Union types: discriminate across table types and between enums | strings

[pdesaulniers]

As mentioned in the tutorial, the is operator can only discriminate across primitive types and at most one table type. The same limitation exists between enums and strings.

It would be nice if there wasn't such limitation.

[hishamhm]

discriminate across table types

This will require metatable-equipped records (or objects (or some sort of tagged unions)). For reference, see related discussion in #97.

between enums | strings

This will require reifying the enum in the generated code for runtime lookup. Not done yet, but doable. This one seems pretty low-priority, though.

[Fang-]

or some sort of tagged unions

Considering they're mentioned here, I'll refrain from opening a separate issue for this, but: supporting tagged unions seems very important to me. Coming from a language that supports them, they're rightfully used all over the place, and can make up for unimplemented complex union cases like the mentioned string | MyEnum.
#124 seems to be pushing in this direction already, could be worth pursuing that further.

(Regardless, excited about this project! Thanks @hishamhm and all contributors involved for the great work so far. (: )

[makspll]

+1, I am embedding Teal in Rust, and the limitation of not being able to use multiple UserData types in an union is a massive problem for me, I have to basically resort to using 'Any' everywhere (mostly metamethods such as Add, Mul etc..) which then defeats the purpose of using teal in the first place :p

[lenscas]

I think a decent compromise would be to allow table | otherTable (or userdata | otherUserData) but do not allow is to disambiguate between them.

This should allow people to create their own functions to disambiguate between the variants (which sadly would require as) and would also make documenting api's that are not written in teal easier (Like libraries written in lua but also apis written in C and Rust which might use a library to generate the documentation)

[hishamhm]

Half of this is implemented: one can now equip record types with an is handler that allows discriminating between them.

Discriminating between enums and strings is still to-do!