rand(sa, ::Int) returns Vector

[jariji]

Does this have to return Vector or could it return StructArray?

julia> sa = StructArray(a=[10,20,30], b=[100,200,300])
3-element StructArray(::Vector{Int64}, ::Vector{Int64}) with eltype @NamedTuple{a::Int64, b::Int64}:
 (a = 10, b = 100)
 (a = 20, b = 200)
 (a = 30, b = 300)

julia> rand(sa, 2)
2-element Vector{@NamedTuple{a::Int64, b::Int64}}:
 (a = 30, b = 300)
 (a = 20, b = 200)

[aplavin]

Agree that it would make sense – but unfortunately there doesn't appear to be a consensus :(
#269
JuliaLang/julia#49156

[jariji]

After reading the Julia thread I think they're right. If rand(x, ::Int) is going to work on range then rand shouldn't preserve type. It's better to have strict rules than best-effort.

For a type-preserving rand, another function should be introduced, which will be undefined on UnitRange.

[aplavin]

Lots of function do type preservation only when it makes sense, returning the closest reasonable type – don't see how rand is special in that regard.

[jariji]

map and filter come to mind, about which my opinion is the same. What else?

[aplavin]

There are so many it's hard to decide where to start :)
Two very different examples:
sqrt preserves floats but returns float when passed an int
vcat preserves many array kinds (incl structarrays), but returns a vector when passed a range