N] is meant to convey.

I also think it’s difficult to understand that this is possible from the docs for add_edge/5 because the edge() type is not documented. It’s also confusing to have edge “names” differ from edge “labels” and not document the “names” anywhere.

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The default edge names aren’t meant to convey anything: not giving them a name implies that you don’t care what they’re called.

The funny-looking names were most likely chosen because they’re more compact in memory and are unlikely to clash with user-named edges.

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Submitted pull request to hopefully clarify docs a bit.

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Using the {from, to} naming scheme will not work in general, because the :digraph module supports having any number of distinct edges between each pair of vertices.

What is the utility of that feature? Is it to allow using labels to sort of classify connections in different schemas or something? Something like a graph of cars that includes vertices

v1 = {:toyota, :yellow}
v2 = {:toyota, :red}
v3 = {:ford, :yellow}
v4 = {:also_ford, :yellow}

Could have edges {v1, v2} label: :toyota, {v1, v3} label: :yellow, {v3, v4} label: :yellow, {v3, v4} label: :ford?

I don’t know the original motivation for that feature, but one algorithm that needs it is Karger’s algorithm.

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