AD skeleton

src/Flux.jl

@@ -20,6 +20,7 @@ include("core.jl")
 import .FluxCore: graph
 
 include("utils.jl")
+include("grad/track.jl")
 include("params.jl")
 
 include("compiler/code.jl")

src/grad/track.jl

@@ -0,0 +1,24 @@
+struct Call{F,As<:Tuple}
+  func::F
+  args::As
+end
+
+Call(f, args...) = Call{typeof(f),typeof(args)}(f, args)
+
+back!(c::Call, Δ) = back!(c.func, Δ, c.args...)
+back!(::Void, Δ) = nothing
+
+mutable struct Var{T}
+  f::Call
+  x::T
+  Δ::T
+end
+
+Var(x::T, Δ::T) where {T} = Var(Call(nothing), x, Δ)
+Var(x::AbstractArray) = Var(x, zeros(x))
+Var(x::Number) = Var(x, zero(x))
+
+function back!(x::Var, Δ)
+  x.Δ .+= Δ
+  back!(x.f, Δ)
+end