From d22ab50b5bced55e1a46995ffd020e51c0bcb007 Mon Sep 17 00:00:00 2001
From: Jeff Bezanson <jeff.bezanson@gmail.com>
Date: Fri, 2 Jun 2017 19:26:59 -0400
Subject: [PATCH] add `merge` and `structdiff` for named tuples

---
 base/namedtuple.jl | 41 +++++++++++++++++++++++++++++++++++++++++
 test/namedtuple.jl | 10 ++++++++++
 2 files changed, 51 insertions(+)

diff --git a/base/namedtuple.jl b/base/namedtuple.jl
index b8a5d417c96f6..cd647845f5a1e 100644
--- a/base/namedtuple.jl
+++ b/base/namedtuple.jl
@@ -89,3 +89,44 @@ end
         namedtuple($NT, $(args...))
     end
 end
+
+# a version of `in` for the older world these generated functions run in
+function sym_in(x, itr)
+    for y in itr
+        y === x && return true
+    end
+    return false
+end
+
+@generated function merge(a::NamedTuple{an}, b::NamedTuple{bn}) where {an, bn}
+    names = Symbol[an...]
+    for n in bn
+        if !sym_in(n, an)
+            push!(names, n)
+        end
+    end
+    vals = map(names) do n
+        if sym_in(n, bn)
+            :(getfield(b, $(Expr(:quote, n))))
+        else
+            :(getfield(a, $(Expr(:quote, n))))
+        end
+    end
+    names = (names...,)
+    :(namedtuple(NamedTuple{$names}, $(vals...)))
+end
+
+@generated function structdiff(a::NamedTuple{an},
+                               b::Union{NamedTuple{bn},Type{NamedTuple{bn}}}) where {an,bn}
+    names = Symbol[]
+    for n in an
+        if !sym_in(n, bn)
+            push!(names, n)
+        end
+    end
+    vals = map(names) do n
+        :(getfield(a, $(Expr(:quote, n))))
+    end
+    names = (names...,)
+    :(namedtuple(NamedTuple{$names}, $(vals...)))
+end
diff --git a/test/namedtuple.jl b/test/namedtuple.jl
index 6738fa3f94f63..b689546367676 100644
--- a/test/namedtuple.jl
+++ b/test/namedtuple.jl
@@ -51,3 +51,13 @@
 @test map(+, (x=1, y=2), (x=10, y=20)) == (x=11, y=22)
 @test map(string, (x=1, y=2)) == (x="1", y="2")
 @test map(round, (x=1//3, y=Int), (x=3, y=2//3)) == (x=0.333, y=1)
+
+@test merge((a=1, b=2), (a=10,)) == (a=10, b=2)
+@test merge((a=1, b=2), (a=10, z=20)) == (a=10, b=2, z=20)
+@test merge((a=1, b=2), (z=20,)) == (a=1, b=2, z=20)
+
+@test Base.structdiff((a=1, b=2), (b=3,)) == (a=1,)
+@test Base.structdiff((a=1, b=2, z=20), (b=3,)) == (a=1, z=20)
+@test Base.structdiff((a=1, b=2, z=20), (b=3, q=20, z=1)) == (a=1,)
+@test Base.structdiff((a=1, b=2, z=20), (b=3, q=20, z=1, a=0)) == NamedTuple()
+@test Base.structdiff((a=1, b=2, z=20), NamedTuple{(:b,)}) == (a=1, z=20)