[jamesisaac]

Yeah I realise you'd never reach the same level of soundness due to the limitations of the underlying JS (although presumably you could get close with a subset?). But that's why it's an interesting challenge, and I think what Flow has shown is that it's possible to get a lot closer than had previously been imagined.

If everyone just accepted the argument 4 years ago that "JS will never be sound" then maybe today TS would still just be Java style `interface` annotations for classes. It's not like the Flow team has reached a ceiling at this point... there's still plenty on their roadmap that would continue to improve soundness and expressiveness.

> You can’t have anything approaching ocaml correctness when in typescript all objects with the same shape are interchangeable.

Could you elaborate? Flow has recently switched to exact objects by default[1], which I would have thought would be enough for a sound approach?

[1] https://medium.com/flow-type/on-the-roadmap-exact-objects-by...

[tigershark]

For example in typescript this is valid:

    type A = {name: string}
    type B = {name: string}
    function print(obj: A) { alert(obj.name);}
    let a: A = {name: “hello”};
    let b: B = {name: “world”}
    print(a);
    print(b);

This is because of typescript structural equality and I think that the same applies to flow given your link. Obviously if I want a function to accept an email I don’t want the same function to accept an address, but in typescript you can’t guarantee it because you have no way to get rid of structural equality as far as I understood.

[jamesisaac]

Ah I see. That can be achieved since Flow 0.51 with opaque types[1][2]. It seems like TypeScript hasn't yet caught up with this functionality[3].

[1] https://medium.com/flow-type/hiding-implementation-details-w...

[2] https://flow.org/en/docs/types/opaque-types/

[3] https://github.com/Microsoft/TypeScript/issues/15807#issueco...

[tigershark]

Yup that seems to help.

[zeugmasyllepsis]

I'm certainly not well versed in OCaml, but my understanding is that it is also structurally typed. This particular example would also be permitted in OCaml, correct?

[octachron]

No, records are nominally typed in OCaml, thus the two types `A` and `B` would be deemed incompatible.

[wtetzner]

But objects in OCaml are structurally typed (well, row typed).