Monads IO

🚀 Efficient Monads for JS: Maybe (Option) and Either (Result)

Why use this lib

1. Small and Tree-Shakable. Either - 3kb minified, Maybe - 3kb minified, can be imported separately
2. No dependencies.
3. Memory-Efficient. 8 bytes overhead per instance (only class pointer)
4. Tested. 100% coverage
5. Practical. Just 2 wrappers: Either and Maybe - easy for non-fp people

Credits

Huge credit to @JSMonk. This library is based on JSMonk/sweet-monads

Installation

• Using npm

  npm i monads-io

• Using Yarn

  yarn add monads-io

• Using pnpm

  pnpm add monads-io

Usage

Either

The Either type represents values with two possibilities: a value of type Either a b is either Left a or Right b. (source)

1. Makes error path of function strongly typed
2. Separates errors from exceptions
3. Minimal memory overhead (see benchmarks)

import {
  Either,
  fromPromise,
  fromTryAsync,
  left,
  mergeInOne,
  right
} from "monads-io/either";

class NetworkError extends Error {}
class HttpError extends Error {}
class JsonParsingError extends Error {}
class NotFoundError extends Error {}

type FetchError = NetworkError | HttpError | JsonParsingError;

type ID = string;
type User = { id: ID; username: string; name: string /* ... */ };
type Post = { id: ID; userId: User["id"]; body: string /* ... */ };

async function getJson<T>(url: string): Promise<Either<FetchError, T>> {
  const response = await fromPromise(
    fetch(`https://jsonplaceholder.typicode.com/${url}`),
    (cause) => new NetworkError("Unable to connect", { cause })
  );

  const okResponse = response.chain((response) => {
    if (response.ok) return right(response);

    return left(
      new HttpError(
        `Response status is ${response.status} ${response.statusText}`,
        { cause: response }
      )
    );
  });

  const json = await okResponse.asyncChain((response) => {
    return fromTryAsync(
      async () => (await response.json()) as T,
      (cause) => new JsonParsingError("Unable to parse JSON", { cause })
    );
  });

  return json;
}

async function getUserByUsername(username: string) {
  const users = await getJson<User[]>(`/users?username=${username}`);

  return users.chain((users) => {
    const user = users[0];

    if (!user) {
      return left(new NotFoundError(`User not found`, { cause: { username } }));
    }

    return right(user);
  });
}

const getPosts = (userId: string) =>
  getJson<Post[]>(`/posts?ownerId=${userId}`);

class PageLoadError extends Error {
  /* ... */

  constructor(public returnStatus: number, message: string, cause?: unknown) {
    super(message, { cause });
  }
}

async function getUserPageData(username: string) {
  const user = await getUserByUsername(username);
  const posts = await user.asyncChain((user) => getPosts(user.id));

  return mergeInOne([user, posts])
    .map(([user, posts]) => ({ user, posts }))
    .mapLeft((error) => {
      if (error instanceof NotFoundError) {
        return new PageLoadError(404, "User not found", error);
      }

      // error: FetchError
      console.log("Error fetching data for User Page", error);
      return new PageLoadError(500, "Internal server error", error);
    });
}

Maybe

The Maybe monad represents computations which might "go wrong" by not returning a value. (source)

1. Allows to separate empty/present state from undefined
2. Minimal memory overhead (see benchmarks)

// Real world example
// This maybe is not tree-shakable. Used in NodeJS code
import * as Maybe from "monads-io/maybe";

export async function getTargets(
api: TelegramAPI,
tokens: formattedText,
{ mentionLimit = 1, message = undefined as message | undefined } = {}
): Promise<Map<number, chat | undefined>> {
const mentions = getMentions(tokens).slice(0, mentionLimit);

const targets = new Map<number, chat | undefined>();
let replyTarget: [number, chat | undefined] | undefined;
const { messagesService, chatsService } = getServices(api);

...

// 1. Get message
// 2. Get message reply id (0 = no reply)
// 3. Get reply message by message id
// 4. Get reply message sender
// 5. Get his/her profile
// 6. Set local variable to profile

const reply = await Maybe.fromNullable(message)
  .filter((message) => message.reply_to_message_id !== 0)
  .asyncChain((message) =>
    messagesService.getReply(message.chat_id, message.id)
  );

const sender = await reply
  .map(MemberId.fromMessage)
  .tap(({ memberId }) => {
    replyTarget = [memberId, undefined];
  })
  .asyncChain(({ memberId }) => chatsService.getById(memberId));

sender.tap((sender) => {
  replyTarget = [sender.id, sender];
});

...

return replyTarget ? new Map([replyTarget, ...targets]) : targets;
}

Identity

The Identity monad is a monad that does not embody any computational strategy. It simply applies the bound function to its input without any modification. (source)

Example

import * as Identity from "monads-io/identity";

// Before
app.use(express.static(path.resolve(getDirname(import.meta.url), "../public")));

// After
Identity.from(import.meta.url)
  .map(getDirname)
  .map((dir) => path.resolve(dir, "../public"))
  .map(express.static)
  .map(app.use);