tracked tuples

src/tracker/back.jl

@@ -1,3 +1,5 @@
+init_grad(x) = zero(x)
+
 scan(c::Call) = foreach(scan, c.args)
 
 function scan(x::Tracked)
@@ -5,7 +7,7 @@ function scan(x::Tracked)
   if ref == 1
     scan(x.f)
   else
-    isdefined(x, :grad) || (x.grad = zeros(x.data))
+    isdefined(x, :grad) || (x.grad = init_grad(x.data))
   end
   return
 end
@@ -19,13 +21,13 @@ back_(f, y, args...) = back(f, args...)
 back_(c::Call, y, Δ) = back_(c.func, y, Δ, c.args...)
 back_(::Call{Void}, y, Δ) = nothing
 
-accum!(x::Tracked, Δ) = (x.grad += Δ)
-accum!(x::Tracked{<:AbstractArray}, Δ) = (x.grad .+= Δ)
+accum!(x, Δ) = x .+ Δ
+accum!(x::AbstractArray, Δ) = (x .+= Δ)
 
 function back(x::Tracked, Δ)
   ref = x.ref -= 1
   if isdefined(x, :grad)
-    accum!(x, Δ)
+    x.grad = accum!(x.grad, Δ)
     ref == 0 && back_(x.f, x.data, x.grad)
   else
     ref == 0 && back_(x.f, x.data, Δ)

src/tracker/scalar.jl

@@ -61,3 +61,28 @@ for (M, f, arity) in DiffRules.diffrules()
     end
   end
 end
+
+# Tuples
+
+struct TrackedTuple{T<:Tuple}
+  tracker::Tracked{T}
+end
+
+tracker(xs::TrackedTuple) = xs.tracker
+
+accum!(x::Tuple, Δ::Tuple) = accum!.(x, Δ)
+init_grad(x::Tuple) = init_grad.(x)
+
+track(f::Call, xs::Tuple) = TrackedTuple(Tracked(f, xs))
+
+function Base.show(io::IO, xs::TrackedTuple)
+  show(io, data(xs))
+  print(io, " (tracked)")
+end
+
+Base.length(x::TrackedTuple) = length(data(x))
+
+Base.getindex(xs::TrackedTuple, i::Integer) = track(getindex, xs, i)
+
+back(::typeof(getindex), Δ, t, i) =
+  back(t, ntuple(j -> i == j ? Δ : 0, length(t)))