Under what conditions can addLabeledEdge fail?

[kozross]

According to the docs, addLabeledEdge has the following type:

addLabeledEdge :: (MLabeledEdge g, PrimMonad m, MonadRef m) => 
    g m -> Vertex -> Vertex -> MEdgeLabel g -> m (Maybe Edge) 

Under what conditions will the resulting Maybe Edge be Nothing? Specifically, I assume that, unless both Vertexes are in g m, this will not create an Edge. Am I correct in this assumption?

[travitch]

The edge will not be added if either vertex does not exist in the graph. Also, it allows simple graphs to reject duplicate edges if the edge already exists.

[kozross]

@travitch Thanks for the fast response!

Based on your response, if I wanted to 'link together' two different graphs using an edge, it seems like I would first need to 'copy over' the vertices and edges of one graph to the other, and then add said edge. Is that correct, or am I missing an easier way?

[travitch]

Correct, that is currently the only method for combining graphs. Unlike fgl, there is no monoid instance for merging graphs because vertex creation is tightly-controlled. I could imagine some interesting combinators for merging graphs, but I don't know what would be best or most useful yet.

[noughtmare]

I could imagine some interesting combinators for merging graphs, but I don't know what would be best or most useful yet.

It might be useful to look at what the algebraic-graphs package does.