Monads for JavaScript developers

with practical use cases

@MichalZalecki

Let's define a "Monad"!

MONAD

/ˈməʊnad/ or /ˈmɒnad/

In functional programming, monads are a way to build computer programs by joining simple components in robust ways.

‒ wikipedia.org

What is a "MONAD"?

• Design Pattern
• Specification
• Algebraic Structure
• Functor
• Object

Fantasy Land Specification

Specification for interoperability of common algebraic structures in JavaScript

I"M SO PROUD OF MYSELF 💪

1 commit, 1 character change

Identity

class Identity {
  static of(value) {
    return new Identity(value);
  }

  constructor(public value) {}

  chain(fn) {
    return fn(this.value);
  }

  ap(m) {
    return m.map(x => x(this.value));
  }

  map(fn) {
    return Identity.of(fn(this.value));
  }
}

Either

class Left {
  constructor(public value) {}
  map(_fn) {
    return this;
  }
}

class Right {
  map(fn) {
    try {
      return Right.of(fn(this.value));
    } catch (err) {
      return Left.of(err);
    }
  }
}

type Either = Left | Right;

EXAMPLE

Maybe

class Nothing {
  map(_fn) {
    return this;
  }
}

class Just {
  map(fn) {
    try {
      return Just.of(fn(this.value));
    } catch (err) {
      return new Nothing();
    }
  }
}

type Maybe = Nothing | Just;

EXAMPLE

IO

class IO {  
  static of(value, ...args) {
    return new IO(value, ...args);
  }

  map(fn) {
    return IO.of(fn(this.value));
  }

  run() {
    this.value(...this.args);
  }

  equals(m) {
    return deepEqual(this, m);
  }
}

EXAMPLE

LIST

function* lazyMap(iterable, fn) {
  for (let elem of iterable) {
    yield fn(elem);
  }
}

export class List {
  map(fn) {
    return List.of(lazyMap(this.value, fn));
  }

  *[Symbol.iterator]() {
    yield* this.value;
  }
}

EXAMPLE

Continuation

Promise.resolve(URL)
  .then(axios.get)
  .then(appendPosts)
  .catch((err) => { ... });

Continuation.of(URL)
  .chain(axios.get)
  .chain(appendPosts)
  .chain(m => if (m instanceof Error) {})

https://github.com/MichalZalecki/paradajs

Thank YOu!

Questions?

@MichalZalecki