可能在FlowType中具有不兼容的别名？

Question

I'm thinking about something like Haskell, where it's a compile-time error to implicitly transfer between different types.

I'd like to be able to do something like:

type Username = string;
type Password = string;

function login(username: Username, password: Password): void {
   // do some stuff
}

const username: Username = getUsername();
const password: Password = getPassword();

login(password, username); // ideally, should error

Trying this seems to work just fine. I'm wondering if maybe I need something besides a type alias. I think it's possible if I wrap it in an object, but I don't know.

Is such a thing possible?

Answer 1

You can emulate this using classes:

declare class Password {}

function passwordFromString(str: string): Password {
  return (str: any);
}

function passwordToString(pass: Password): string {
  return (pass: any);
}

function checkPassword(pass: Password): boolean {
  if (passwordToString(pass) === 'foo') {
    return true;
  }

  return false;
}

Answer 2

"Opaque types" are what this is called in Hack. Some discussion about implementing them in Flow here: https://github.com/facebook/flow/issues/1056#issuecomment-154876614

Answer 3

Opaque types have been officially implemented in flow. Check out the docs here: https://flow.org/en/docs/types/opaque-types/