avoid back! fallback

src/Tracker/Tracker.jl

@@ -3,6 +3,7 @@ module Tracker
 export track, back!
 
 data(x) = x
+istracked(x) = false
 
 struct Call{F,As<:Tuple}
   func::F
@@ -14,8 +15,7 @@ Call(f, args...) = Call{typeof(f),typeof(args)}(f, args)
 (c::Call)() = c.func(data.(c.args)...)
 
 back!(c::Call, Δ) = back!(c.func, Δ, c.args...)
-
-back!(f, Δ) = nothing
+back!(::Call{Void}, Δ) = nothing
 
 struct TrackedArray{T,N,A} <: AbstractArray{T,N}
   f::Call
@@ -37,6 +37,7 @@ TrackedArray(c::Call) = TrackedArray(c, c())
 TrackedArray(x::AbstractArray) = TrackedArray(Call(nothing), x)
 
 track(xs) = TrackedArray(xs)
+istracked(x::TrackedArray) = true
 data(x::TrackedArray) = x.x
 grad(x::TrackedArray) = x.Δ
 
@@ -45,6 +46,13 @@ function back!(x::TrackedArray, Δ)
   back!(x.f, Δ)
 end
 
+macro back!(x, Δ)
+  quote
+    x = $(esc(x))
+    istracked(x) && back!(x, $(esc(Δ)))
+  end
+end
+
 # Fallthrough methods
 
 for f in :[Base.size, Base.ndims].args

src/Tracker/lib.jl

@@ -5,7 +5,7 @@ Base.getindex(xs::TrackedArray, i...) = TrackedArray(Call(getindex, xs, i...))
 function back!(::typeof(getindex), Δ, xs::TrackedArray, i...)
   Δ′ = zeros(xs)
   Δ′[i...] = Δ
-  back!(xs, Δ′)
+  @back!(xs, Δ′)
 end
 
 Base.:-(xs::TrackedArray) = TrackedArray(Call(-, xs))
@@ -15,8 +15,8 @@ a::TrackedMatrix * b::AbstractMatrix = TrackedArray(Call(*, a, b))
 a::AbstractMatrix * b::TrackedMatrix = TrackedArray(Call(*, a, b))
 
 function back!(::typeof(*), Δ, a::AbstractMatrix, b::AbstractMatrix)
-  back!(a, A_mul_Bt(Δ, data(b)))
-  back!(b, At_mul_B(data(a), Δ))
+  @back!(a, A_mul_Bt(Δ, data(b)))
+  @back!(b, At_mul_B(data(a), Δ))
 end
 
 # Broadcasting
@@ -39,7 +39,7 @@ end
 
 function back!(b::Broadcasted, Δ, args...)
   Δargs = ntuple(i -> Δ .* getindex.(partials.(b.data), i), length(args))
-  back!.(args, Δargs)
+  map((x, Δ) -> @back!(x, Δ), args, Δargs)
   return
 end