[Vanit]

It's kind of amusing reading this as a Typescript dev because I'm so used to Typescript's expressiveness I couldn't really work out which trick it was trying to tell me was novel.

The answer is structural typing (aka duck typing).

[pd-andy]

Not quite. I guess in some ways phantom types achieve the opposite of structural typing.

In a language with structural typing, two types are considered equivalent if they share the same structure. With phantom types we can have a single structure and disambiguate between different uses without touching the underling structure. There's no difference between Id(User) and Id(Post) at runtime, the annotation is purely a compile-time restraint.

In fact, because of typescript's structural typing, the only way to use phantom types in ts is to have some dummy field of type `never` that the type system can use to disambiguate between the two [1].

[1]: https://gist.github.com/GoNZooo/243b23702df1fae38c966ae18832...

[mushishi]

Holy smokes, that would be irritating :D

Btw. is there a difference between these two:

    (s: string): PlainText => <PlainText><any>s;
    (s: string): PlainText => s as any as PlainText;

Syntax-wise I'd rather use the latter for it's more clear in typed JSX-context.

[pd-andy]

As far as I know those two are equivalent. Definitely slightly clunkier, but is what it is.

[timdeve]

Yes, exactly, it's because of Structural Typing that you have to implement Phantom Types in that hacky way rather than Structural Typing being the "trick".