Implementing simple Monads

README.md

Implementing simple Monads

From the book "Mastering Functional Programming"

monads.js

const VALUE = Symbol('value');

class Container {
  constructor(x) {
    this[VALUE] = x;
  }
  
  static of(x) {
    return new Container(x);
  }

  map(fn) {
    return fn(this[VALUE]);
  }

  toString() {
    return `${this.constructor.name} wrapping ${this[VALUE]}`
  }

  valueOf() {
    return this[VALUE];
  }
}

// *****************

class Functor extends Container {
  static of(x) {
    return new Functor(x);
  }

  map(fn) {
    return Functor.of(fn(this[VALUE]));
  }
}

// *****************

class Nothing extends Functor {
  isNothing() {
    return true;
  }

  toString() {
    return "Nothing()";
  }

  map(_) {
    return this;
  }

  orElse(v) {
    return v; 
  }
}

class Just extends Functor {
  static of(x) {
    return new Just(x);
  }

  isNothing() {
    return false;
  }

  map(fn) {
    return Maybe.of(fn(this[VALUE]));
  }

  orElse(_) {
    return this.valueOf(); 
  }
}

class Maybe extends Functor {
  constructor(x) {
    return (x === undefined || x === null) ? // !!
      new Nothing()
      : new Just(x);
  }

  static of(x) {
    return new Maybe(x);
  }
}

// *****************

class Monad extends Functor {
  static of(x) {
    return new Monad(x);
  }

  map(fn) {
    return Monad.of(fn(this[VALUE]));
  }

  flatten() {
    const value = this[VALUE];
    return (value instanceof Container) ? value.flatten() : this; // !! Note we're returning `this` (the container), not the unwrapped value
  }

  chain(fn) {
    return this.map(fn).flatten();
  }
}

// *****************

// TODO: Recreate Nothing, Just and Maybe so they extend Monad rather than Functor, so they have flatten and chain

// *****************

class Left extends Monad {
  isLeft() {
    return true;
  }

  map(_) {
    return this;
  }
}

class Right extends Monad {
  isLeft() {
    return false;
  }

  map(fn) {
    return Either.of(null, fn(this[VALUE]));
  }
}

class Either extends Monad {
  constructor(left, right) {
    return right == undefined ? // !! If right is neither null nor undefined, return a new Right (regardless of left)
      new Left(left)
      : new Right(right);
  }

  static of(left, right) {
    return new Either(left, right);
  }
}

// *****************

class Try extends Either {
  constructor(thunk, message) {
    try {
      return new Either(null, thunk());
    } catch (err) {
      return new Either(message ?? err, null)
    }
  }

  static of (thunk, message) {
    return new Try(thunk, message);
  }
}

// *****************

const cont1 = Container.of(42);
const funct1 = Functor.of(42);
const just1 = Just.of(42);
const maybe1 = Maybe.of(42);
const maybe2 = Maybe.of(null);
const monad1 = Monad.of(42);
const monadOfMonad1 = Monad.of(monad1);
const either1 = Either.of(null, 42)
const either2 = Either.of(new Error("Some error"), 42)
const either3 = Either.of(new Error("Some error"), null)
const try1 = Try.of(() => 42);
const try2 = Try.of(() => {throw new Error("Oh shit")});
const try3 = Try.of(() => {throw new Error("Oh shit")}, "Custom error message");

console.log(
  // cont1.toString() // Container wrapping 42
  // cont1.valueOf() // 42
  // cont1.map(x => x*2) // 84

  // funct1.toString() // Functor wrapping 42
  // funct1.map(x => x*2) // Functor { [Symbol(value)]: 84 }

  // just1 // Just { [Symbol(value)]: 42 }

  // maybe1 // Just { [Symbol(value)]: 42 }
  // maybe2 // Nothing { [Symbol(value)]: undefined }
  // maybe1.map(x => x * 2) // Just { [Symbol(value)]: 84 }
  // maybe1.map(_ => null) // Nothing { [Symbol(value)]: undefined }

  // maybe1.orElse("Some default or placeholder value") // 42
  // maybe2.orElse("Some default or placeholder value") // Some default or placeholder value

  // monad1 // Monad { [Symbol(value)]: 42 }
  // monadOfMonad1 // Monad { [Symbol(value)]: Monad { [Symbol(value)]: 42 } }
  // monadOfMonad1.flatten() // Monad { [Symbol(value)]: 42 }
  // monadOfMonad1.chain(x => x * 2) // Monad { [Symbol(value)]: 84 }

  // either1 // Right { [Symbol(value)]: 42 }
  // either2 // Right { [Symbol(value)]: 42 }
  // either3 // Left {
  //   [Symbol(value)]: Error: Some error
  //       at Object.<anonymous> (/Users/jfhector/Desktop/continuation_test/monads/monads.js:153:27)
  //       at Module._compile (internal/modules/cjs/loader.js:1072:14)
  //       at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
  //       at Module.load (internal/modules/cjs/loader.js:937:32)
  //       at Function.Module._load (internal/modules/cjs/loader.js:778:12)
  //       at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)
  //       at internal/main/run_main_module.js:17:47
  // }
  // ^ NOTE: This error wasn't thrown

  // either1.map(x => x * 2) // Right { [Symbol(value)]: 84 }
  // either3.map(x => x * 2) // Left {
  //   [Symbol(value)]: Error: Some error
  //       at Object.<anonymous> (/Users/jfhector/Desktop/continuation_test/monads/monads.js:153:27)
  //       at Module._compile (internal/modules/cjs/loader.js:1072:14)
  //       at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
  //       at Module.load (internal/modules/cjs/loader.js:937:32)
  //       at Function.Module._load (internal/modules/cjs/loader.js:778:12)
  //       at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)
  //       at internal/main/run_main_module.js:17:47
  // }
  // ^ NOTE: This error wasn't thrown

  // either2.chain(x => x * 2) // Right { [Symbol(value)]: 84 }

  // try1 // Right { [Symbol(value)]: 42 }
  // try2 // Left {
  //   [Symbol(value)]: Error: Oh shit
  //       at /Users/jfhector/Desktop/continuation_test/monads/monads.js:171:34
  //       at new Try (/Users/jfhector/Desktop/continuation_test/monads/monads.js:147:31)
  //       at Function.of (/Users/jfhector/Desktop/continuation_test/monads/monads.js:154:12)
  //       at Object.<anonymous> (/Users/jfhector/Desktop/continuation_test/monads/monads.js:171:18)
  //       at Module._compile (internal/modules/cjs/loader.js:1072:14)
  //       at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
  //       at Module.load (internal/modules/cjs/loader.js:937:32)
  //       at Function.Module._load (internal/modules/cjs/loader.js:778:12)
  //       at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12)
  //       at internal/main/run_main_module.js:17:47
  // }
  // ^ NOTE: This error wasn't thrown

  // try3 // Left { [Symbol(value)]: 'Custom error message' }
);