Defined to_vec for ChainRulesCore differential types

Project.toml

@@ -1,6 +1,6 @@
 name = "FiniteDifferences"
 uuid = "26cc04aa-876d-5657-8c51-4c34ba976000"
-version = "0.11.1"
+version = "0.11.2"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
@@ -11,8 +11,8 @@ Richardson = "708f8203-808e-40c0-ba2d-98a6953ed40d"
 
 [compat]
 ChainRulesCore = "0.9"
-julia = "1"
 Richardson = "1.2"
+julia = "1"
 
 [extras]
 Random = "9a3f8284-a2c9-5f02-9a11-845980a1fd5c"

src/to_vec.jl

@@ -139,3 +139,26 @@ function to_vec(d::Dict)
     end
     return d_vec, Dict_from_vec
 end
+
+
+# ChainRulesCore Differentials
+function FiniteDifferences.to_vec(x::Composite{P}) where{P}
+    x_canon = canonicalize(x)  # to be safe, fill in every field and put in primal order.
+    x_inner = ChainRulesCore.backing(x_canon)
+    x_vec, back_inner = FiniteDifferences.to_vec(x_inner)
+    function Composite_from_vec(y_vec)
+        y_back = back_inner(y_vec)
+        return Composite{P, typeof(y_back)}(y_back)
+    end
+    return x_vec, Composite_from_vec
+end
+
+
+function FiniteDifferences.to_vec(x::AbstractZero)
+    function AbstractZero_from_vec(z)
+        length(z) == 1  || throw(DimensionMismatch("tried to go back to $x from $z"))
+        iszero(first(z)) || throw(DomainError(first(z)))
+        return x
+    end
+    return [false], AbstractZero_from_vec
+end

test/to_vec.jl

@@ -18,6 +18,14 @@ struct FillVector <: AbstractVector{Float64}
     len::Int
 end
 
+# For testing Composite{ThreeFields}
+struct ThreeFields
+    a
+    b
+    c
+end
+
+
 Base.size(x::FillVector) = (x.len,)
 Base.getindex(x::FillVector, n::Int) = x.x
 
@@ -105,6 +113,43 @@ end
         end
     end
 
+    @testset "ChainRulesCore Differentials" begin
+        @testset "Composite{Tuple}" begin
+            @testset "basic" begin
+                x_tup = (1.0, 2.0, 3.0)
+                x_comp = Composite{typeof(x_tup)}(x_tup...)
+                test_to_vec(x_comp)
+            end
+
+            @testset "nested" begin
+                x_inner = (2, 3)
+                x_outer = (1, x_inner)
+                x_comp = Composite{typeof(x_outer)}(1, Composite{typeof(x_inner)}(2, 3))
+                test_to_vec(x_comp)
+            end
+        end
+
+        @testset "Composite Struct" begin
+            @testset "NamedTuple basic" begin
+                nt = (; a=1.0, b=20.0)
+                comp = Composite{typeof(nt)}(; nt...)
+                test_to_vec(comp)
+            end
+
+            @testset "Struct" begin
+                test_to_vec(Composite{ThreeFields}(; a=10.0, b=20.0, c=30.0))
+                test_to_vec(Composite{ThreeFields}(; a=10.0, b=20.0,))
+                test_to_vec(Composite{ThreeFields}(; a=10.0, c=30.0))
+                test_to_vec(Composite{ThreeFields}(; c=30.0, a=10.0, b=20.0))
+            end
+        end
+
+        @testset "AbstractZero" begin
+            test_to_vec(Zero())
+            test_to_vec(DoesNotExist())
+        end
+    end
+
     @testset "FillVector" begin
         x = FillVector(5.0, 10)
         x_vec, from_vec = to_vec(x)