allow passing multiple arguments to append! & prepend! (#36227)

E.g. append!([1], [2], [3]) == [1, 2, 3].
The equivalent operation for sets (`union!`) and dictionaries (`merge!`)
already supports multiple arguments, as well as `push!`.

For `prepend!`, order is maintained in the same way as in `pushfirst!`:
`prepend!([3], [1], [2]) == [1, 2, 3] == pushfirst!([3], 1, 2)`.

NEWS.md

@@ -92,6 +92,8 @@ New library features
 * New constructor `NamedTuple(iterator)` that constructs a named tuple from a key-value pair iterator.
 * A new `reinterpret(reshape, T, a::AbstractArray{S})` reinterprets `a` to have eltype `T` while potentially
   inserting or consuming the first dimension depending on the ratio of `sizeof(T)` and `sizeof(S)`.
+* New `append!(vector, collections...)` and `prepend!(vector, collections...)` methods accept multiple
+  collections to be appended or prepended ([#36227]).
 
 Standard library changes
 ------------------------

base/array.jl

@@ -944,19 +944,23 @@ function push!(a::Array{Any,1}, @nospecialize item)
 end
 
 """
-    append!(collection, collection2) -> collection.
+    append!(collection, collections...) -> collection.
 
-For an ordered container `collection`, add the elements of `collection2` to the end of it.
+For an ordered container `collection`, add the elements of each `collections`
+to the end of it.
+
+!!! compat "Julia 1.6"
+    Specifying multiple collections to be appended requires at least Julia 1.6.
 
 # Examples
 ```jldoctest
-julia> append!([1],[2,3])
+julia> append!([1], [2, 3])
 3-element Vector{Int64}:
  1
  2
  3
 
-julia> append!([1, 2, 3], [4, 5, 6])
+julia> append!([1, 2, 3], [4, 5], [6])
 6-element Vector{Int64}:
  1
  2
@@ -981,6 +985,8 @@ end
 append!(a::AbstractVector, iter) = _append!(a, IteratorSize(iter), iter)
 push!(a::AbstractVector, iter...) = append!(a, iter)
 
+append!(a::AbstractVector, iter...) = foldl(append!, iter, init=a)
+
 function _append!(a, ::Union{HasLength,HasShape}, iter)
     n = length(a)
     i = lastindex(a)
@@ -999,17 +1005,32 @@ function _append!(a, ::IteratorSize, iter)
 end
 
 """
-    prepend!(a::Vector, items) -> collection
+    prepend!(a::Vector, collections...) -> collection
+
+Insert the elements of each `collections` to the beginning of `a`.
+
+When `collections` specifies multiple collections, order is maintained:
+elements of `collections[1]` will appear leftmost in `a`, and so on.
 
-Insert the elements of `items` to the beginning of `a`.
+!!! compat "Julia 1.6"
+    Specifying multiple collections to be prepended requires at least Julia 1.6.
 
 # Examples
 ```jldoctest
-julia> prepend!([3],[1,2])
+julia> prepend!([3], [1, 2])
 3-element Vector{Int64}:
  1
  2
  3
+
+julia> prepend!([6], [1, 2], [3, 4, 5])
+6-element Vector{Int64}:
+ 1
+ 2
+ 3
+ 4
+ 5
+ 6
 ```
 """
 function prepend! end
@@ -1029,6 +1050,8 @@ end
 prepend!(a::Vector, iter) = _prepend!(a, IteratorSize(iter), iter)
 pushfirst!(a::Vector, iter...) = prepend!(a, iter)
 
+prepend!(a::AbstractVector, iter...) = foldr((v, a) -> prepend!(a, v), iter, init=a)
+
 function _prepend!(a, ::Union{HasLength,HasShape}, iter)
     require_one_based_indexing(a)
     n = length(iter)

test/arrayops.jl

@@ -1613,6 +1613,14 @@ end
     @test append!([1,2], g) == [1,2] == push!([1,2], g...)
     @test prepend!([1,2], g) == [1,2] == pushfirst!([1,2], g...)
 
+    # multiple items
+    A = [1]
+    @test append!(A, [2, 3], [4], [5, 6]) === A
+    @test A == [1, 2, 3, 4, 5, 6]
+    A = [1]
+    @test prepend!(A, [2, 3], [4], [5, 6]) === A
+    @test A == [2, 3, 4, 5, 6, 1]
+
     # offset array
     @test append!([1,2], OffsetArray([9,8], (-3,))) == [1,2,9,8]
     @test prepend!([1,2], OffsetArray([9,8], (-3,))) == [9,8,1,2]