Add operator kwarg to Cols (#58)

Author: bkamins

Project.toml

@@ -1,7 +1,7 @@
 name = "DataAPI"
 uuid = "9a962f9c-6df0-11e9-0e5d-c546b8b5ee8a"
 authors = ["quinnj <[email protected]>"]
-version = "1.13.0"
+version = "1.14.0"
 
 [compat]
 julia = "1"

src/DataAPI.jl

@@ -183,9 +183,9 @@ Between(x::Union{Int, Symbol}, y::AbstractString) = Between(x, Symbol(y))
 Between(x::AbstractString, y::Union{Int, Symbol}) = Between(Symbol(x), y)
 
 """
-    All(cols...)
+    All()
 
-Select the union of the selections in `cols`. If `cols == ()`, select all columns.
+Select all columns.
 """
 struct All{T<:Tuple}
     cols::T
@@ -198,17 +198,24 @@ struct All{T<:Tuple}
 end
 
 """
-    Cols(cols...)
-    Cols(f::Function)
+    Cols(cols...; operator=union)
+    Cols(f::Function; operator=union)
 
-Select the union of the selections in `cols`. If `cols == ()`, select no columns.
+Select columns matching specifications in `cols`. If `cols == ()`, select no columns.
 
 If the only positional argument is a `Function` `f` then select the columns whose
 names passed to the `f` predicate as strings return `true`.
+
+When multiple `cols` selectors are passed then the sets of columns selected by them
+are passed to `operator` as positional arguments.
+`operator` should be a set operation function, like `union`, `intersect`, `setdiff`, and `symdiff`
+defined in Base Julia. By default `operator=union` in which case all columns matching
+at least one selector are returned.
 """
 struct Cols{T<:Tuple}
     cols::T
-    Cols(args...) = new{typeof(args)}(args)
+    operator
+    Cols(args...; operator=union) = new{typeof(args)}(args, operator)
 end
 
 """

test/runtests.jl

@@ -249,6 +249,8 @@ end
             (Ref(DataAPI.BroadcastedSelector(v())) .=> [sum, float])
     end
 
+    @test DataAPI.Cols(:a, operator=union).operator == union
+    @test DataAPI.Cols(:a, operator=intersect).operator == intersect
 end
 
 @testset "unwrap" begin