Loosen signature of read! to accept views?

[giordano]

On slack it was pointed out that you can't write a binary file to a slice of an Array:

julia> x = rand(3, 3)
3×3 Array{Float64,2}:
 0.899822  0.0439746   0.961134
 0.748855  0.0687037   0.238223
 0.774079  0.00639737  0.224812

julia> open("test.bin", "w") do io
           write(io, [1.0, 2.0, 3.0])
       end;

julia> open("test.bin", "r") do io
           read!(io, x[:,3])
       end
3-element Array{Float64,1}:
 1.0
 2.0
 3.0

julia> x
3×3 Array{Float64,2}:
 0.899822  0.0439746   0.961134
 0.748855  0.0687037   0.238223
 0.774079  0.00639737  0.224812

That's of course because the slice creates a copy, so it would be great if we could call read!(io, @view x[:,3]), but the signature of read! doesn't accept it.

Would it make sense to loosen the signature of read! to be something like

read!(s::IO, a::Union{Array{T}, SubArray{T,N,<:Array{T}} where N}) where T

? In this way we could do:

julia> open("test.bin", "r") do io
           read!(io, @view x[:,3])
       end
3-element view(::Array{Float64,2}, :, 3) with eltype Float64:
 1.0
 2.0
 3.0

julia> x
3×3 Array{Float64,2}:
 0.899822  0.0439746   1.0
 0.748855  0.0687037   2.0
 0.774079  0.00639737  3.0

Are there perhaps other functions that could benefit from similar loosening?

[tlnagy]

Glad to see this brought up again. I ran into this about 2 years ago: https://discourse.julialang.org/t/efficient-way-to-read-large-array-from-binary-file-in-slices/6847

There's also this issue: #32524