Some Array rules

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "1.4.0"
+version = "1.5.0"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/rulesets/Base/array.jl

@@ -1,3 +1,40 @@
+#####
+##### constructors
+#####
+
+ChainRules.@non_differentiable (::Type{T} where {T<:Array})(::UndefInitializer, args...)
+
+function rrule(::Type{T}, x::AbstractArray) where {T<:Array}
+    project_x = ProjectTo(x)
+    Array_pullback(ȳ) = (NoTangent(), project_x(ȳ))
+    return T(x), Array_pullback
+end
+
+#####
+##### `vect`
+#####
+
+@non_differentiable Base.vect()
+
+# Don't worry about projection here. The data passes straight through, so if a cotangent has
+# the wrong type for some reason, it must be the fault of another rule somewhere.
+function rrule(::typeof(Base.vect), X...)
+    function vect_pullback(ȳ)
+        X̄ = ntuple(n -> ȳ[n], length(X))
+        return (NoTangent(), X̄...)
+    end
+    return Base.vect(X...), vect_pullback
+end
+
+# # Edge case: Numbers get promoted to other numbers, so we need to project.
+# function rrule(::typeof(Base.vect), X::Number...)
+#     project
+#     function vect_pullback(ȳ)
+#         X̄ = ntuple(n -> )
+#     end
+#     return Base.vect(X...), vect_pullback
+# end
+
 #####
 ##### `reshape`
 #####

test/rulesets/Base/array.jl

@@ -1,3 +1,29 @@
+@testset "constructors" begin
+
+    # We can't use test_rrule here (as it's currently implemented) because the elements of
+    # the array have arbitrary values. The only thing we can do is ensure that we're getting
+    # `ZeroTangent`s back, and that the forwards pass produces the correct thing still.
+    @testset "undef" begin
+        val, pullback = rrule(Array{Float64}, undef, 5)
+        @test size(val) == (5, )
+        @test val isa Array{Float64, 1}
+        @test pullback(randn(5)) == (NoTangent(), NoTangent(), NoTangent())
+    end
+    @testset "from existing array" begin
+        test_rrule(Array, randn(2, 5))
+        test_rrule(Array, Diagonal(randn(5)))
+        test_rrule(Matrix, Diagonal(randn(5)))
+        test_rrule(Matrix, transpose(randn(4)))
+        test_rrule(Array{ComplexF64}, randn(3))
+    end
+end
+
+@testset "vect" begin
+    test_rrule(Base.vect)
+    test_rrule(Base.vect, 5.0, 4.0, 3.0)
+    test_rrule(Base.vect, randn(2, 2), randn(3, 3); check_inferred=false)
+end
+
 @testset "reshape" begin
     test_rrule(reshape, rand(4, 5), (2, 10))
     test_rrule(reshape, rand(4, 5), 2, 10)