gradient hook

src/tracker/Tracker.jl

@@ -58,6 +58,16 @@ include("scalar.jl")
 include("array.jl")
 include("numeric.jl")
 
+"""
+    hook(f, x) -> x′
+
+Hook into gradient backpropagation. `x` is unmodified, but when backpropagating
+`f` will be applied to the incoming gradient. For example, `hook(-, x)` will reverse
+the sign of the gradient applied to `x`.
+"""
+hook(f, x) = istracked(x) ? track(hook, f, x) : x
+back(::typeof(hook), Δ, f, x) = @back(x, f(Δ))
+
 param(x::Number) = TrackedReal(float(x))
 param(xs::AbstractArray) = TrackedArray(float.(xs))