feat(examples): Add an example using the Julia type system.

Building over #32, this provides an example implementation for a tree
based on the Julia type system.

Project.toml

@@ -5,12 +5,14 @@ version = "0.4.5"
 
 [compat]
 Aqua = "0.8"
+InteractiveUtils = "1"
 Test = "1"
 julia = "1"
 
 [extras]
 Aqua = "4c88cf16-eb10-579e-8560-4a9242c79595"
+InteractiveUtils = "b77e0a4c-d291-57a0-90e8-8db25a27a240"
 Test = "8dfed614-e22c-5e08-85e1-65c5234f0b40"
 
 [targets]
-test = ["Aqua", "Test"]
+test = ["Aqua", "Test", "InteractiveUtils"]

docs/src/index.md

@@ -203,3 +203,4 @@ repr_node
 - [`OneTree`](https://github.com/JuliaCollections/AbstractTrees.jl/blob/master/test/examples/onetree.jl)
 - [`FSNode`](https://github.com/JuliaCollections/AbstractTrees.jl/blob/master/test/examples/fstree.jl)
 - [`BinaryNode`](https://github.com/JuliaCollections/AbstractTrees.jl/blob/master/test/examples/binarytree.jl)
+- [`TypeTree`](https://github.com/JuliaCollections/AbstractTrees.jl/blob/master/test/examples/juliatypes.jl)

test/examples/juliatypes.jl

@@ -0,0 +1,38 @@
+using AbstractTrees
+
+@static if Base.VERSION ≥ v"1.5"
+    using InteractiveUtils: subtypes, supertype, supertypes
+else
+    using InteractiveUtils: subtypes, supertype
+    function supertypes(T::Type)
+        S = supertype(T)
+        return S === T ? (T,) : (T, supertypes(S)...)
+    end
+end
+
+"""
+    TypeTree
+
+The first thing you might think of is using Type{T} as a node directly. However,
+this will create many difficulties with components of AbstractTrees.jl that rely
+on the type system (in particular `childrentype`). A simple workaround is to 
+use a wrapper type.
+"""
+struct TypeTree
+    t::Type
+end
+function AbstractTrees.children(t::TypeTree)
+    t.t === Function ? Vector{TypeTree}() : map(x->TypeTree(x), filter(x -> x !== Any,subtypes(t.t)))
+end
+AbstractTrees.printnode(io::IO,t::TypeTree) = print(io,t.t)
+AbstractTrees.nodevalue(t::TypeTree) = t.t
+AbstractTrees.parent(t::TypeTree) = TypeTree(supertype(t.t))
+AbstractTrees.ParentLinks(::Type{TypeTree}) = StoredParents()
+
+module JuliaTypesExamples
+abstract type AbstractSuperType end
+struct DirectDescendant <: AbstractSuperType end
+abstract type AbstractFooBar <: AbstractSuperType end
+struct Foo <: AbstractFooBar end
+struct Bar <: AbstractFooBar end
+end

test/runtests.jl

@@ -8,4 +8,4 @@ if Base.VERSION >= v"1.6"
     @testset "Printing" begin include("printing.jl") end
 end
 
-Aqua.test_all(AbstractTrees)
+Aqua.test_all(AbstractTrees)

test/trees.jl

@@ -116,3 +116,31 @@ include(joinpath(@__DIR__, "examples", "binarytree.jl"))
     @test nodevalue.(sbfs) == [0, 1, 2, 3]
 end
 
+include(joinpath(@__DIR__, "examples", "juliatypes.jl"))
+
+@testset "TypeTree" begin
+    t = TypeTree(JuliaTypesExamples.AbstractSuperType)
+
+    ls = collect(Leaves(t))
+    @test issetequal(nodevalue.(ls), [JuliaTypesExamples.DirectDescendant, JuliaTypesExamples.Foo, JuliaTypesExamples.Bar])
+
+    predfs = nodevalue.(collect(PreOrderDFS(t)))
+    for (i,T) in enumerate(predfs)
+        @test !any(T .<: predfs[i+1:end])
+    end
+
+    postdfs = nodevalue.(collect(PostOrderDFS(t)))
+    for (i,T) in enumerate(postdfs)
+        @test !any(T .<: postdfs[begin:i-1])
+    end
+
+    sbfs = nodevalue.(collect(StatelessBFS(t)))
+    for (i,T) in enumerate(sbfs)
+        parents = collect(supertypes(T)[2:end])
+        for j in (i+1):length(sbfs)
+            later_parents = collect(supertypes(sbfs[j])[2:end])
+            @test (parents ∩ later_parents) == parents
+        end
+    end
+end
+