使用Flow类型系统执行运行时检查？

Question

In my application, a UUID is just a string that conforms to a particular regex:

'0b4ba6ba-496f-11e8-a21b-06f9c13aa914' // UUID

'hello world' // Not UUID

Now, I can check the format at run-time like this:

const isUuid = (x : string) : boolean => {
  const pattern = /^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$/;
  return pattern.test(x);
};

This is a run-time check, but I can leverage the type-system to ensure that the check is performed on all code paths. Basically, I want to create a string-like type that is used to represent a string that has passed the isUuid check.

type Uuid = ?

let s : string = '0b4ba6ba-496f-11e8-a21b-06f9c13aa914';

let x : Uuid = s; // Type error

let y : Uuid = ensureUuid(s); // Type checked, but may throw at run-time

However, I want my existing code that takes UUIDs as strings to continue to work.

Is this possible in Flow?

// @flow

type Uuid = string;

const ensureUuid = (x : string) : Uuid => {
  if (!isUuid(x)) {
    throw new TypeError(x + ' is not a Uuid');
  }

  return x;
};

Answer 1

This is possible using the opaque keyword:

Opaque type aliases are type aliases that do not allow access to their underlying type outside of the file in which they are defined.

Docs: https://flow.org/en/docs/types/opaque-types/

You will need two files.

First uuid.js :

// @flow

export opaque type Uuid = string;

export const is = (x : string) : boolean => {
  const pattern = /^[0-9a-fA-F]{8}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{4}-[0-9a-fA-F]{12}$/;

  return pattern.test(x);
};

export const create = (x : string) : Uuid => {
  if (is(x)) {
    return x;
  }

  throw new TypeError('"' + x + '" is not a valid UUID');
};

Then the consumer, index.js :

// @flow

import * as uuid from './uuid';

import type { Uuid } from './uuid';

const s : string = '0b4ba6ba-496f-11e8-a21b-06f9c13aa914';

// const q : Uuid = s; // Type error!

const r : Uuid = uuid.create(s); // Type checked, but might throw a run-time error

// const h : Uuid = 'not allowed!';
// const i : Uuid = ('not allowed!' : Uuid);

With this setup, a Uuid instance can only be created via uuid.create .