Implementing order functions where missings is the smallest value Jul…

…iaData#142

README.md

@@ -28,6 +28,8 @@ This package provides additional functionality for working with `missing` values
 - `Missings.replace` to wrap a collection in a (possibly indexable) iterator replacing `missing` with another value
 - `Missings.fail` to wrap a collection in a (possibly indexable) iterator throwing an error if `missing` is encountered
 - `skipmissings` to loop through a collection of iterators excluding indices where any iterators are `missing`
+- `missingsmallest(f)` to create a partial order function that behaves as `f` and making `missing` the smallest value
+- `missingsless` the standard `isless` function modified so that `missing` is less than any other element
 
 ## Contributing and Questions
 

src/Missings.jl

@@ -2,7 +2,7 @@ module Missings
 
 export allowmissing, disallowmissing, ismissing, missing, missings,
        Missing, MissingException, levels, coalesce, passmissing, nonmissingtype,
-       skipmissings, emptymissing
+       skipmissings, emptymissing, missingsmallest, missingsless
 
 using Base: ismissing, missing, Missing, MissingException
 
@@ -514,4 +514,32 @@ julia> emptymissing(first)([1], 2)
 """
 emptymissing(f) = (x, args...; kwargs...) -> isempty(x) ? missing : f(x, args...; kwargs...)
 
+# Variant of `isless` where `missing` is the smallest value
+"""
+    missingsmallest(f::Function)
+
+Creates a function of two arguments `x` and `y` that tests whether `x` is less
+than `y` such that `missing` is always less than the other argument. In other
+words, modifies the partial order function `f` such that `missing` is the
+smallest possible value, and all other non-`missing` values are compared
+according to `f`.
+
+See also: [`missingsless`](@ref), the standard order where `missing` is the
+smallest possible value.
+
+# Examples
+```
+julia> missingsmallest(isless)(missing, Inf)
+true
+
+julia> missingsless(-Inf, missing)
+false
+```
+"""
+missingsmallest(f) = (x, y) -> ismissing(y) ? false : ismissing(x) ? true : f(x, y)
+
+" The standard partial order `isless` modified so that `missing` is always the
+smallest possible value."
+const missingsless = missingsmallest(isless)
+
 end # module

test/runtests.jl

@@ -257,4 +257,11 @@ struct CubeRooter end
     @test emptymissing(fun)(3, 1, c=2) == (1, 2)
 end
 
+@testset "missingsmallest" begin
+    @test missingsless(missing, Inf) == true
+    @test missingsless(-Inf, missing) == false
+    @test missingsless(3, 4) == true
+    @test missingsless(-Inf, Inf) == true 
+end
+
 end