Add conditional and passmissing

src/Missings.jl

@@ -1,7 +1,7 @@
 module Missings
 
 export allowmissing, disallowmissing, ismissing, missing, missings,
-       Missing, MissingException, levels, coalesce
+       Missing, MissingException, levels, coalesce, passmissing
 
 using Base: ismissing, missing, Missing, MissingException
 
@@ -165,4 +165,24 @@ function levels(x)
     levs
 end
 
+struct PassMissing{F} <: Function end
+
+(::PassMissing{F})(xs...;kw...) where {F} =
+     any(ismissing, xs) || any(ismissing, values(values(kw))) ? missing : F(xs...; kw...)
+
+"""
+    passmissing(f)
+
+Return a function that returns `missing` if any of its positional or keyword arguments
+are `missing` and otherwise applies `f` to those arguments.
+
+# Examples
+```jldoctest
+julia> passmissing(sqrt).([missing, 4])
+2-element Array{Union{Missing, Float64},1}:
+  missing
+   2.0
+"""
+passmissing(f::Base.Callable) = PassMissing{f}()
+
 end # module

test/runtests.jl

@@ -355,4 +355,8 @@ using Test, Dates, InteractiveUtils, SparseArrays, Missings
 
     # MissingException
     @test sprint(showerror, MissingException("test")) == "MissingException: test"
+
+    # Lifting
+    @test isequal(passmissing(sqrt).([missing, 4]), [missing, 2.0])
+    @test isequal(passmissing(parse)(Int, "a", base=missing), missing)
 end