add function graph_property_type (#10)

Author: nsajko

docs/src/index.md

@@ -11,10 +11,11 @@ Documentation for [GraphProperties](https://github.com/JuliaGraphs/GraphProperti
 
 ```@docs
 GraphProperty
+graph_property_type
 PropertyComparison
 ```
 
 ```@autodocs
 Modules = [GraphProperties]
-Filter = !(t -> t in (GraphProperty, PropertyComparison))
+Filter = !(t -> t in (GraphProperty, graph_property_type, PropertyComparison))
 ```

src/GraphProperties.jl

@@ -2,6 +2,7 @@ module GraphProperties
 
 export
     GraphProperty,
+    graph_property_type,
     PropertyComparison
 
 """
@@ -13,6 +14,17 @@ The only type parameter is the type of each value of the property.
 """
 abstract type GraphProperty{T} end
 
+"""
+    graph_property_type(::GraphProperty{T}) -> T
+
+Get the type of each value of the property.
+
+It is not allowed to add methods to this function.
+"""
+function graph_property_type(::GraphProperty{T}) where {T}
+    T
+end
+
 """
     PropertyComparison(comparison, property, value)
 

test/runtests.jl

@@ -10,6 +10,12 @@ using Test
         @test FractionalChromaticNumber() isa GraphProperty{Real}
         @test DegreeSequence() isa GraphProperty{AbstractVector{<:Integer}}
     end
+    @testset "`graph_property_type`" begin
+        @test Bool === @inferred graph_property_type(GraphIsCograph())
+        @test Integer === @inferred graph_property_type(Degeneracy())
+        @test Real === @inferred graph_property_type(Strength())
+        @test AbstractVector{<:Integer} == @inferred graph_property_type(DegreeSequence())
+    end
 end
 
 using Aqua: Aqua