If you mean type aliases, that's a good idea within a single codebase, but the distinction between different types of strings doesn't survive serialization so it wouldn't have prevented this error. They really needed to return either null or a structure that includes a tag indicating success or failure (aka Result<T>).

At least in python there's a difference between type aliases and typing.NewType (the difference described here: https://gist.github.com/simonwhitaker/3a24d669b26b4f373690c6...)

That's what I was thinking of. I assume something similar exists in other languages.

In python, aliases are separate from typing.NewType, but you're right: it's only enforced within a codebase and then only if the dev cares to run the typechecker.

So yeah, I guess it either needs to happen at runtime, or we just do signaling and user data on separate file descriptors.

This is presuming that the type system is nomitive rather than structural and there is not serialisation/deserialisation or type refinement occurring. A structural difference is much more preferable.