Constructors for 2.0

Here is a thread for discussing the constructors for Graphs 2.0

Current constructors

Constructors for SimpleGraph

• SimpleGraph{T}() : empty graph
• SimpleGraph{T}(nv::Integer) : graph with no edges
• SimpleGraph{T}(nv::Integer, ne::Integer) : random graph (erdos-renyi)
• SimpleGraph(::Type{T}) : empty graph
• SimpleGraph(nv::Integer, ne::Integer, edgestream::Channel) : add edges from the stream until stream end or we reach ne edges.
• SimpleGraphFromIterator(edgelist)
• SimpleGraph(adjmx::AbstractMatrix)
• SimpleGraph{T}(g::AbstractGraph)
• SimpleGraph(edge_list::Vector{SimpleGraphEdge{T}})

Goals for 2.0

We will have a new concrete graph type Graph{V} which can represent arbitrary vertices.
We would need new constructors to be able to construct such new graphs.

Instead of passing nv, we should provide a list (or even an iterator ?) of the vertices of the graph.

As the type of vertices can be arbitrary, the type signatures of generators can quickly become a mess / incompatible

Also we will need to support multigraphs

My proposal

• Graph{V}() and Graph(::Type{V})should be unambiguous
• Remove SimpleGraph{T}(nv::Integer, ne::Integer) (much better to call explicitly erdos-renyi)
• I don't really get this one SimpleGraph(nv::Integer, ne::Integer, edgestream::Channel), I think It does not add much to SimpleGraphFromIterator
• Keep Graph{T<:Integer}(nv::Integer)
• Use keywords for other generators:
  • Graph{V<:Integer}(adjmx=adjmx)
  • (Multi)Graph{V}(edge_list=edge_list)
  • (Multi)Graph{V}(vertices=vertices)
  • (Multi)Graph{V}(vertices=vertices::Vector{V})
  • Graph{V}(vertices=vertices::Vector{V}, adjmx=adjmx)
  • (Multi)Graph{V}(vertices=vertices::Vector{V}, edge_list=edge_list)

Is this proposal sound?
Do you have some other ideas of generators? A better proposal?

Start by reviewing the listed SimpleGraph constructors and the proposed Graph and MultiGraph signatures. Compare how vertex collections, adjacency matrices, edge lists, generators, and multigraphs should be represented in the 2.0 API. Done means reaching agreement on a consistent constructor design; this issue is a discussion thread rather than an implementation task.