add emptymissing (#143)

Project.toml

@@ -1,6 +1,6 @@
 name = "Missings"
 uuid = "e1d29d7a-bbdc-5cf2-9ac0-f12de2c33e28"
-version = "1.0.2"
+version = "1.1.0"
 
 [deps]
 DataAPI = "9a962f9c-6df0-11e9-0e5d-c546b8b5ee8a"

src/Missings.jl

@@ -2,7 +2,7 @@ module Missings
 
 export allowmissing, disallowmissing, ismissing, missing, missings,
        Missing, MissingException, levels, coalesce, passmissing, nonmissingtype,
-       skipmissings
+       skipmissings, emptymissing
 
 using Base: ismissing, missing, Missing, MissingException
 
@@ -459,7 +459,7 @@ 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
 ```
@@ -488,4 +488,30 @@ function filter(f, itr::SkipMissingsofArrays)
     y
 end
 
+"""
+    emptymissing(f)
+
+Return a function that takes at least one positional argument `x` and
+returns `missing` if `x` is empty (`isempty(x)` returns `true`)
+and otherwise returns `f(x, args...; kwargs...)`, where `args` are following
+positional arguments passed to `f` and `kwargs` are its keyword arguments.
+
+# Examples
+```
+julia> emptymissing(last)([])
+missing
+
+julia> emptymissing(last)([1, 2, 3])
+3
+
+julia> emptymissing(first)([], 2)
+missing
+
+julia> emptymissing(first)([1], 2)
+1-element Vector{Int64}:
+1
+```
+"""
+emptymissing(f) = (x, args...; kwargs...) -> isempty(x) ? missing : f(x, args...; kwargs...)
+
 end # module

test/runtests.jl

@@ -246,4 +246,15 @@ struct CubeRooter end
             @inferred Union{Missing, Int} reduce(+, mx)
         end
     end
+
+@testset "emptymissing" begin
+    @test emptymissing(last)([]) === missing
+    @test emptymissing(last)([1, 2, 3]) === 3
+    @test emptymissing(sum)(skipmissing(missings(Int, 3))) === missing
+    @test emptymissing(sum)(skipmissing([1, 2, 3])) === 6
+    fun(a, b; c) = (b, c)
+    @test emptymissing(fun)([], 1, c=2) === missing
+    @test emptymissing(fun)(3, 1, c=2) == (1, 2)
+end
+
 end