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Add flexibility in dispatch for iszero_tuple #667

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d01e00a
Add flexibility in dispatch for `iszero_tuple`
timholy Sep 28, 2023
63a008b
Test that `iszero_tuple` can be specialized
timholy Aug 26, 2026
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3 changes: 3 additions & 0 deletions src/dual.jl
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Original file line number Diff line number Diff line change
Expand Up @@ -25,6 +25,9 @@ end
##########
Base.ArithmeticStyle(::Type{<:Dual{T,V}}) where {T,V} = Base.ArithmeticStyle(V)

unwrap_dual(::Type{Dual{T,V,N}}) where {T,V,N} = unwrap_dual(V)
unwrap_dual(::Type{V}) where V = V

##############
# Exceptions #
##############
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6 changes: 5 additions & 1 deletion src/partials.jl
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Expand Up @@ -166,7 +166,11 @@ end
@inline rand_tuple(::AbstractRNG, ::Type{Tuple{}}) = tuple()
@inline rand_tuple(::Type{Tuple{}}) = tuple()

@generated function iszero_tuple(tup::NTuple{N,V}) where {N,V}
iszero_tuple(tup::NTuple{N,V}) where {N,V} = _iszero_tuple(unwrap_dual(V), tup)

# default implementation; specific number types (e.g., Interval from IntervalArithmetic)
# can add specializations.
@generated function _iszero_tuple(::Type{V0}, tup::NTuple{N,V}) where {V0,N,V}
ex = Expr(:&&, [:(z == tup[$i]) for i=1:N]...)
return quote
z = zero(V)
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36 changes: 36 additions & 0 deletions test/PartialsTest.jl
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Expand Up @@ -164,4 +164,40 @@ show(io, MIME("text/plain"), Partials((1, 2, 3)))
str = String(take!(io))
@test str == "3-element $(ForwardDiff.Partials{3,Int}):\n 1\n 2\n 3"

###################################
# Specialization of iszero_tuple #
###################################

# A scalar type that does not support `==`, standing in for types (e.g., intervals)
# that reserve `==` for a different meaning and supply their own `iszero_tuple`
# implementation.
struct Fuzzy{T<:Real} <: Real
val::T
end
Base.:(==)(::Fuzzy, ::Fuzzy) = throw(ArgumentError("`==` is not supported for Fuzzy"))
Base.zero(::Type{Fuzzy{T}}) where {T} = Fuzzy(zero(T))

fuzzy_iszero(x::Fuzzy) = iszero(x.val)
fuzzy_iszero(d::ForwardDiff.Dual) =
fuzzy_iszero(ForwardDiff.value(d)) && all(fuzzy_iszero, ForwardDiff.partials(d).values)

ForwardDiff._iszero_tuple(::Type{<:Fuzzy}, tup::NTuple{N,V}) where {N,V} = all(fuzzy_iszero, tup)

@testset "iszero_tuple specialization" begin
@test ForwardDiff.unwrap_dual(Fuzzy{Float64}) === Fuzzy{Float64}
@test ForwardDiff.unwrap_dual(ForwardDiff.Dual{:t1,Fuzzy{Float64},1}) === Fuzzy{Float64}
@test ForwardDiff.unwrap_dual(ForwardDiff.Dual{:t2,ForwardDiff.Dual{:t1,Fuzzy{Float64},1},1}) === Fuzzy{Float64}

# Without the specialization these would throw, since the default implementation uses `==`
z, o = Fuzzy(0.0), Fuzzy(1.0)
@test iszero(Partials((z, z)))
@test !iszero(Partials((z, o)))

# Higher-order derivatives: dispatch is on the type wrapped by the `Dual`s
dz = ForwardDiff.Dual{:t1}(z, Partials((z,)))
dnz = ForwardDiff.Dual{:t1}(z, Partials((o,)))
@test iszero(Partials((dz, dz)))
@test !iszero(Partials((dz, dnz)))
end

end # module