Add methods to reduce, specialized for merge on vector of Dict (#…

…26440)

base/dict.jl

@@ -691,6 +691,12 @@ end
 
 filter!(f, d::Dict) = filter_in_one_pass!(f, d)
 
+function reduce(::typeof(merge), items::Vector{<:Dict})
+ K = mapreduce(keytype, promote_type, items)
+ V = mapreduce(valtype, promote_type, items)
+ return reduce(merge!, items; init=Dict{K,V}())
+end
+
 function map!(f, iter::ValueIterator{<:Dict})
  dict = iter.dict
  vals = dict.vals

test/dict.jl

@@ -931,6 +931,23 @@ end
  @test d1 == Dict("A" => 1, "B" => 15, "C" => 28)
 end
 
+@testset "Dict reduce merge" begin
+ function check_merge(i::Vector{<:Dict}, o)
+ r1 = reduce(merge, i)
+ r2 = merge(i...)
+ t = typeof(o)
+ @test r1 == o
+ @test r2 == o
+ @test typeof(r1) == t
+ @test typeof(r2) == t
+ end
+ check_merge([Dict(1=>2), Dict(1.0=>2.0)], Dict(1.0=>2.0))
+ check_merge([Dict(1=>2), Dict(2=>Complex(1.0, 1.0))],
+ Dict(2=>Complex(1.0, 1.0), 1=>Complex(2.0, 0.0)))
+ check_merge([Dict(1=>2), Dict(3=>4)], Dict(3=>4, 1=>2))
+ check_merge([Dict(3=>4), Dict(:a=>5)], Dict(:a => 5, 3 => 4))
+end
+
 @testset "misc error/io" begin
  d = Dict('a'=>1, 'b'=>1, 'c'=> 3)
  @test_throws ErrorException 'a' in d