Fix rrule_via_ad return type (#86)

* fix rrule_via_ad return type

* Update src/stage1/generated.jl

Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>

* add length method

* Update src/stage1/generated.jl

Co-authored-by: Jeff Bezanson <jeff.bezanson@gmail.com>

Author: mcabbott

src/stage1/generated.jl

@@ -52,6 +52,7 @@ Base.getindex(o::OpticBundle, i::Int) = i == 1 ? o.x :
 Base.iterate(o::OpticBundle) = (o.x, nothing)
 Base.iterate(o::OpticBundle, ::Nothing) = (o.clos, missing)
 Base.iterate(o::OpticBundle, ::Missing) = nothing
+Base.length(o::OpticBundle) = 2
 
 # Desturucture using `getfield` rather than iterate to make
 # inference happier
@@ -227,7 +228,7 @@ function (::∂⃖{N})(f::T, args...) where {T, N}
 end
 
 function ChainRulesCore.rrule_via_ad(::DiffractorRuleConfig, f::T, args...) where {T}
-    ∂⃖{1}()(f, args...)
+    Tuple{Any, Any}(∂⃖{1}()(f, args...))
 end
 
 @Base.pure function (::∂⃖{1})(::typeof(Core.apply_type), head, args...)

test/runtests.jl

@@ -208,7 +208,9 @@ x43 = rand(10, 10)
 @test Diffractor.gradient(x->loss(svd(x), x[:,1], x[:,2]), x43) isa Tuple{Matrix{Float64}}
 
 # PR # 45 - Calling back into AD from ChainRules
-y45, back45 = rrule_via_ad(DiffractorRuleConfig(), x -> log(exp(x)), 2)
+r45 = rrule_via_ad(DiffractorRuleConfig(), x -> log(exp(x)), 2)
+@test r45 isa Tuple
+y45, back45 = r45
 @test y45 ≈ 2.0
 @test back45(1) == (ZeroTangent(), 1.0)