add reverse method for Composite

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "0.9.21"
+version = "0.9.22"
 
 [deps]
 LinearAlgebra = "37e2e46d-f89d-539d-b4ee-838fcccc9c8e"

src/differentials/composite.jl

@@ -90,6 +90,10 @@ end
 Base.iterate(comp::Composite, args...) = iterate(backing(comp), args...)
 Base.length(comp::Composite) = length(backing(comp))
 Base.eltype(::Type{<:Composite{<:Any, T}}) where T = eltype(T)
+function Base.reverse(comp::Composite)
+    rev_backing = reverse(backing(comp))
+    Composite{typeof(rev_backing), typeof(rev_backing)}(rev_backing)
+end
 
 function Base.map(f, comp::Composite{P, <:Tuple}) where P
     vals::Tuple = map(f, backing(comp))

test/differentials/composite.jl

@@ -44,7 +44,6 @@ end
         @test hash(Composite{Foo}(y=2.5, x=Zero())) == hash(Composite{Foo}(y=2.5))
     end
 
-
     @testset "indexing, iterating, and properties" begin
         @test keys(Composite{Foo}(x=2.5)) == (:x,)
         @test propertynames(Composite{Foo}(x=2.5)) == (:x,)
@@ -75,6 +74,19 @@ end
         @test collect(Composite{Tuple{Float64,}}(2.0)) == [2.0]
     end
 
+    @testset "reverse" begin
+        c = Composite{Tuple{Int, Int, String}}(1, 2, "something")
+        cr = Composite{Tuple{String, Int, Int}}("something", 2, 1)
+        @test reverse(c) === cr
+
+        # can't reverse a named tuple or a dict
+        @test_throws MethodError reverse(Composite{Foo}(;x=1.0, y=2.0))
+
+        d = Dict(:x => 1, :y => 2.0)
+        cdict = Composite{Foo, typeof(d)}(d)
+        @test_throws MethodError reverse(Composite{Foo}()) 
+    end
+
     @testset "unset properties" begin
         @test Composite{Foo}(; x=1.4).y === Zero()
     end