add merge and structdiff for named tuples

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

test/namedtuple.jl

@@ -45,3 +45,13 @@
 @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)