Arraylike: AbstractArrays without an eltype

[bramtayl]

Would be useful for many packages, JuliennedArrays, LightQuery, SplitApplyCombine, MappedArrays, Query, etc. Base already has an AbstractEltypelessArray: the lazy broadcast machinery, it would just be a matter of formalizing the interface. Given that traits aren't coming any time soon, it couldn't be <: AbstractArray, so we would have to duplicate all of the machinery for AbstactArrays. I could try to build a prototype if people are interested?

[JeffBezanson]

I think it would be more realistic to use eltype Any, and try to get things working as well as possible with such arrays.

[bramtayl]

Oh, ok, if that would be easier.

[bramtayl]

Ok, while I'm at it, it would be nice to have abstractarray machinery for all sorts of things, zips of arrays, generators of arrays

[bramtayl]

Actually, we could just change most AbstractArray functions to be Union{AbstractArray, AbstractEltypelessArray} and nip the problem in the bud, I think.

[bramtayl]

Took a shower, came back, and realized that because this is a meaningful subtype relationship, we don't need traits or Unions. PR coming.

[bramtayl]

"""
    abstract type Arraylike{Dimensions} end

Things that have a shape, but not a compile-time [`eltype`](@ref). Use
[`AbstractArray`] if you know the eltype at compile-time.
"""
abstract type Arraylike{Dimensions} end

abstract type AbstractArray{Item, Dimensions} <: Arraylike{Dimensions} end

TADA problem solved

[bramtayl]

Welp, its not that easy because AbstractArray is implemented in C according to boot.jl. I'll punt on making a PR.

[JeffBezanson]

The type is not compile time.

[tkf]

👍 for this direction. I needed to emulate Arraylike using Union when implementing mapreduce for Broadcasted #31020. There was a small discussion in #31020 (comment)

[bramtayl]

@JeffBezanson this is the stickingpoint for adding JuliennedArrays functionality to base. I was looking through src to see how hard it would be to add Arraylike to base. I see that Signed is added in base/boot.jl, and Int64 is added in src/builtins.c, but where is the code that says Int64 <: Signed?

[bramtayl]

See discussion in #34196