From dbe7765455dc9cb54ad7f1f0dee8c050f7b340ee Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sat, 11 Jan 2020 17:18:21 -0500
Subject: [PATCH 1/7] Eagerly evaluate scalers rules

Master behavior
```julia
julia> @scalar_rule(one(x), Zero())

julia> frule(one, 1, Zero(), [1, 2])
(1, Zero())

julia> frule(one, 1, Zero(), One())
(1, Zero())
```

Desirable behavior
```julia
julia> @scalar_rule(one(x), Zero())

julia> frule(one, 1, Zero(), [1, 2])
(1, [0, 0])

julia> frule(one, 1, Zero(), One())
(1, Thunk(var"#8#10"())
)
```
---
 src/differential_arithmetic.jl | 9 +++++----
 src/rule_definition_tools.jl   | 3 ++-
 2 files changed, 7 insertions(+), 5 deletions(-)

diff --git a/src/differential_arithmetic.jl b/src/differential_arithmetic.jl
index 2d4434a93..3ee1723bf 100644
--- a/src/differential_arithmetic.jl
+++ b/src/differential_arithmetic.jl
@@ -57,11 +57,12 @@ end
 Base.:+(a::AbstractThunk, b::AbstractThunk) = unthunk(a) + unthunk(b)
 Base.:*(a::AbstractThunk, b::AbstractThunk) = unthunk(a) * unthunk(b)
 for T in (:Any,)
-    @eval Base.:+(a::AbstractThunk, b::$T) = unthunk(a) + b
-    @eval Base.:+(a::$T, b::AbstractThunk) = a + unthunk(b)
+    # we want to eagerly compute the result when thunk meets other types
+    @eval Base.:+(a::AbstractThunk, b::$T) = extern(a) + b
+    @eval Base.:+(a::$T, b::AbstractThunk) = a + extern(b)
 
-    @eval Base.:*(a::AbstractThunk, b::$T) = unthunk(a) * b
-    @eval Base.:*(a::$T, b::AbstractThunk) = a * unthunk(b)
+    @eval Base.:*(a::AbstractThunk, b::$T) = extern(a) * b
+    @eval Base.:*(a::$T, b::AbstractThunk) = a * extern(b)
 end
 
 ################## Composite ##############################################################
diff --git a/src/rule_definition_tools.jl b/src/rule_definition_tools.jl
index d631f6131..65e41c39e 100644
--- a/src/rule_definition_tools.jl
+++ b/src/rule_definition_tools.jl
@@ -209,7 +209,8 @@ function propagation_expr(Δs, ∂s)
     # This is basically Δs ⋅ ∂s
     ∂s = map(esc, ∂s)
 
-    ∂_mul_Δs = ntuple(i->:($(∂s[i]) * $(Δs[i])), length(∂s))
+    # this is neccssary since we want to eagerly evaluate the result
+    ∂_mul_Δs = [:(@thunk($(∂s[i])) * $(Δs[i])) for i in 1:length(∂s)]
     return :(+($(∂_mul_Δs...)))
 end
 

From 9abe813f926470d6c19dcc100e97a90f4a49ac26 Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sat, 11 Jan 2020 17:23:45 -0500
Subject: [PATCH 2/7] New release

---
 Project.toml | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/Project.toml b/Project.toml
index 2ed20dd34..18bb57a35 100644
--- a/Project.toml
+++ b/Project.toml
@@ -1,6 +1,6 @@
 name = "ChainRulesCore"
 uuid = "d360d2e6-b24c-11e9-a2a3-2a2ae2dbcce4"
-version = "0.5.0"
+version = "0.5.1"
 
 [compat]
 julia = "^1.0"

From cf2bc6e3328ec8fcd93379769937d776276b9fae Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sat, 11 Jan 2020 17:33:41 -0500
Subject: [PATCH 3/7] Add tests

---
 test/rules.jl | 18 +++++++++++++-----
 1 file changed, 13 insertions(+), 5 deletions(-)

diff --git a/test/rules.jl b/test/rules.jl
index f280ebc59..1845f5739 100644
--- a/test/rules.jl
+++ b/test/rules.jl
@@ -8,6 +8,9 @@ cool(x, y) = x + y + 1
 dummy_identity(x) = x
 @scalar_rule(dummy_identity(x), One())
 
+nice(x) = 1
+@scalar_rule(nice(x), Zero())
+
 #######
 
 _second(t) = Base.tuple_type_head(Base.tuple_type_tail(t))
@@ -31,11 +34,16 @@ _second(t) = Base.tuple_type_head(Base.tuple_type_tail(t))
     @test cool_methods == only_methods
 
     frx, cool_pushforward = frule(cool, 1, dself, 1)
-    @test frx == 2
-    @test cool_pushforward == 1
+    @test frx === 2
+    @test cool_pushforward === 1
     rrx, cool_pullback = rrule(cool, 1)
     self, rr1 = cool_pullback(1)
-    @test self == NO_FIELDS
-    @test rrx == 2
-    @test rr1 == 1
+    @test self === NO_FIELDS
+    @test rrx === 2
+    @test rr1 === 1
+
+    frx, nice_pushforward = frule(nice, 1, dself, 1)
+    @test nice_pushforward === 0
+    rrx, nice_pullback = rrule(nice, 1)
+    @test (NO_FIELDS, 0) === nice_pullback(1)
 end

From d329ed8fbdd527b8a319479bf6b32b23d2bcb09d Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sun, 12 Jan 2020 14:36:07 -0500
Subject: [PATCH 4/7] Revert "Eagerly evaluate scalers rules"

This reverts commit dbe7765455dc9cb54ad7f1f0dee8c050f7b340ee.
---
 src/differential_arithmetic.jl | 9 ++++-----
 src/rule_definition_tools.jl   | 3 +--
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/src/differential_arithmetic.jl b/src/differential_arithmetic.jl
index 3ee1723bf..2d4434a93 100644
--- a/src/differential_arithmetic.jl
+++ b/src/differential_arithmetic.jl
@@ -57,12 +57,11 @@ end
 Base.:+(a::AbstractThunk, b::AbstractThunk) = unthunk(a) + unthunk(b)
 Base.:*(a::AbstractThunk, b::AbstractThunk) = unthunk(a) * unthunk(b)
 for T in (:Any,)
-    # we want to eagerly compute the result when thunk meets other types
-    @eval Base.:+(a::AbstractThunk, b::$T) = extern(a) + b
-    @eval Base.:+(a::$T, b::AbstractThunk) = a + extern(b)
+    @eval Base.:+(a::AbstractThunk, b::$T) = unthunk(a) + b
+    @eval Base.:+(a::$T, b::AbstractThunk) = a + unthunk(b)
 
-    @eval Base.:*(a::AbstractThunk, b::$T) = extern(a) * b
-    @eval Base.:*(a::$T, b::AbstractThunk) = a * extern(b)
+    @eval Base.:*(a::AbstractThunk, b::$T) = unthunk(a) * b
+    @eval Base.:*(a::$T, b::AbstractThunk) = a * unthunk(b)
 end
 
 ################## Composite ##############################################################
diff --git a/src/rule_definition_tools.jl b/src/rule_definition_tools.jl
index 65e41c39e..d631f6131 100644
--- a/src/rule_definition_tools.jl
+++ b/src/rule_definition_tools.jl
@@ -209,8 +209,7 @@ function propagation_expr(Δs, ∂s)
     # This is basically Δs ⋅ ∂s
     ∂s = map(esc, ∂s)
 
-    # this is neccssary since we want to eagerly evaluate the result
-    ∂_mul_Δs = [:(@thunk($(∂s[i])) * $(Δs[i])) for i in 1:length(∂s)]
+    ∂_mul_Δs = ntuple(i->:($(∂s[i]) * $(Δs[i])), length(∂s))
     return :(+($(∂_mul_Δs...)))
 end
 

From 0a66a95b3c56a82c660c03113039fcf3d0026639 Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sun, 12 Jan 2020 14:41:38 -0500
Subject: [PATCH 5/7] Redefine * between ::Zero and ::Any

---
 src/differential_arithmetic.jl | 10 +++++++---
 test/differentials/zero.jl     | 14 +++++++-------
 2 files changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/differential_arithmetic.jl b/src/differential_arithmetic.jl
index 2d4434a93..299616532 100644
--- a/src/differential_arithmetic.jl
+++ b/src/differential_arithmetic.jl
@@ -32,15 +32,19 @@ Base.:*(::DoesNotExist, ::Zero) = Zero()
 Base.:*(::Zero, ::DoesNotExist) = Zero()
 
 
-Base.:+(::Zero, b::Zero) = Zero()
+Base.:+(::Zero, ::Zero) = Zero()
 Base.:*(::Zero, ::Zero) = Zero()
 for T in (:One, :AbstractThunk, :Any)
     @eval Base.:+(::Zero, b::$T) = b
     @eval Base.:+(a::$T, ::Zero) = a
 
-    @eval Base.:*(::Zero, ::$T) = Zero()
-    @eval Base.:*(::$T, ::Zero) = Zero()
+    if T !== :Any
+        @eval Base.:*(::Zero, ::$T) = Zero()
+        @eval Base.:*(::$T, ::Zero) = Zero()
+    end
 end
+Base.:*(::Zero, x) = zero(x)
+Base.:*(x, ::Zero) = zero(x)
 
 
 Base.:+(a::One, b::One) = extern(a) + extern(b)
diff --git a/test/differentials/zero.jl b/test/differentials/zero.jl
index 54c16367c..24d939d97 100644
--- a/test/differentials/zero.jl
+++ b/test/differentials/zero.jl
@@ -1,15 +1,15 @@
 @testset "Zero" begin
     z = Zero()
     @test extern(z) === false
-    @test z + z == z
-    @test z + 1 == 1
-    @test 1 + z == 1
-    @test z * z == z
-    @test z * 1 == z
-    @test 1 * z == z
+    @test z + z === z
+    @test z + 1 === 1
+    @test 1 + z === 1
+    @test z * z === z
+    @test z * 1 === 0
+    @test 1 * z === 0
     for x in z
         @test x === z
     end
     @test broadcastable(z) isa Ref{Zero}
-    @test conj(z) == z
+    @test conj(z) === z
 end

From 6a142594490c26ffb1e6a33852c493a4e7567a41 Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sun, 12 Jan 2020 15:37:48 -0500
Subject: [PATCH 6/7] Make it nicer

---
 src/differential_arithmetic.jl | 10 +++-------
 1 file changed, 3 insertions(+), 7 deletions(-)

diff --git a/src/differential_arithmetic.jl b/src/differential_arithmetic.jl
index 299616532..fc7680803 100644
--- a/src/differential_arithmetic.jl
+++ b/src/differential_arithmetic.jl
@@ -38,14 +38,10 @@ for T in (:One, :AbstractThunk, :Any)
     @eval Base.:+(::Zero, b::$T) = b
     @eval Base.:+(a::$T, ::Zero) = a
 
-    if T !== :Any
-        @eval Base.:*(::Zero, ::$T) = Zero()
-        @eval Base.:*(::$T, ::Zero) = Zero()
-    end
+    @eval Base.:*(::Zero, x::$T) = zero(x)
+    @eval Base.:*(x::$T, ::Zero) = zero(x)
 end
-Base.:*(::Zero, x) = zero(x)
-Base.:*(x, ::Zero) = zero(x)
-
+Base.zero(::AbstractDifferential) = Zero()
 
 Base.:+(a::One, b::One) = extern(a) + extern(b)
 Base.:*(::One, ::One) = One()

From df2eee0c9b27d35fce19bb20bb1312da95b22bea Mon Sep 17 00:00:00 2001
From: Yingbo Ma <mayingbo5@gmail.com>
Date: Sun, 12 Jan 2020 15:47:14 -0500
Subject: [PATCH 7/7] Add tests and move zero(::AbstractDifferential) to the
 right folder

---
 src/differential_arithmetic.jl | 1 -
 src/differentials/zero.jl      | 2 ++
 test/differentials/zero.jl     | 4 ++++
 3 files changed, 6 insertions(+), 1 deletion(-)

diff --git a/src/differential_arithmetic.jl b/src/differential_arithmetic.jl
index fc7680803..985f1204f 100644
--- a/src/differential_arithmetic.jl
+++ b/src/differential_arithmetic.jl
@@ -41,7 +41,6 @@ for T in (:One, :AbstractThunk, :Any)
     @eval Base.:*(::Zero, x::$T) = zero(x)
     @eval Base.:*(x::$T, ::Zero) = zero(x)
 end
-Base.zero(::AbstractDifferential) = Zero()
 
 Base.:+(a::One, b::One) = extern(a) + extern(b)
 Base.:*(::One, ::One) = One()
diff --git a/src/differentials/zero.jl b/src/differentials/zero.jl
index 3e5312a72..2249a0b95 100644
--- a/src/differentials/zero.jl
+++ b/src/differentials/zero.jl
@@ -11,3 +11,5 @@ Base.Broadcast.broadcastable(::Zero) = Ref(Zero())
 
 Base.iterate(x::Zero) = (x, nothing)
 Base.iterate(::Zero, ::Any) = nothing
+
+Base.zero(::AbstractDifferential) = Zero()
diff --git a/test/differentials/zero.jl b/test/differentials/zero.jl
index 24d939d97..30f89a76f 100644
--- a/test/differentials/zero.jl
+++ b/test/differentials/zero.jl
@@ -12,4 +12,8 @@
     end
     @test broadcastable(z) isa Ref{Zero}
     @test conj(z) === z
+    @test zero(@thunk(3)) === z
+    @test zero(One()) === z
+    @test zero(DoesNotExist()) === z
+    @test zero(Composite{Tuple{Int,Int}}((1, 2))) === z
 end