Union-type data in Numba

[jpivarski]

Currently, union-type data can pass through Numba, but it can't be operated on (__getitem__). This is a particularly hard problem because Numba has no dynamic types (unlike C++, which at least has runtime polymorphism/virtual methods). It could be represented by a new ArrayView that holds multiple possibilities, some MultiArrayView, and it can raise exceptions at runtime if the tag is inappropriate for any one of those views. However, the view that does not raise an exception, which ultimately resolves to a number, has MultiArrayView type in Numba but must satisfy all the operations of a number. "All the operations of a number" is an extremely wide-open set—this is the hard part, figuring out how to make a new type that satisfies everything a number satisfies.

We might need help from Numba experts on this one.

[jpivarski]

Old references to this same issue: #85 (first consideration) and #95 (was going to include UnionArray, but I saw how hard that would be).

[jpivarski]

I don't think it would be possible.