[bugfix] Base.eltype works on graph types as well as instances (#144)

* Cover bug in tests

* Change eltype to act on type

* Implement eltype on AbstractGraph instead

* Fix eltype extension

* Proper subtyping

* Proper subtyping on nonbacktracking

* Remove useless exception since eltype is no longer in interface

* Fix typos

* Remove eltype in generic test graph tyoe

---------

Co-authored-by: Guillaume Dalle <22795598+gdalle@users.noreply.github.com>

Author: lmondada

docs/src/ecosystem/interface.md

@@ -5,7 +5,6 @@ This section is designed to guide developers who wish to write their own graph s
 All Graphs.jl functions rely on a standard API to function. As long as your graph structure is a subtype of [`AbstractGraph`](@ref) and implements the following API functions with the given return values, all functions within the Graphs.jl package should just work:
 
 - [`edges`](@ref)
-- [Base.eltype](https://docs.julialang.org/en/latest/base/collections/#Base.eltype)
 - [`edgetype`](@ref) (example: `edgetype(g::CustomGraph) = Graphs.SimpleEdge{eltype(g)})`)
 - [`has_edge`](@ref)
 - [`has_vertex`](@ref)

src/SimpleGraphs/simpledigraph.jl

@@ -24,7 +24,6 @@ function SimpleDiGraph(
     return SimpleDiGraph{T}(ne, fadjlist, badjlist)
 end
 
-eltype(x::SimpleDiGraph{T}) where {T} = T
 
 # DiGraph{UInt8}(6), DiGraph{Int16}(7), DiGraph{Int8}()
 """

src/SimpleGraphs/simplegraph.jl

@@ -19,8 +19,6 @@ function SimpleGraph(ne, fadjlist::Vector{Vector{T}}) where {T}
     return SimpleGraph{T}(ne, fadjlist)
 end
 
-eltype(x::SimpleGraph{T}) where {T} = T
-
 # Graph{UInt8}(6), Graph{Int16}(7), Graph{UInt8}()
 """
     SimpleGraph{T}(n=0)

src/Test/Test.jl

@@ -49,9 +49,6 @@ end
 Graphs.is_directed(::Type{<:GenericGraph}) = false
 Graphs.is_directed(::Type{<:GenericDiGraph}) = true
 
-Base.eltype(g::GenericGraph) = eltype(g.g)
-Base.eltype(g::GenericDiGraph) = eltype(g.g)
-
 Graphs.edges(g::GenericGraph) = (GenericEdge(e) for e in Graphs.edges(g.g))
 Graphs.edges(g::GenericDiGraph) = (GenericEdge(e) for e in Graphs.edges(g.g))
 

src/interface.jl

@@ -118,11 +118,11 @@ Return the type of graph `g`'s edge
 edgetype(g::AbstractGraph) = _NI("edgetype")
 
 """
-    eltype(g)
+    eltype(G)
 
 Return the type of the graph's vertices (must be <: Integer)
 """
-eltype(g::AbstractGraph) = _NI("eltype")
+eltype(::Type{<:AbstractGraph{T}}) where {T} = T
 
 """
     nv(g)

src/linalg/nonbacktracking.jl

@@ -85,7 +85,7 @@ end
 
 size(nbt::Nonbacktracking) = (nbt.m, nbt.m)
 size(nbt::Nonbacktracking, i::Number) = size(nbt)[i]
-eltype(nbt::Nonbacktracking) = Float64
+eltype(::Type{<:Nonbacktracking}) = Float64
 issymmetric(nbt::Nonbacktracking) = false
 
 function *(nbt::Nonbacktracking, x::Vector{T}) where {T<:Number}

test/interface.jl

@@ -18,7 +18,7 @@ mutable struct DummyEdge <: AbstractEdge{Int} end
         @test_throws Graphs.NotImplementedError edgefun2edges(dummyedge, dummyedge)
     end
 
-    for graphfunbasic in [nv, ne, vertices, edges, is_directed, edgetype, eltype]
+    for graphfunbasic in [nv, ne, vertices, edges, is_directed, edgetype]
         @test_throws Graphs.NotImplementedError graphfunbasic(dummygraph)
     end
 

test/linalg/spectral.jl

@@ -61,6 +61,7 @@ Matrix(nbt::Nonbacktracking) = Matrix(sparse(nbt))
         @test size(n10, 1) == n10.m
         @test size(n10, 2) == n10.m
         @test eltype(n10) == Float64
+        @test eltype(typeof(n10)) == Float64
         @test !issymmetric(n10)
 
         contract!(z, n10, v)

test/simplegraphs/simplegraphs.jl

@@ -89,7 +89,7 @@ using Random: Random
         T = @inferred(eltype(g))
         @test @inferred(nv(SimpleGraph{T}(6))) == 6
 
-        @test @inferred(eltype(SimpleGraph(T))) == T
+        @test @inferred(eltype(SimpleGraph{T})) == T
         @test @inferred(eltype(SimpleGraph{T}(adjmx1))) == T
 
         ga = SimpleGraph(10)
@@ -156,7 +156,7 @@ using Random: Random
         T = @inferred(eltype(g))
         @test @inferred(nv(SimpleDiGraph{T}(6))) == 6
 
-        @test @inferred(eltype(SimpleDiGraph(T))) == T
+        @test @inferred(eltype(SimpleDiGraph{T})) == T
         @test @inferred(eltype(SimpleDiGraph{T}(adjmx2))) == T
 
         ga = SimpleDiGraph(10)
@@ -271,7 +271,7 @@ using Random: Random
             edge_set_any = Set{Any}(edge_list)
 
             g1 = @inferred SimpleDiGraph(edge_list)
-            # we can't infer the return type of SimpleDiGraphFromIterator at the moment 
+            # we can't infer the return type of SimpleDiGraphFromIterator at the moment
             g2 = SimpleDiGraphFromIterator(edge_list)
             g3 = SimpleDiGraphFromIterator(edge_iter)
             g4 = SimpleDiGraphFromIterator(edge_set)
@@ -312,7 +312,7 @@ using Random: Random
             ))
         end
 
-        # check if multiple edges && multiple self-loops result in the 
+        # check if multiple edges && multiple self-loops result in the
         # correct number of edges & vertices
         # edges using integers < 1 should be ignored
         g_undir = SimpleGraph(0)