Merge pull request #270 from JuliaDiff/ox/scalardiv

Add scalar-array / and \

Project.toml

@@ -1,6 +1,6 @@
 name = "ChainRules"
 uuid = "082447d4-558c-5d27-93f4-14fc19e9eca2"
-version = "0.7.20"
+version = "0.7.21"
 
 [deps]
 ChainRulesCore = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"

src/rulesets/LinearAlgebra/dense.jl

@@ -163,6 +163,25 @@ function rrule(::typeof(*), B::AbstractArray{<:Real}, A::Real)
     return A * B, times_pullback
 end
 
+#####
+##### `\`, `/` matrix-scalar_rule
+
+function rrule(::typeof(/), A::AbstractArray{<:Real}, b::Real)
+    Y = A/b
+    function slash_pullback(Ȳ)
+        return (NO_FIELDS, @thunk(Ȳ/b), @thunk(-dot(Ȳ, Y)/b))
+    end
+    return Y, slash_pullback
+end
+
+function rrule(::typeof(\), b::Real, A::AbstractArray{<:Real})
+    Y = b\A
+    function backslash_pullback(Ȳ)
+        return (NO_FIELDS, @thunk(-dot(Ȳ, Y)/b), @thunk(Ȳ/b))
+    end
+    return Y, backslash_pullback
+end
+
 
 #####
 ##### `pinv`

test/rulesets/LinearAlgebra/dense.jl

@@ -178,6 +178,13 @@
             end
         end
     end
+    @testset "/ and \\ Scalar-AbstractArray" begin
+        A = randn(3, 4, 5)
+        Ā = randn(3, 4, 5)
+        Ȳ = randn(3, 4, 5)
+        rrule_test(/, Ȳ, (A, Ā), (7.2, 2.3))
+        rrule_test(\, Ȳ, (7.2, 2.3), (A, Ā))
+    end
     @testset "norm" begin
         for dims in [(), (5,), (3, 2), (7, 3, 2)]
             A = randn(dims...)