alt Hacker NewsIt's worth pointing out that the two examples that you're writing are actually strictly different, and not just "better syntax for the same thing". (This is assuming `String | Int` works as in Python, and the second example works as in Rust.)
To understand the difference, `String | String` is just `String`. It's a union, not a sum type. There's no tag or identifier, so you cannot distinguish whether it's the first or the second string.
If this sounds pedantic, this has pretty important ramifications, especially once generics get involved.
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ekimekim • today at 1:45 AM
To provide a concrete example, this bit me in a typescript codebase:
type Option<T> = T | undefined
function f<T>(value: T): Option<T> { ... }
let thing: string | undefined = undefined;
let result = f(thing);
The mistake here is in how Option is defined, but it's a footgun you need to be aware of.
Hijacking your comment because this is a common point that's made on the superiority of Swift syntax against the union syntax.
At least with |, you're attempting to model the state space. You're saying "this is one of these things." You might get the exhaustiveness wrong, but you're in the right ballpark. As it's normally done right now, the Swift developer with five optional properties is modeling state as "maybe this, maybe that, maybe both, who knows, good luck." which is just worse than a bar. If you need to discriminate explicitly, add a `__kind` field!