add default constructors for AbstractRange

[putianyi889]

Related posts

• Support convert(AbstractArray{T}, ::AbstractRange) and variants #27138
• Implement AbstractVector{T}(r::AbstractRange), fix #27138 #28956

[inkydragon]

Is it necessary to add constructors for abstract types?

julia> methods(AbstractRange)
# 0 methods for type constructor

julia> supertype(AbstractRange)
AbstractVector (alias for AbstractArray{T, 1} where T)

julia>  methods(AbstractVector)
# 0 methods for type constructor

julia> supertype(AbstractVector)
Any

[putianyi889]

As you can see from added tests, it does eltype conversion for general ranges.
AbstractArray has constructors.

[jishnub]

@putianyi889 This seems like a good idea. Is this ready from your side?
We would probably want more tests.

[putianyi889]

Is this ready from your side?

confirm.

We would probably want more tests.

not sure how many. maybe add 2-3 segments to every conversion test?

[jishnub]

Could you add tests for other range types, such as StepRangeLen and StepRange, including cases where the elements are floating-point types?

[mcabbott]

Here are some failures from this T(first(r)):T(step(r)):T(last(r)) construction:

julia> convert(AbstractArray{Float32}, 0 * range(1, 2, 10)) |> length
ERROR: ArgumentError: range step cannot be zero

julia> convert(AbstractArray{Float16}, range(1/43^2, 1, 43)) |> length
42

I assume that range(start=T(first(r)), stop=T(last(r)), length=length(r)) would avoid these.

[putianyi889]

Thanks for pointing that out!

range would fix StepRangeLen, but changes OrdinalRange to StepRangeLen as well. Anyway, either OrdinalRange or StepRangeLen needs a specialized method.

As for the general fallback, I would keep the colon syntax until there are other issues.

[mcabbott]

Wait the priority surely is to get the right answer?

If necessary, there can be specialised paths which try also to preserve certain types, like UnitRange and StepRange and LinRange, when possible. But the default path should not rely on a:b:c colon magic to keep away from factually wrong answers.