Implement promotion rules for CategoricalArray (#384)

These are rarely needed, but an exception is when storing `CategoricalArray`
objects within arrays, as the `[a1, a2]` syntax uses promotion to choose
the element type of the result. This previously failed as it hit a fallback
`promote_result` method defined for `AbstractArray` in Base in range.jl,
which tried to convert `CategoricalArray`s to `Array` by calling nonexistent
`convert` methods for `CategoricalValue`.

src/array.jl

@@ -302,6 +302,19 @@ CategoricalMatrix(A::CategoricalArray{T, 2, R};
  ordered::Bool=_isordered(A)) where {T, R} =
  CategoricalArray{T, 2, R}(A, levels=levels, ordered=ordered)
 
+## Promotion methods
+
+Base.promote_rule(::Type{<:CategoricalArray{S}},
+ ::Type{<:CategoricalArray{T}}) where {S, T} =
+ CategoricalArray{cat_promote_type(S, T)}
+Base.promote_rule(::Type{<:CategoricalArray{S, N}},
+ ::Type{<:CategoricalArray{T, N}}) where {S, T, N} =
+ CategoricalArray{cat_promote_type(S, T), N}
+Base.promote_rule(::Type{<:CategoricalArray{S, N, R1}},
+ ::Type{<:CategoricalArray{T, N, R2}}) where
+ {S, T, N, R1<:Integer, R2<:Integer} =
+ CategoricalArray{cat_promote_type(S, T), N, promote_type(R1, R2)}
+
 ## Conversion methods
 
 # From AbstractArray

test/13_arraycommon.jl

@@ -2224,4 +2224,54 @@ end
  end
 end
 
+@testset "promotion" begin
+ @test promote_type(CategoricalVector{Int},
+ CategoricalVector{String}) ==
+ CategoricalVector{Union{Int, String}}
+ @test promote_type(CategoricalVector{Int, UInt32},
+ CategoricalVector{String, UInt32}) ==
+ CategoricalVector{Union{Int, String}, UInt32}
+ @test promote_type(CategoricalArray{Int, UInt32},
+ CategoricalArray{String, UInt32}) ==
+ CategoricalArray{Union{Int, String}, UInt32}
+ @test promote_type(CategoricalVector{Int, UInt32},
+ CategoricalMatrix{String, UInt32}) ==
+ CategoricalArray{Union{Int, String}}
+ @test promote_type(CategoricalVector{Int, UInt8},
+ CategoricalVector{String, UInt16}) ==
+ CategoricalVector{Union{Int, String}, UInt16}
+
+ @test promote_type(CategoricalVector{Int8},
+ CategoricalVector{Float64}) ==
+ CategoricalVector{Float64}
+ @test promote_type(CategoricalVector{Int8, UInt32},
+ CategoricalVector{Float64, UInt32}) ==
+ CategoricalVector{Float64, UInt32}
+ @test promote_type(CategoricalArray{Int8, UInt32},
+ CategoricalArray{Float64, UInt32}) ==
+ CategoricalArray{Float64, UInt32}
+ @test promote_type(CategoricalVector{Int8, UInt32},
+ CategoricalMatrix{Float64, UInt32}) ==
+ CategoricalArray{Float64}
+ @test promote_type(CategoricalVector{Int8, UInt8},
+ CategoricalVector{Float64, UInt16}) ==
+ CategoricalVector{Float64, UInt16}
+
+ @test [CategoricalVector([1, 2]),
+ CategoricalVector(["a", "b"])] isa
+ Vector{CategoricalVector{Union{Int, String}, UInt32}}
+ @test [CategoricalVector([1, missing]),
+ CategoricalVector(["a", "b"])] isa
+ Vector{CategoricalVector{Union{Int, String, Missing}, UInt32}}
+ @test [CategoricalVector([1, missing]),
+ CategoricalVector(["a", missing])] isa
+ Vector{CategoricalVector{Union{Int, String, Missing}, UInt32}}
+ @test [CategoricalVector([Int8(1), missing]),
+ CategoricalVector([Int16(2)])] isa
+ Vector{CategoricalVector{Union{Int16, Missing}, UInt32}}
+ @test [CategoricalVector([1, 2]),
+ CategoricalMatrix(["a" "b"])] isa
+ Vector{CategoricalArray{Union{Int, String}}}
+end
+
 end