triangular dispatch (left-to-right template parameter chaining)

[timholy]

If it wouldn't cause the very foundations of the type system to crumble, it seems like it would be nice to be able to do this:

colorspace{C<:ColorValue,A<:AbstractArray{C}}(img::A) = string(C)

Use case: given an inheritance pattern

Image{T,N,A<:AbstractArray} <: AbstractImageIndexed{T,N} <: AbstractImage{T,N} <:AbstractArray{T,N}

this:

colorspace{C<:ColorValue}(img::AbstractArray{C}) = string(C)

has problems because methods like this:

colorspace(img::AbstractImage) = get(img.properties, "colorspace", "Unknown")

take precedence. One needs to use constructs like this:

colorspace{C<:ColorValue}(img::Union(Array{C},SubArray{C},Image{C})) = string(C)

Even this:

colorspace{C<:ColorValue}(img::Union(AbstractArray{C},AbstractImage{C})) = string(C)

fails to take precedence. But I have more concrete image types defined than just Image, so it would be nice to have this work more generally without having to list all of them.

There's also an example in the FAQ that might benefit from this. In that case one could solve the problem using an inner constructor, but this type-chaining would have been more natural.

[lindahua]

+1

[JeffBezanson]

It wouldn't cause the foundations of the type system to crumble, but it would cause them to be more difficult to implement :)

True that there are lots of uses of this; I have wanted it a few times myself.

In this particular case, AbstractImage is still more specific than AbstractArray{C} even with this feature. It should work to define the method for AbstractImage{C}.

[timholy]

That's a helpful tip re AbstractImage, thanks.

[filmackay]

+typemax(Uint128)

I believe this is referred to as "triangular dispatch"? See this discussion.

[andreasnoack]

Following up on #6620 here. This feature seems necessary for defining QR for arbitrary AbstractMatrix.

[andreasnoack]

bump

[quinnj]

Superseded by #6984? Specifically the proposal here?

[quinnj]

Closing in favor of Jeff's proposal mentioned here. Feel free to reopen if that wouldn't cover the issue here.