Accumulate NamedTuple + Tangent (#88)

* accumulate NamedTuple + Tangent

* fixup

* don't test on 1.8

Author: mcabbott

.github/workflows/CI.yml

@@ -17,7 +17,9 @@ jobs:
       matrix:
         version:
           - '1.7'  # Lowest claimed support in Project.toml
-          - '1'    # Latest Release
+          # - '1'    # Latest Release  # Testing on 1.8 gives this message:
+          # ┌ Warning: ir verification broken. Either use 1.9 or 1.7
+          # └ @ Diffractor ~/work/Diffractor.jl/Diffractor.jl/src/stage1/recurse.jl:889
           - 'nightly'
         os:
           - ubuntu-latest

src/extra_rules.jl

@@ -266,3 +266,6 @@ end
 function ChainRulesCore.rrule(::DiffractorRuleConfig, ::Type{InplaceableThunk}, add!!, val)
     val, Δ->(NoTangent(), NoTangent(), Δ)
 end
+
+Base.real(z::ZeroTangent) = z  # TODO should be in CRC
+Base.real(z::NoTangent) = z

src/runtime.jl

@@ -5,11 +5,25 @@ struct DiffractorRuleConfig <: RuleConfig{Union{HasReverseMode,HasForwardsMode}}
 @Base.constprop :aggressive accum(a::Tuple, b::Tuple) = map(accum, a, b)
 @Base.constprop :aggressive @generated function accum(x::NamedTuple, y::NamedTuple)
     fnames = union(fieldnames(x), fieldnames(y))
+    isempty(fnames) && return :((;))  # code below makes () instead
     gradx(f) = f in fieldnames(x) ? :(getfield(x, $(quot(f)))) : :(ZeroTangent())
     grady(f) = f in fieldnames(y) ? :(getfield(y, $(quot(f)))) : :(ZeroTangent())
     Expr(:tuple, [:($f=accum($(gradx(f)), $(grady(f)))) for f in fnames]...)
 end
 @Base.constprop :aggressive accum(a, b, c, args...) = accum(accum(a, b), c, args...)
-@Base.constprop :aggressive accum(a::NoTangent, b) = b
-@Base.constprop :aggressive accum(a, b::NoTangent) = a
-@Base.constprop :aggressive accum(a::NoTangent, b::NoTangent) = NoTangent()
+@Base.constprop :aggressive accum(a::AbstractZero, b) = b
+@Base.constprop :aggressive accum(a, b::AbstractZero) = a
+@Base.constprop :aggressive accum(a::AbstractZero, b::AbstractZero) = NoTangent()
+
+using ChainRulesCore: Tangent, backing
+
+function accum(x::Tangent{T}, y::NamedTuple) where T
+  # @warn "gradient is both a Tangent and a NamedTuple" x y
+  _tangent(T, accum(backing(x), y))
+end
+accum(x::NamedTuple, y::Tangent) = accum(y, x)
+# This solves an ambiguity, but also avoids Tangent{ZeroTangent}() which + does not:
+accum(x::Tangent{T}, y::Tangent) where T = _tangent(T, accum(backing(x), backing(y)))
+
+_tangent(::Type{T}, z) where T = Tangent{T,typeof(z)}(z)
+_tangent(::Type, ::NamedTuple{()}) = NoTangent()

test/runtests.jl

@@ -162,15 +162,7 @@ end
 # Make sure that there's no infinite recursion in kwarg calls
 g_kw(;x=1.0) = sin(x)
 f_kw(x) = g_kw(;x)
-@test bwd(f_kw)(1.0) == bwd(sin)(1.0)  broken=true
-#=
-MethodError: no method matching +(::Tangent{var"#g_kw#47"{var"#g_kw#11#48"}, NamedTuple{(Symbol("#g_kw#11"),), Tuple{ZeroTangent}}}, ::Tangent{Diffractor.KwFunc{var"#g_kw#47"{var"#g_kw#11#48"}, var"#g_kw#47##kw"}, NamedTuple{(:kwf,), Tuple{ZeroTangent}}})
-...
-  [2] elementwise_add(a::NamedTuple{(:contents,), Tuple{Tangent{var"#g_kw#47"{var"#g_kw#11#48"}, NamedTuple{(Symbol("#g_kw#11"),), Tuple{ZeroTangent}}}}}, b::NamedTuple{(:contents,), Tuple{Tangent{Diffractor.KwFunc{var"#g_kw#47"{var"#g_kw#11#48"}, var"#g_kw#47##kw"}, NamedTuple{(:kwf,), Tuple{ZeroTangent}}}}})
-    @ ChainRulesCore ~/.julia/packages/ChainRulesCore/ctmSK/src/tangent_types/tangent.jl:287
-  [3] +(a::Tangent{Core.Box, NamedTuple{(:contents,), Tuple{Tangent{var"#g_kw#47"{var"#g_kw#11#48"}, NamedTuple{(Symbol("#g_kw#11"),), Tuple{ZeroTangent}}}}}}, b::Tangent{Core.Box, NamedTuple{(:contents,), Tuple{Tangent{Diffractor.KwFunc{var"#g_kw#47"{var"#g_kw#11#48"}, var"#g_kw#47##kw"}, NamedTuple{(:kwf,), Tuple{ZeroTangent}}}}}})
-    @ ChainRulesCore ~/.julia/packages/ChainRulesCore/ctmSK/src/tangent_arithmetic.jl:130
-=#
+@test bwd(f_kw)(1.0) == bwd(sin)(1.0)
 
 function f_crit_edge(a, b, c, x)
     # A function with two critical edges. This used to trigger an issue where