From 9eb8080ebb1fdd4d41fdb6552a81c811843d1860 Mon Sep 17 00:00:00 2001
From: Neven Sajko <s@purelymail.com>
Date: Tue, 7 Oct 2025 21:45:59 +0200
Subject: [PATCH] add function `graph_property_type`

---
 docs/src/index.md      |  3 ++-
 src/GraphProperties.jl | 12 ++++++++++++
 test/runtests.jl       |  6 ++++++
 3 files changed, 20 insertions(+), 1 deletion(-)

diff --git a/docs/src/index.md b/docs/src/index.md
index fc0e3cb..0a3e1b4 100644
--- a/docs/src/index.md
+++ b/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))
 ```
diff --git a/src/GraphProperties.jl b/src/GraphProperties.jl
index bbc1f78..99a081f 100644
--- a/src/GraphProperties.jl
+++ b/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)
 
diff --git a/test/runtests.jl b/test/runtests.jl
index a961354..5cf7abc 100644
--- a/test/runtests.jl
+++ b/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