alt Hacker NewsI think your Option/String example is a real-world tradeoff, but it’s not a slam-dunk “untagged > tagged.”
For API evolution, T | null can be a pragmatic “relax/strengthen contract” knob with less mechanical churn than Option<T> (because many call sites don’t care and just pass values through). That said, it also makes it easier to accidentally reintroduce nullability and harder to enforce handling consistently, the failure mode is “it compiles, but someone forgot the check.”
In practice, once the union has more than “nullable vs present”, people converge to discriminated unions ({ kind: "ok", ... } | { kind: "err", ... }) because the explicit tag buys exhaustiveness and avoids ambiguous narrowing. So I’d frame untagged unions as great for very narrow cases (nullability / simple widening), and tagged/discriminated unions as the reliability default for domain states.
For reliability, I’d rather pay the mechanical churn of Option<T> during API evolution than pay the ongoing risk tax of “nullable everywhere.
My post argues for paying costs that are one-time and compiler-enforced (refactors) vs costs that are ongoing and human-enforced (remembering null checks).
I believe there is a misunderstanding. The compiler can check untagged unions just as much as it can check tagged unions. I don't think there is any problem with "ambiguous narrowing", or "reliability". There is also no risk of "nullable everywhere": If the type of x is Foo|Null, the compiler forces you to write a null check before you can access x.bar(). If the type of x is Foo, x is not nullable. So you don't have to remember null checks (or checks for other types): the compiler will remember them. There is no difference to tagged unions in this regard.