MicroCollections.jl provides immutable empty and singleton collections.
julia> using MicroCollections
julia> vec0() # or EmptyVector()
0-element EmptyVector{Union{}}
julia> vec0(Int) # or EmptyVector{Int}()
0-element EmptyVector{Int64}
julia> vec1(1) # or SingletonVector((1,))
1-element SingletonVector{Int64}:
1
julia> EmptyDict()
EmptyDict{Union{},Union{}}()
julia> EmptyDict{Symbol,Char}()
EmptyDict{Symbol,Char}()
julia> SingletonDict(:a => 0)
SingletonDict{Symbol,Int64} with 1 entry:
:a => 0
julia> EmptySet()
EmptySet{Union{}}()
julia> EmptySet{Int64}()
EmptySet{Int64}()
julia> SingletonSet((1,))
SingletonSet{Int64} with 1 element:
1With BangBang.jl, MicroCollections.jl is useful for constructing singleton solutions that can be combined with a reduce:
julia> using BangBang.Experimental: mergewith!!
julia> @assert mapreduce(
x -> SingletonDict(abs(x) % 10 => 1), mergewith!!(+), 1:1000,
) == Dict(
0 => 100,
1 => 100,
2 => 100,
3 => 100,
4 => 100,
5 => 100,
6 => 100,
7 => 100,
8 => 100,
9 => 100,
)