Add defaultarray function to figure out optimal array type based on the element type

[piever]

When collecting an iterable into a column based table, it is not obvious what array type to use for the various elements. For example CategoricalValue from CategoricalArrays should clearly be collected in a CategoricalArray that is optimized for that type, WeakRefString from WeakRefStrings belongs in a StringArray (which is optimized to store those), DataValue naturally belongs to a DataValueArray etc.

This makes it very hard to write code that would collect an iterable into its "optimized container" without depending on all the above packages (or using Requires like here), which in my view is a design that does not scale. I feel that this could be solved by adding a defaultarray(T, sz) = Array{T}(undef, sz) function in Base that the various packages (CategoricalArrays, WeakRefStrings, DataValueArrays) could then overload. In this way one could write a collect optimized for the element type without any dependency.

[nalimilan]

In Base itself, it could make sense to have defaultarray(Bool, dims) return a BitArray.

[piever]

I'm unfamiliar with the broadcasting machinery, but if as I imagine there is something similar to defaultarray there to determine whether to collect things as Array or as BitArray, we could just expose that interface and allow packages to overload it for their custom type (so that broadcasting a function that returns a CategoricalValue would return a CategoricalArray).

[davidanthoff]

This could go further: [a, b, c] array construction could also use this.

[nalimilan]

I'm unfamiliar with the broadcasting machinery, but if as I imagine there is something similar to defaultarray there to determine whether to collect things as Array or as BitArray, we could just expose that interface and allow packages to overload it for their custom type (so that broadcasting a function that returns a CategoricalValue would return a CategoricalArray).

Good idea. Currently broadcast relies on similar(::Broadcasted{DefaultArrayStyle{N}}, ::Type) for this, but that's really equivalent to using a custom function since it's completely different from similar(::AbstractArray, ::Type).