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);

Now imagine the definition of Option is in some library or other file and you don't realize how it works. You are thinking of the Option as its own structure and expect f to return Option<string | undefined>. But Option<string | undefined> = string | undefined | undefined = string | undefined = Option<string>.

The mistake here is in how Option is defined, but it's a footgun you need to be aware of.

The mistake here is that you assume that Option is a struct. For TypeScript, this is an idiomatic definition of something that is optional though.

I guess I just want to be able to do something like this in Swift:

    let x: String | Int
    
    switch x {
        case let value as String:
            // handle String value here
        case let value as Int:
            // handle Int value here
    }

There's one more thing about TypeScript-style union types: string literals. I think it's great to be able to do

    type Options = "option_1" | "option_2" ... "option_n"

And subsequently I could use

    let t: Options
    switch t {
        case "option_1":
            // handle `"option_1"` case here
        ...
        case "option_n":
            // handle `"option_n"` case here
    }

I think this is more programmer friendly than requiring an out-of-line definition of a new `enum`. Sometimes you just want to write some code, you know?

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!

So long as you have structurally typed structs (as TypeScript does), you can do stuff like {foo: string} | {bar: string} to the same effect as type constructors if and when you actually need it.

At the same time, how often do you really need (Just (Just (Just ...)))?