Merge pull request #465 from mcabbott/patch-1

Splat arguments in gradient `rrule`

Author: mkitti

src/chainrules/chainrules.jl

@@ -1,12 +1,16 @@
 export rrule
 """
     ChainRulesCore.rrule(itp::AbstractInterpolation, x...)
+
 ChainRulesCore.jl `rrule` for integration with automatic differentiation libraries.
+Note that it gives the gradient only with respect to the evaluation point `x`,
+and not the data inside `itp`.
 """
 function ChainRulesCore.rrule(itp::AbstractInterpolation, x...)
     y = itp(x...)
-    function pullback(Δy)
-        (ChainRulesCore.NoTangent(), Δy * Interpolations.gradient(itp, x...))
+    function interpolate_pullback(Δy)
+        nope = ChainRulesCore.@not_implemented "`Interpolations.gradient` does not calculate a gradient with respect to the original data, only the evaluation point"
+        (nope, (Δy * Interpolations.gradient(itp, x...))...)
     end
-    y, pullback
-end
+    y, interpolate_pullback
+end

test/chainrules.jl

@@ -9,9 +9,9 @@ using Zygote
     itp = interpolate(y,BSpline(Linear()))
 
     if VERSION ≥ v"1.3"
-        @test Zygote.gradient(itp, 1)[1] == Interpolations.gradient(itp, 1)
+        @test Zygote.gradient(itp, 1) == Tuple(Interpolations.gradient(itp, 1))
     else
-        @test_skip Zygote.gradient(itp, 1)[1] == Interpolations.gradient(itp, 1)
+        @test_skip Zygote.gradient(itp, 1) == Tuple(Interpolations.gradient(itp, 1))
     end
 
     # 2D example
@@ -21,8 +21,8 @@ using Zygote
     itp2 = interpolate(y2, BSpline(Cubic(Line(OnGrid()))))
 
     if VERSION ≥ v"1.3"
-        @test Zygote.gradient(itp2,1,2)[1] == Interpolations.gradient(itp2,1,2)
+        @test Zygote.gradient(itp2,1,2) == Tuple(Interpolations.gradient(itp2,1,2))
     else
-        @test_skip Zygote.gradient(itp2,1,2)[1] == Interpolations.gradient(itp2,1,2)
+        @test_skip Zygote.gradient(itp2,1,2) == Tuple(Interpolations.gradient(itp2,1,2))
     end
-end
+end