> What he's talking about is returning a union type. You have a single return value, it's type is either a valid result (i.e. a string) or an error. The compiler expects you to do runtime type checks (basically) to assert whether your return value is actually a value or not.

Not quite. I am suggesting a tagged union. Not a runtime type check.

You'd check the tag.

Some language only support the runtime type-checked version of union types. That's a bit silly. For example, it makes it much harder to write a function that may either error out itself, or produce an error message as its bona fide value.