From c89318449adc7094c7ca6bf70bd52c66ae140834 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Mon, 18 Jul 2022 20:08:40 -0400
Subject: [PATCH 1/3] handle Base.tail & friens

---
 Project.toml                        | 2 +-
 src/tangent_types/abstract_zero.jl  | 4 ++++
 src/tangent_types/tangent.jl        | 7 +++++++
 src/tangent_types/thunks.jl         | 4 ++++
 test/tangent_types/abstract_zero.jl | 4 ++++
 test/tangent_types/tangent.jl       | 7 +++++++
 test/tangent_types/thunks.jl        | 7 +++++++
 7 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/Project.toml b/Project.toml
index bc1ec7b45..3b97c30ab 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "1.15.2"
+version = "1.15.3"
 
 [deps]
 Compat = "34da2185-b29b-5c13-b0c7-acf172513d20"
diff --git a/src/tangent_types/abstract_zero.jl b/src/tangent_types/abstract_zero.jl
index e52d84819..986fc9854 100644
--- a/src/tangent_types/abstract_zero.jl
+++ b/src/tangent_types/abstract_zero.jl
@@ -16,6 +16,10 @@ Base.iszero(::AbstractZero) = true
 Base.iterate(x::AbstractZero) = (x, nothing)
 Base.iterate(::AbstractZero, ::Any) = nothing
 
+Base.first(x::AbstractZero) = x
+Base.tail(x::AbstractZero) = x
+Base.last(x::AbstractZero) = x
+
 Base.Broadcast.broadcastable(x::AbstractZero) = Ref(x)
 Base.Broadcast.broadcasted(::Type{T}) where {T<:AbstractZero} = T()
 
diff --git a/src/tangent_types/tangent.jl b/src/tangent_types/tangent.jl
index f187cb3f2..1e8914dba 100644
--- a/src/tangent_types/tangent.jl
+++ b/src/tangent_types/tangent.jl
@@ -96,6 +96,12 @@ function Base.show(io::IO, tangent::Tangent{P}) where {P}
     end
 end
 
+Base.first(tangent::Tangent{P,T}) where {P,T<:Union{Tuple,NamedTuple}} = first(backing(canonicalize(tangent)))
+Base.last(tangent::Tangent{P,T}) where {P,T<:Union{Tuple,NamedTuple}} = last(backing(canonicalize(tangent)))
+
+Base.tail(tangent::Tangent{P}) where {P<:Tuple} = Tangent{_tailtype(P)}(Base.tail(backing(tangent))...)
+@generated _tailtype(::Type{P}) where {P<:Tuple} = Tuple{P.parameters[2:end]...}
+
 function Base.getindex(tangent::Tangent{P,T}, idx::Int) where {P,T<:Union{Tuple,NamedTuple}}
     back = backing(canonicalize(tangent))
     return unthunk(getfield(back, idx))
@@ -127,6 +133,7 @@ end
 
 Base.iterate(tangent::Tangent, args...) = iterate(backing(tangent), args...)
 Base.length(tangent::Tangent) = length(backing(tangent))
+
 Base.eltype(::Type{<:Tangent{<:Any,T}}) where {T} = eltype(T)
 function Base.reverse(tangent::Tangent)
     rev_backing = reverse(backing(tangent))
diff --git a/src/tangent_types/thunks.jl b/src/tangent_types/thunks.jl
index 3735307b0..8baa006e8 100644
--- a/src/tangent_types/thunks.jl
+++ b/src/tangent_types/thunks.jl
@@ -24,6 +24,10 @@ end
     return element, (underlying_object, new_state)
 end
 
+Base.first(x::AbstractThunk) = first(unthunk(x))
+Base.last(x::AbstractThunk) = last(unthunk(x))
+Base.tail(x::AbstractThunk) = Base.tail(unthunk(x))
+
 Base.:(==)(a::AbstractThunk, b::AbstractThunk) = unthunk(a) == unthunk(b)
 
 Base.:(-)(a::AbstractThunk) = -unthunk(a)
diff --git a/test/tangent_types/abstract_zero.jl b/test/tangent_types/abstract_zero.jl
index 43e433b6c..e3d8642e4 100644
--- a/test/tangent_types/abstract_zero.jl
+++ b/test/tangent_types/abstract_zero.jl
@@ -86,6 +86,10 @@
         @test z[1:3] === z
         @test z[1, 2] === z
         @test getindex(z) === z
+        
+        @test first(z) === z
+        @test last(z) === z
+        @test Base.tail(z) === z
     end
 
     @testset "NoTangent" begin
diff --git a/test/tangent_types/tangent.jl b/test/tangent_types/tangent.jl
index 176dd4985..e37a45a68 100644
--- a/test/tangent_types/tangent.jl
+++ b/test/tangent_types/tangent.jl
@@ -78,6 +78,13 @@ end
         @test getindex(Tangent{Tuple{Float64}}(@thunk 2.0^2), 1) == 4.0
         @test getproperty(Tangent{Tuple{Float64}}(2.0), 1) == 2.0
         @test getproperty(Tangent{Tuple{Float64}}(@thunk 2.0^2), 1) == 4.0
+        
+        tang3 = Tangent{Tuple{Float64, String, Vector{Float64}}}(1.0, NoTangent(), @thunk [3.0] .+ 4)
+        @test @inferred(first(tang3)) === tang3[1] === 1.0
+        @test @inferred(last(tang3)) isa Thunk
+        @test unthunk(last(tang3)) == [7.0]
+        @test Tuple(@inferred Base.tail(tang3))[1] === NoTangent()
+        @test Tuple(Base.tail(tang3))[end] isa Thunk
 
         NT = NamedTuple{(:a, :b),Tuple{Float64,Float64}}
         @test getindex(Tangent{NT}(; a=(@thunk 2.0^2)), :a) == 4.0
diff --git a/test/tangent_types/thunks.jl b/test/tangent_types/thunks.jl
index 4b3ab4eb4..68c2cc53a 100644
--- a/test/tangent_types/thunks.jl
+++ b/test/tangent_types/thunks.jl
@@ -16,6 +16,13 @@
 
         @test nothing === iterate(@thunk ()) == iterate(())
     end
+    
+    @testset "first, last, tail" begin
+        @test first(@thunk (1,2,3) .+ 4) === 5
+        @test last(@thunk (1,2,3) .+ 4) === 7
+        @test Base.tail(@thunk (1,2,3) .+ 4) === (6, 7)
+        @test Base.tail(@thunk NoTangent() * 5) === NoTangent()
+    end
 
     @testset "show" begin
         rep = repr(Thunk(rand))

From 652d88c24fac48079bfdd286e29ae65b0ae960c4 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Tue, 19 Jul 2022 07:42:06 -0400
Subject: [PATCH 2/3] empty cases

---
 src/tangent_types/tangent.jl  | 4 +++-
 test/tangent_types/tangent.jl | 5 ++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/tangent_types/tangent.jl b/src/tangent_types/tangent.jl
index 1e8914dba..6d83596f5 100644
--- a/src/tangent_types/tangent.jl
+++ b/src/tangent_types/tangent.jl
@@ -99,8 +99,10 @@ end
 Base.first(tangent::Tangent{P,T}) where {P,T<:Union{Tuple,NamedTuple}} = first(backing(canonicalize(tangent)))
 Base.last(tangent::Tangent{P,T}) where {P,T<:Union{Tuple,NamedTuple}} = last(backing(canonicalize(tangent)))
 
-Base.tail(tangent::Tangent{P}) where {P<:Tuple} = Tangent{_tailtype(P)}(Base.tail(backing(tangent))...)
+Base.tail(t::Tangent{P}) where {P<:Tuple} = Tangent{_tailtype(P)}(Base.tail(backing(canonicalize(t)))...)
 @generated _tailtype(::Type{P}) where {P<:Tuple} = Tuple{P.parameters[2:end]...}
+Base.tail(t::Tangent{<:Tuple{Any}}) = NoTangent()
+Base.tail(t::Tangent{<:Tuple{}}) = NoTangent()
 
 function Base.getindex(tangent::Tangent{P,T}, idx::Int) where {P,T<:Union{Tuple,NamedTuple}}
     back = backing(canonicalize(tangent))
diff --git a/test/tangent_types/tangent.jl b/test/tangent_types/tangent.jl
index e37a45a68..96946288b 100644
--- a/test/tangent_types/tangent.jl
+++ b/test/tangent_types/tangent.jl
@@ -72,12 +72,15 @@ end
         end
         @test Tangent{Foo}(; x=2.5).x == 2.5
 
-        @test keys(Tangent{Tuple{Float64}}(2.0)) == Base.OneTo(1)
+        tang1 = Tangent{Tuple{Float64}}(2.0)
+        @test keys(tang1) == Base.OneTo(1)
         @test propertynames(Tangent{Tuple{Float64}}(2.0)) == (1,)
         @test getindex(Tangent{Tuple{Float64}}(2.0), 1) == 2.0
         @test getindex(Tangent{Tuple{Float64}}(@thunk 2.0^2), 1) == 4.0
         @test getproperty(Tangent{Tuple{Float64}}(2.0), 1) == 2.0
         @test getproperty(Tangent{Tuple{Float64}}(@thunk 2.0^2), 1) == 4.0
+        @test NoTangent() === @inferred Base.tail(tang1)
+        @test NoTangent() === @inferred Base.tail(Tangent{Tuple{}}())
         
         tang3 = Tangent{Tuple{Float64, String, Vector{Float64}}}(1.0, NoTangent(), @thunk [3.0] .+ 4)
         @test @inferred(first(tang3)) === tang3[1] === 1.0

From 3db6428c79d7307ea92b5c9356187daffaab3f41 Mon Sep 17 00:00:00 2001
From: Michael Abbott <32575566+mcabbott@users.noreply.github.com>
Date: Tue, 19 Jul 2022 07:42:27 -0400
Subject: [PATCH 3/3] tail on NamedTuples too

---
 src/tangent_types/tangent.jl  | 5 +++++
 test/tangent_types/tangent.jl | 8 ++++++++
 2 files changed, 13 insertions(+)

diff --git a/src/tangent_types/tangent.jl b/src/tangent_types/tangent.jl
index 6d83596f5..d13e75c76 100644
--- a/src/tangent_types/tangent.jl
+++ b/src/tangent_types/tangent.jl
@@ -104,6 +104,11 @@ Base.tail(t::Tangent{P}) where {P<:Tuple} = Tangent{_tailtype(P)}(Base.tail(back
 Base.tail(t::Tangent{<:Tuple{Any}}) = NoTangent()
 Base.tail(t::Tangent{<:Tuple{}}) = NoTangent()
 
+Base.tail(t::Tangent{P}) where {P<:NamedTuple} = Tangent{_tailtype(P)}(; Base.tail(backing(canonicalize(t)))...)
+_tailtype(::Type{NamedTuple{S,P}}) where {S,P} = NamedTuple{Base.tail(S), _tailtype(P)}
+Base.tail(t::Tangent{<:NamedTuple{<:Any, <:Tuple{Any}}}) = NoTangent()
+Base.tail(t::Tangent{<:NamedTuple{<:Any, <:Tuple{}}}) = NoTangent()
+
 function Base.getindex(tangent::Tangent{P,T}, idx::Int) where {P,T<:Union{Tuple,NamedTuple}}
     back = backing(canonicalize(tangent))
     return unthunk(getfield(back, idx))
diff --git a/test/tangent_types/tangent.jl b/test/tangent_types/tangent.jl
index 96946288b..004dd71cf 100644
--- a/test/tangent_types/tangent.jl
+++ b/test/tangent_types/tangent.jl
@@ -99,6 +99,14 @@ end
         @test getproperty(Tangent{NT}(; a=(@thunk 2.0^2)), :b) == ZeroTangent()
         @test getproperty(Tangent{NT}(; b=(@thunk 2.0^2)), 1) == ZeroTangent()
         @test getproperty(Tangent{NT}(; b=(@thunk 2.0^2)), 2) == 4.0
+        
+        @test first(Tangent{NT}(; a=(@thunk 2.0^2))) isa Thunk
+        @test unthunk(first(Tangent{NT}(; a=(@thunk 2.0^2)))) == 4.0
+        @test last(Tangent{NT}(; a=(@thunk 2.0^2))) isa ZeroTangent
+        
+        ntang1 = @inferred Base.tail(Tangent{NT}(; b=(@thunk 2.0^2)))
+        @test ntang1 isa Tangent{<:NamedTuple{(:b,)}}
+        @test NoTangent() === @inferred Base.tail(ntang1)
 
         # TODO: uncomment this once https://github.com/JuliaLang/julia/issues/35516
         if VERSION >= v"1.8-"