Thank you so much for coming to me with a real-life issue you encountered. I am glad that you pointed this out and gave me a chance to make Yog better.
Okay, so let me give an example and see if I understood your issue better 
please call me out if there’s a gap in my understanding.
Let’s say you have the following graph:
graph =
Yog.undirected()
|> Yog.add_node(1, "Home")
|> Yog.add_node(2, "Office")
|> Yog.add_node(3, "Mall")
|> Yog.add_node(4, "backyard")
|> Yog.add_edge!(from: 1, to: 2, with: %{distance: 10, by: "Car"})
|> Yog.add_edge!(from: 2, to: 3, with: %{distance: 5, by: "Walk"})
|> Yog.add_edge!(from: 1, to: 3, with: %{distance: 10, by: "Bus"})
|> Yog.add_edge!(from: 1, to: 4, with: %{distance: 0.1, by: "Foot"})
|> Yog.add_edge!(from: 2, to: 1, with: %{distance: 8, by: "Train"})
|> Yog.add_edge!(from: 2, to: 4, with: %{distance: 11, by: "Train"})
Now, I made the with (i.e. the Edge metadata) non-numeric, so we have some edge specific data. So, the shortest path, let’s use Dijkstra would be (with custom zero, add, compare configuration so the algorithm knows the math behind our edge data):
Dijkstra.shortest_path(
graph,
2,
4,
0,
fn total, b ->
total + b.distance
end,
&Yog.Utils.compare/2
)
Which would give us the Path %Yog.Pathfinding.Path{nodes: [2, 1, 4], weight: 8.1, algorithm: :dijkstra, metadata: %{}}
Which I don’t want. Because I want to draw “Icon” based on “Car”, “Bus”, “Foot” etc.
I’m surprised I didn’t put a convenience function on Path for this! And I thank you for calling it out!
For NOW, how I’d do this is:
edges =
path.nodes
|> Enum.chunk_every(2, 1, :discard)
|> Enum.map(fn [u, v] ->
# O(1) because `out_edges` is a map.
{u, v, graph.out_edges[u][v]}
end)
And you’d get something like: [{2, 1, %{distance: 8, by: "Train"}}, {1, 4, %{distance: 0.1, by: "Foot"}}]
Now, I would not expect the client of this function to write this, I’d rather add a function, hydrate_path on Path that is like:
def hydrate_path(graph, node_ids) do
node_ids
|> Enum.chunk_every(2, 1, :discard)
|> Enum.map(fn [u, v] ->
{u, v, graph.out_edges[u][v]}
end)
end
I hope I understood your problem right, and could help out. Sorry for not having this as a function already.
Also, this works on simple graphs, multi-graphs are work in progress for now (and it does have edge_id).
