add @? macro

src/Missings.jl

@@ -2,7 +2,7 @@ module Missings
 
 export allowmissing, disallowmissing, ismissing, missing, missings,
        Missing, MissingException, levels, coalesce, passmissing, nonmissingtype,
-       skipmissings
+       skipmissings, @?
 
 using Base: ismissing, missing, Missing, MissingException
 
@@ -283,7 +283,7 @@ struct SkipMissings{V, T}
     others::T
 end
 
-Base.@propagate_inbounds function _anymissingindex(others::Tuple{Vararg{AbstractArray}}, i)    
+Base.@propagate_inbounds function _anymissingindex(others::Tuple{Vararg{AbstractArray}}, i)
    for oth in others
         oth[i] === missing && return true
     end
@@ -292,7 +292,7 @@ Base.@propagate_inbounds function _anymissingindex(others::Tuple{Vararg{Abstract
 end
 
 @inline function _anymissingiterate(others::Tuple, state)
-    for oth in others 
+    for oth in others
         y = iterate(oth, state)
         y !== nothing && first(y) === missing && return true
     end
@@ -303,7 +303,7 @@ end
 const SkipMissingsofArrays = SkipMissings{V, T} where
     {V <: AbstractArray, T <: Tuple{Vararg{AbstractArray}}}
 
-function Base.show(io::IO, mime::MIME"text/plain", itr::SkipMissings{V}) where V 
+function Base.show(io::IO, mime::MIME"text/plain", itr::SkipMissings{V}) where V
     print(io, SkipMissings, '{', V, '}', '(', itr.x, ')', " comprised of " *
           "$(length(itr.others) + 1) iterators")
 end
@@ -360,7 +360,7 @@ end
 @inline function Base.getindex(itr::SkipMissingsofArrays, i)
     @boundscheck checkbounds(itr.x, i)
     @inbounds xi = itr.x[i]
-    if xi === missing || @inbounds _anymissingindex(itr.others, i) 
+    if xi === missing || @inbounds _anymissingindex(itr.others, i)
         throw(MissingException("the value at index $i is missing for some element"))
     end
     return xi
@@ -405,9 +405,9 @@ Base.mapreduce_impl(f, op, A::SkipMissingsofArrays, ifirst::Integer, ilast::Inte
     A = itr.x
     if ifirst == ilast
         @inbounds a1 = A[ifirst]
-        if a1 === missing 
+        if a1 === missing
             return nothing
-        elseif _anymissingindex(itr.others, ifirst) 
+        elseif _anymissingindex(itr.others, ifirst)
             return nothing
         else
             return Some(Base.mapreduce_first(f, op, a1))
@@ -461,7 +461,7 @@ end
 Return a vector similar to the array wrapped by the given `SkipMissings` iterator
 but skipping all elements with a `missing` value in one of the iterators passed
 to `skipmissing` and elements for which `f` returns `false`. This method
-only applies when all iterators passed to `skipmissings` are arrays. 
+only applies when all iterators passed to `skipmissings` are arrays.
 
 # Examples
 ```
@@ -490,4 +490,17 @@ function filter(f, itr::SkipMissingsofArrays)
     y
 end
 
+if VERSION >= v"1.3"
+    include("unionmissing.jl")
+else
+    union_macro_ex = :(
+    macro _(typ)
+            :(Union{$(esc(typ)), Missing})
+    end
+    )
+
+    union_macro_ex.args[1].args[1] = :?
+    eval(union_macro_ex)
+end
+
 end # module

src/unionmissing.jl

@@ -0,0 +1,28 @@
+"""
+    @?(ex)
+
+Wrap an expression `ex` into `Union{Missing, ex}`.
+
+Note that Julia will pass a whole expression `ex` following `@? to it`.
+Therefore using parantheses to correctly specify the `ex` is sometimes required.
+
+```jldoctest
+julia> Vector{@?Int}
+Vector{Union{Missing, Int64}} (alias for Array{Union{Missing, Int64}, 1})
+
+julia> Union{String, @?Int}
+Union{Missing, Int64, String}
+
+julia> Union{@?(Int), String}
+Union{Missing, Int64, String}
+
+julia> @?(Int)[1, 2, 3]
+3-element Vector{Union{Missing, Int64}}:
+ 1
+ 2
+ 3
+```
+"""
+macro var"?"(typ)
+    :(Union{$(esc(typ)), Missing})
+end

test/runtests.jl

@@ -193,6 +193,11 @@ struct CubeRooter end
     @test passmissing(Int) === Missings.PassMissing{Type{Int}}(Int)
     @test passmissing(cuberoot) === Missings.PassMissing{CubeRooter}(cuberoot)
 
+    @testset "@?" begin
+        @test (@?Int) === Union{Int, Missing}
+        @test @?(Int)[] isa Vector{Union{Int, Missing}}
+    end
+
     @testset "deprecated" begin
         x = [1, 2, missing, 4]
         y = ["a", "b", "c", missing]