Add Empty

src/BangBang.jl

@@ -4,6 +4,7 @@ module BangBang
 @doc read(joinpath(dirname(@__DIR__), "README.md"), String) BangBang
 
 export @!,
+       Empty,
        append!!,
        delete!!,
        empty!!,
@@ -33,7 +34,7 @@ function ismutable end
 function push!! end
 
 include("NoBang/NoBang.jl")
-using .NoBang: ImmutableContainer, singletonof
+using .NoBang: Empty, ImmutableContainer, singletonof
 
 include("core.jl")
 include("base.jl")

src/NoBang/NoBang.jl

@@ -1,6 +1,6 @@
 module NoBang
 
-export singletonof
+export Empty, singletonof
 
 using Base.Iterators: Pairs
 using Base: ImmutableDict
@@ -17,6 +17,7 @@ include("singletoncontainers.jl")
 include("base.jl")
 include("linearalgebra.jl")
 include("singletonof.jl")
+include("emptycontainers.jl")
 
 function __init__()
     @require StaticArrays="90137ffa-7385-5640-81b9-e52037218182" begin

src/NoBang/emptycontainers.jl

@@ -0,0 +1,35 @@
+"""
+    Empty(T)
+
+Create a proxy of an empty container of type `T`.
+
+# Examples
+```jldoctest
+julia> using BangBang
+
+julia> push!!(Empty(Vector), 1)
+1-element Array{Int64,1}:
+ 1
+
+julia> append!!(Empty(Dict), (:a=>1, :b=>2))
+Dict{Symbol,Int64} with 2 entries:
+  :a => 1
+  :b => 2
+
+julia> using DataFrames: DataFrame
+
+julia> @assert push!!(Empty(DataFrame), (a=1, b=2)) == DataFrame(a=[1], b=[2])
+
+julia> using StructArrays: StructVector
+
+julia> @assert push!!(Empty(StructVector), (a=1, b=2)) == StructVector(a=[1], b=[2])
+
+julia> using TypedTables: Table
+
+julia> @assert push!!(Empty(Table), (a=1, b=2)) == Table(a=[1], b=[2])
+```
+"""
+struct Empty{T} end
+Empty(T::Type) = Empty{T}()
+
+push(::Empty{T}, x) where T = singletonof(T, x)