Add isordered

Project.toml

@@ -1,7 +1,10 @@
 name = "DataAPI"
 uuid = "9a962f9c-6df0-11e9-0e5d-c546b8b5ee8a"
 authors = ["quinnj <quinn.jacobd@gmail.com>"]
-version = "1.4.0"
+version = "1.5.0"
+
+[deps]
+Dates = "ade2ca70-3891-5945-98fb-dc099432e06a"
 
 [compat]
 julia = "1"

src/DataAPI.jl

@@ -1,5 +1,7 @@
 module DataAPI
 
+import Dates
+
 """
     defaultarray(T, N)
 
@@ -89,6 +91,9 @@ If the collection is not sortable then the order of levels is unspecified.
 
 Contrary to [`unique`](@ref), this function may return values which do not
 actually occur in the data, and does not preserve their order of appearance in `x`.
+
+If a type of `A` implements `levels` with a meaningful order then it should also
+implement `isordered` as by default `isordered` returns `false`.
 """
 function levels(x)
     T = Base.nonmissingtype(eltype(x))
@@ -100,6 +105,26 @@ function levels(x)
     levs
 end
 
+"""
+    isordered(A)
+
+Test whether entries in `A` can be compared using `<`, `>` and similar operators,
+using the ordering of `levels(A)`.
+
+If a type of `A` implements `levels` with a meaningful order then it should also
+implement `isordered` as by default `isordered` returns `false`.
+"""
+function isordered end
+
+isordered(::AbstractArray{<:Union{Missing, AbstractString}}) = true
+isordered(::AbstractArray{<:Union{Missing, Real}}) = true
+isordered(::AbstractArray{<:Union{Missing, Symbol}}) = true
+isordered(::AbstractArray{<:Union{Missing, AbstractChar}}) = true
+isordered(::AbstractArray{<:Union{Missing, Dates.Period}}) = true
+isordered(::AbstractArray{<:Union{Missing, Dates.TimeType}}) = true
+isordered(::AbstractArray{Missing}) = true
+isordered(::AbstractArray{<:Any}) = false
+
 """
     Between(first, last)
 

test/runtests.jl

@@ -84,5 +84,17 @@ end
     end
 
 end
+
+@testset "isordered" begin
+
+    @test DataAPI.isordered([1])
+    @test DataAPI.isordered(Union{Real,Missing}[1])
+    @test DataAPI.isordered(["1"])
+    @test DataAPI.isordered(['1'])
+    @test !DataAPI.isordered(Any[1])
+    @test !DataAPI.isordered(["1", '1'])
+    @test !DataAPI.isordered(Union{Char, String}["1", '1'])
+
+end
 
 end # @testset "DataAPI"