Merge pull request #502 from JuliaDiff/mz/-Tangent

Implement `-Tangent`

Author: mzgubic

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.10.2"
+version = "1.11.0"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"

src/tangent_arithmetic.jl

@@ -39,10 +39,11 @@ for T in (:AbstractThunk, :Tangent, :Any)
     @eval LinearAlgebra.dot(x::NotImplemented, ::$T) = x
     @eval LinearAlgebra.dot(::$T, x::NotImplemented) = x
 end
+# unary :- is the same as multiplication by -1
+Base.:-(x::NotImplemented) = x
 
 # subtraction throws an exception: in AD we add tangents but do not subtract them
 # subtraction happens eg. in gradient descent which can't be performed with `NotImplemented`
-Base.:-(x::NotImplemented) = throw(NotImplementedException(x))
 Base.:-(x::NotImplemented, ::NotImplemented) = throw(NotImplementedException(x))
 for T in (:ZeroTangent, :NoTangent, :AbstractThunk, :Tangent, :Any)
     @eval Base.:-(x::NotImplemented, ::$T) = throw(NotImplementedException(x))
@@ -144,6 +145,8 @@ end
 Base.:+(a::Dict, d::Tangent{P}) where {P} = merge(+, a, backing(d))
 Base.:+(a::Tangent{P}, b::P) where {P} = b + a
 
+Base.:-(tangent::Tangent{P}) where {P} = map(-, tangent)
+
 # We intentionally do not define, `Base.*(::Tangent, ::Tangent)` as that is not meaningful
 # In general one doesn't have to represent multiplications of 2 differentials
 # Only of a differential and a scaling factor (generally `Real`)

test/tangent_types/notimplemented.jl

@@ -22,6 +22,7 @@
         @test ni2 + ni === ni2
 
         # multiplication and dot product
+        @test -ni == ni
         for a in (true, x, thunk)
             @test ni * a === ni
             @test a * ni === ni
@@ -46,7 +47,6 @@
 
         # unsupported operations
         E = ChainRulesCore.NotImplementedException
-        @test_throws E -ni
         for a in (true, x, NoTangent(), ZeroTangent(), thunk)
             @test_throws E ni - a
             @test_throws E a - ni

test/tangent_types/tangent.jl

@@ -321,6 +321,12 @@ end
         @test c * t == c * 2
     end
 
+    @testset "-Tangent" begin
+        t = Tangent{Foo}(; x=1.0, y=-2.0)
+        @test -t == Tangent{Foo}(; x=-1.0, y=2.0)
+        @test -1.0 * t == -t
+    end
+
     @testset "scaling" begin
         @test (
             2 * Tangent{Foo}(; y=1.5, x=2.5) ==