Enable graphs with different Vertex and Edge types

[zenna]

In working on a constraint solving package, I wanted to use Graphs.jl to create graphs with different types of Vertices. I attempted to do this using an abstract Vertex type, and vertices are subtypes of that abstract type.

I found myself having to reimplement several methods such as add_vertex! add_edge! with the same body, but slightly different type signature, because they were not designed for this use case. For illustration consider

function add_vertex!{V}(g::GenericGraph{V}, v::V)

Really, I believe I want something like

function add_vertex!{V1, V1 <: V2}(g::GenericGraph{V1}, v::V2)

But this is not valid Julia, so I am not sure how one would achieve this. Or perhaps there is a simple work around?

[milktrader]

Why can't you just do

function add_vertex!{V1, V2}(g::GenericGraph{V1}, v::V2)

and ensure that V1 and V2 are valid types defined somewhere in the module?

[zenna]

I could do that. It is is a looser type constraint (V2 could be anything) but the main problem is that I'll still be duplicating all the code that exists in Graphs.jl already.

[milktrader]

Hmm, this question might be better answered on the julia-users mailing list. https://groups.google.com/forum/#!forum/julia-users

I would try that forum.

[silgon]

I encounter a similar problem. I tried to create vertex and edges types because I wanted to create some fixed variables to the vertices and edges:

type VertexGraph
    id::UInt32
    S::Array{Float64,1}
    V::Float64
    function VertexGraph(id::Integer)
        new(id, Float64[0], 0)
    end
end

type EdgeGraph{V} 
    id::UInt32
    source::V
    target::V
    Q::Float64
    action::Array{Float64, 1}
    ap::Array{Float64, 1}
    function EdgeGraph(id::Integer, source::V, target::V)
        new(id, source, target, 0, [0], [0])
    end
end

The problem is that since there's not such thing as AbstractEdge, therefore you're not allowed to use add_edge! that it only supports the type Edge and ExEdge. It throws the following error:

ERROR: LoadError: MethodError: `source` has no method matching source(::EdgeGraph{VertexGraph}, ::Graphs.GenericGraph{VertexGraph,EdgeGraph{VertexGraph},Array{VertexGraph,1},Array{EdgeGraph{VertexGra
ph},1},Array{Array{EdgeGraph{VertexGraph},1},1}})
Closest candidates are:
  source{V}(!Matched::Graphs.Edge{V}, ::Graphs.AbstractGraph{V,E})
  source{V}(!Matched::Graphs.ExEdge{V}, ::Graphs.AbstractGraph{V,E})

In the meantime I'm using ExEdge but a better solution would be implementing my own types based on an abstract vertex and abstract edge to implement constructors and some extra analysis inside the constructor of the type.