@fp51/opaque-type

@fp51/opaque-types

Inspired from https://codemix.com/opaque-types-in-javascript/

Usage

npm add @fp51/opaque-type

📖 Documentation

Why ?

Semantic type checking

The following naive code will compile properly, except that if you mix createRoom parameters you will introduce a nasty bug at runtime.

type RoomID = string;
type UserID = string;

function createRoom(roomId: RoomID, userId: UserID) {
  // something
}

const roomId: RoomID = ...;
const userId: UserID = ...;
createRoom(userId, roomId);

Opaque at rescue !

This following code won't compile, because Opaque add more specificty to RoomId and UserId, so typescript wont think they are compatible because they are built uppon a string;

import { Opaque } from "@fp51/opaque-type";

type RoomID = Opaque<"RoomId">;
type UserID = Opaque<"UserId">;

function createRoom(roomId: RoomID, userId: UserID) {
  // something
}

const roomId: RoomID = ...;
const userId: UserID = ...;

createRoom(userId, roomId); // TypeError

Here the error message you will get in your editor or when you will try to build your application

Argument of type 'Opaque<"UserId">' is not assignable to parameter of type 'Opaque<"RoomId">'.
  ...

Type opacification (hence the name)

Let say you built a nice piece of code:

// message.ts
type Message = {
  userId: string,
  text: string,
};

export function clearMessageText(message: Message) : Message {
  return message.text = '';
}

export function getMessage():Message {
  return ...
}

You user can endup writting the following code:

import { clearMessageText, getMessage } from './message';

const message = { userId: 'userId', text: 'mytext' };

clearMessageText(message); 

or

import { getMessage } from '..'

const message2 = getMessage();

message2.userId = 'whatever';

Those two pieces of code are problematic, because in these the inner structure of your types are exposed and manipulated.

If you make any change to the Message structure it will be a breaking change for your code user. Meaning that he could find himself to edit dozen or hundreds of line of code.

How can we avoid this protecting our user from creating a code strongly tied to your library internals ?

// message.ts
import { createOpaqueAPI } from '../index';

type $Message = {
  userId: string,
  text: string,
};

const { toOpaque, fromOpaque } = createOpaqueAPI<
  'Message',
  $Message,
>('Message');

type Message = ReturnType<typeof toOpaque>;

export function getMessage(): Message {
  return toOpaque({ userId: 'userId', text: 'text' });
}

export function clearMessageText(message: Message): Message {
  const $message = fromOpaque(message)

  return toOpaque({
    ...$message,
    text: '',
  });
}

Your user won't be able to write code in same fashion, he will have to ignore the internals of your code.

import { getMessage, clearMessageText } from '../index';

const message = getMessage();

const messageCleared = clearMessageText(message); 

This is especially usefull when creating a reusable a library:

• It does protect the user of your code from himself
• It does protect you the maintainer by allowing you to break type structure behind the scène without creating a breaking change.