Julia 0.6 TODO: write awesome promotion rules

[timholy]

Between ColorTypes and #272, we have some useful but imperfect promotion rules that I propose to fix once we can rely on the awesome new type system. Here's one simple example of current failings:

julia> using ColorTypes

julia> promote_type(RGB1{Float32}, RGB1{Float32})
ColorTypes.RGB1{Float32}

julia> promote_type(RGB1{Float32}, RGB1{Float64})
ColorTypes.RGB{Float64}

This behavior is a consequence of these rules. The ideal way to write this would be to separately promote the "inner" type (Float32 or Float64) and the "outer" type (e.g., RGB1{<:Fractional}). I suffered through doing something similar with conversion (e.g., convert(RGB, c) without specifying the "inner" type) on julia 0.4, and it wasn't fun: getting the dispatch working properly for types with only a typevar inner type was basically black art and strongly dependent on the order of function definitions. I basically don't have the time/patience to do this again. But once we can rely on the new type system, this should be much more pleasant.

[kimikage]

Can they move to ColorTypes.jl? (0646395)

Colors.jl/src/Colors.jl

Lines 9 to 11 in 0ceb396

	# TODO: why these types are defined here? Can they move to ColorTypes.jl?
	AbstractAGray{C<:AbstractGray,T} = AlphaColor{C,T,2}
	AbstractGrayA{C<:AbstractGray,T} = ColorAlpha{C,T,2}

Colors.jl/src/promotions.jl

Lines 1 to 6 in 5d6f18a

	Base.promote_rule(::Type{C3}, ::Type{Cgray}) where {C3<:Color3,Cgray<:AbstractGray} = base_colorant_type(C3){promote_type(eltype(C3), eltype(Cgray))}
	Base.promote_rule(::Type{C3}, ::Type{Cagray}) where {C3<:Color3,Cagray<:AbstractAGray} = alphacolor(base_colorant_type(C3)){promote_type(eltype(C3), eltype(Cagray))}
	Base.promote_rule(::Type{C3}, ::Type{Cgraya}) where {C3<:Color3,Cgraya<:AbstractGrayA} = coloralpha(base_colorant_type(C3)){promote_type(eltype(C3), eltype(Cgraya))}
	Base.promote_rule(::Type{C3}, ::Type{Cgray}) where {C3<:Transparent3,Cgray<:AbstractGray} = base_colorant_type(C3){promote_type(eltype(C3), eltype(Cgray))}
	Base.promote_rule(::Type{C3}, ::Type{Cgray}) where {C3<:Transparent3,Cgray<:TransparentGray} = base_colorant_type(C3){promote_type(eltype(C3), eltype(Cgray))}

[timholy]

Those seem like reasonable candidates for moving.

[kimikage]

Bump.
JuliaGraphics/ColorTypes.jl#129
JuliaGraphics/ColorTypes.jl#164

[kimikage]

I wrote awful promotion rules in ColorTypes.jl on Julia 1.3 (cf. JuliaGraphics/ColorTypes.jl#170). 😄
The rules will take effect in the next release.

[timholy]

They're beautiful, not awful. Though I know exactly what you mean. So glad to have this fixed!