[ulysses4ever]

You can use Unions, and in fact that's what Julia do. The issue is performance -- that's what the paper advocates: stable (grounded, more precisely) functions are a clear cut in terms of optimization. Unstable ones is a lottery (sometimes Julia does a decent Job at optimizing them sometimes not).

Rust is an interesting example, maybe I should look into the library. But the compilation model is very different from probably any JIT, including Julia. Optimizations too. For one, unions (enums) in Rust would have much less profound impact on possible optimizations and therefore performance.

[jlrubin]

I think that Julia lacks a native tagged enum, which is what rust has. Unions are more akin to a local trait T that you box and implement for all the return types, which is strictly worse. If instead explicit unions were optimized to be tagged enums you could get the big performance gains of rust.

Calling code would, of course, have to destructure the return. But destructures of a return value can be very efficient using match (jump tables internally where possible).