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Comment by wwwigham | Hacker News Reader

> In TypeScript you can assign an instance of one class to an instance of another as long as they have compatible fields.

Fun fact: If you add a private field to a class it'll behave nominally in TS. This is because private fields kinda require nominal relations to function. So, in a way, it does support "switching" to nominal type checking for classes - the opt in is simply per-class.

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It won't do variance [0] right, will it?

[0] https://flow.org/en/docs/lang/variance/

I think variance can be simulated by typing the private field as an existing covariant/contravariant/invariant type

  class CovariantFoo<T> {
    private phantom!: T
  }

  class ContravariantFoo<T> {
    private phantom!: (_: T) => void
  }

  class InvariantFoo<T> {
    private phantom!: (_: T) => T
  }

(inspired by Rust [0])

[0] https://doc.rust-lang.org/nomicon/phantom-data.html

But variance depends on position where the value is used, not at the time of declaration, superclass/subclass at parameter/return value position can't be correctly encoded like this, can it?

I think most languages define variance at type definition level, notable exceptions being Kotlin which supports both [0][1] and Flow. But yeah, TS doesn't support (variable-)declaration-site variance which I didn't realize you were asking in my previous answer.

[0] https://kotlinlang.org/docs/generics.html#variance

[1] https://kotlinlang.org/docs/generics.html#declaration-site-v...

Doesn't OCaml support it as well?

I'm less familiar with OCaml and don't know off the top of my head. Doing a quick search I was only able to find references to type declaration variance [0], though I learned that Java also supports use-site variance too [1]

[0] https://blog.janestreet.com/a-and-a/

[1] https://blog.jooq.org/tag/use-site-variance/

I believe this is conceptually similar to how "brands" can operate to add HKTs to TypeScript/Flow

https://www.cl.cam.ac.uk/~jdy22/papers/lightweight-higher-ki...

https://cs.emis.de/LIPIcs/volltexte/2015/5231/pdf/21_.pdf

Oh cool, hadn't realized this. Also thanks for your great work! I'm rooting for some of your exploratory PRs!

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