Functional JavaScript

functor.js

// Functors apply a function to a wrapped value and then return a wrapped value
// Functors are chainable
// Functor::(a -> b) -> f(a) -> f(b)
class Wrapper {
  constructor(val) {
    console.log('Created a wrapped value ' + val);
    this.val = val;
  }
  // Functor's `fmap` in Haskell
  map(func) {
    return new Wrapper(func(this.val));
  }
  // Monad's `return` in Haskell
  static of(val) {
    return new Wrapper(val);
  }
}

let value = 'Hello World!';
// Functor
Wrapper.of(value).map(_.words).map(_.size);

monad.js

// Monads apply a function that returns a wrapped values to a wrapped value
// and then return a wrapped value
// Monads are chainable
class Wrapper {
  constructor(val) {
    console.log('Created a wrapped value ' + val);
    this.val = val;
  }
  // Functor's `fmap` in Haskell
  map(func) {
    return new Wrapper(func(this.val));
  }
  // Monad's `>>=` (pronounced bind) in Haskell
  flatMap(func) {
    return func(this.val);
  }
  // Monad's `return` in Haskell
  static of(val) {
    return new Wrapper(val);
  }
}

let value = 2;
// Monad
Wrapper.of(value).flatMap(function(x){ return Wrapper.of(x+2)}).flatMap(function(x) {return Wrapper.of(x*-1)});