Implicit graph structure

[gdalle]

Apparently, the old Graphs.jl package made it possible to define a graph implicitly, where neighbors of a vertex are generated just-in-time instead of being stored statically (source: https://github.com/Shushman/MultiAgentPathFinding.jl).
I see no reason why the current interface would not allow this, is there?

[etiennedeg]

I think the API allows neighbors to be returned as an iterator and it should be enforced in the codebase (see for example this PR). The fact that the documentation says that neighbors returns a list is an inaccuracy of the documentation. I may be wrong though (cc @simonschoelly).

[simonschoelly]

That thing with the list might have been inspired by networkx. I would agree, that one should be able to generate the neighbors implicitly. I would even say, that the current implementation for Simple{Di}Graph is dangerous, as one could accidentally modify the graph this way.

Another example of implict neighbors is https://github.com/JuliaGraphs/SpecialGraphs.jl