Add Experimental.mergewith!!

src/BangBang.jl

@@ -53,6 +53,7 @@ using .NoBang: Empty, ImmutableContainer, SingletonVector, singletonof
 include("core.jl")
 include("base.jl")
 include("linearalgebra.jl")
+include("experimental.jl")
 include("initials.jl")
 include("macro.jl")
 include("dataframes_impl.jl")

src/base.jl

@@ -297,24 +297,12 @@ Dict{Symbol,Float64} with 1 entry:
   :a => 1.5
 ```
 """
-merge!!(dict::Union{AbstractDict,NamedTuple,Empty}, others...) =
-    merge!!(right, dict, others...)
-
-merge!!(combine, dict, other) =
-    foldl(pairs(other); init=dict) do dict, (k, v2)
-        v1 = get(dict, k, _NoValue())
-        setindex!!(dict, v1 isa _NoValue ? v2 : combine(v1, v2), k)
-    end
-
-merge!!(combine, dict, others...) =
-    foldl(others; init=dict) do dict, other
-        merge!!(combine, dict, other)
-    end
+merge!!(dict, others...) = merge!!(right, dict, others...)
+merge!!(combine::Base.Callable, dict, others...) =
+    Experimental.mergewith!!(combine, dict, others...)
 
 right(_, x) = x
 
-struct _NoValue end
-
 #=
 """
     splice!!(sequence, i, [replacement]) -> (sequence′, item)

src/experimental.jl

@@ -0,0 +1,37 @@
+module Experimental
+
+export mergewith!!
+
+using ..BangBang: setindex!!
+
+struct _NoValue end
+
+"""
+    mergewith!!(combine, dict, others...) -> dict′
+    mergewith!!(combine)
+
+Like `merge!!(combine, dict, others...)` but `combine` does not have
+to be a `Function`.
+
+The curried form `mergewith!!(combine)` returns the function
+`(args...) -> mergewith!!(combine, args...)`.
+
+See also:
+[Add mergewith[!](combine, dicts...) by tkf · Pull Request #34296 · JuliaLang/julia](https://github.com/JuliaLang/julia/pull/34296)
+"""
+mergewith!!
+
+mergewith!!(combine, dict, other) =
+    foldl(pairs(other); init=dict) do dict, (k, v2)
+        v1 = get(dict, k, _NoValue())
+        setindex!!(dict, v1 isa _NoValue ? v2 : combine(v1, v2), k)
+    end
+
+mergewith!!(combine, dict, others...) =
+    foldl(others; init=dict) do dict, other
+        mergewith!!(combine, dict, other)
+    end
+
+mergewith!!(combine) = (args...) -> mergewith!!(combine, args...)
+
+end  # module