Apéndice B: Estructuras Algebraicas de Soporte

En este apéndice, encontrarás algunas implementaciones básicas en Javascript de varias estructuras algebraicas descritas en el libro. Ten en cuenta que estas implementaciones pueden no ser las más rápidas o las más eficientes; tienen únicamente una finalidad educativa.

Para encontrar estructuras más preparadas para un entorno en producción, échale un vistazo a ramda, lodash, o folktale.

Ten en cuenta que algunos métodos hacen referencia a funciones definidas en el Apéndice A

Compose

const createCompose = curry((F, G) => class Compose {
  constructor(x) {
    this.$value = x;
  }

  [util.inspect.custom]() {
    return `Compose(${inspect(this.$value)})`;
  }

  // ----- Pointed (Compose F G)
  static of(x) {
    return new Compose(F(G(x)));
  }

  // ----- Functor (Compose F G)
  map(fn) {
    return new Compose(this.$value.map(x => x.map(fn)));
  }

  // ----- Applicative (Compose F G)
  ap(f) {
    return f.map(this.$value);
  }
});

Either

class Either {
  constructor(x) {
    this.$value = x;
  }

  // ----- Pointed (Either a)
  static of(x) {
    return new Right(x);
  }
}

Left

class Left extends Either {
  get isLeft() {
    return true;
  }

  get isRight() {
    return false;
  }

  static of(x) {
    throw new Error('`of` called on class Left (value) instead of Either (type)');
  }

  [util.inspect.custom]() {
    return `Left(${inspect(this.$value)})`;
  }

  // ----- Functor (Either a)
  map() {
    return this;
  }

  // ----- Applicative (Either a)
  ap() {
    return this;
  }

  // ----- Monad (Either a)
  chain() {
    return this;
  }

  join() {
    return this;
  }

  // ----- Traversable (Either a)
  sequence(of) {
    return of(this);
  }

  traverse(of, fn) {
    return of(this);
  }
}

Right

class Right extends Either {
  get isLeft() {
    return false;
  }

  get isRight() {
    return true;
  }

  static of(x) {
    throw new Error('`of` called on class Right (value) instead of Either (type)');
  }

  [util.inspect.custom]() {
    return `Right(${inspect(this.$value)})`;
  }

  // ----- Functor (Either a)
  map(fn) {
    return Either.of(fn(this.$value));
  }

  // ----- Applicative (Either a)
  ap(f) {
    return f.map(this.$value);
  }

  // ----- Monad (Either a)
  chain(fn) {
    return fn(this.$value);
  }

  join() {
    return this.$value;
  }

  // ----- Traversable (Either a)
  sequence(of) {
    return this.traverse(of, identity);
  }

  traverse(of, fn) {
    return fn(this.$value).map(Either.of);
  }
}

Identity

class Identity {
  constructor(x) {
    this.$value = x;
  }

  [util.inspect.custom]() {
    return `Identity(${inspect(this.$value)})`;
  }

  // ----- Pointed Identity
  static of(x) {
    return new Identity(x);
  }

  // ----- Functor Identity
  map(fn) {
    return Identity.of(fn(this.$value));
  }

  // ----- Applicative Identity
  ap(f) {
    return f.map(this.$value);
  }

  // ----- Monad Identity
  chain(fn) {
    return this.map(fn).join();
  }

  join() {
    return this.$value;
  }

  // ----- Traversable Identity
  sequence(of) {
    return this.traverse(of, identity);
  }

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

IO

class IO {
  constructor(fn) {
    this.unsafePerformIO = fn;
  }

  [util.inspect.custom]() {
    return 'IO(?)';
  }

  // ----- Pointed IO
  static of(x) {
    return new IO(() => x);
  }

  // ----- Functor IO
  map(fn) {
    return new IO(compose(fn, this.unsafePerformIO));
  }

  // ----- Applicative IO
  ap(f) {
    return this.chain(fn => f.map(fn));
  }

  // ----- Monad IO
  chain(fn) {
    return this.map(fn).join();
  }

  join() {
    return new IO(() => this.unsafePerformIO().unsafePerformIO());
  }
}

List

class List {
  constructor(xs) {
    this.$value = xs;
  }

  [util.inspect.custom]() {
    return `List(${inspect(this.$value)})`;
  }

  concat(x) {
    return new List(this.$value.concat(x));
  }

  // ----- Pointed List
  static of(x) {
    return new List([x]);
  }

  // ----- Functor List
  map(fn) {
    return new List(this.$value.map(fn));
  }

  // ----- Traversable List
  sequence(of) {
    return this.traverse(of, identity);
  }

  traverse(of, fn) {
    return this.$value.reduce(
      (f, a) => fn(a).map(b => bs => bs.concat(b)).ap(f),
      of(new List([])),
    );
  }
}

Map

class Map {
  constructor(x) {
    this.$value = x;
  }

  [util.inspect.custom]() {
    return `Map(${inspect(this.$value)})`;
  }

  insert(k, v) {
    const singleton = {};
    singleton[k] = v;
    return Map.of(Object.assign({}, this.$value, singleton));
  }

  reduceWithKeys(fn, zero) {
    return Object.keys(this.$value)
      .reduce((acc, k) => fn(acc, this.$value[k], k), zero);
  }

  // ----- Functor (Map a)
  map(fn) {
    return this.reduceWithKeys(
      (m, v, k) => m.insert(k, fn(v)),
      new Map({}),
    );
  }

  // ----- Traversable (Map a)
  sequence(of) {
    return this.traverse(of, identity);
  }

  traverse(of, fn) {
    return this.reduceWithKeys(
      (f, a, k) => fn(a).map(b => m => m.insert(k, b)).ap(f),
      of(new Map({})),
    );
  }
}

Maybe

Ten en cuenta que Maybe también podría definirse con dos clases hijas Just y Nothing, de una manera similar a como lo hicimos con Either. Esta tan solo es una forma diferente.

class Maybe {
  get isNothing() {
    return this.$value === null || this.$value === undefined;
  }

  get isJust() {
    return !this.isNothing;
  }

  constructor(x) {
    this.$value = x;
  }

  [util.inspect.custom]() {
    return this.isNothing ? 'Nothing' : `Just(${inspect(this.$value)})`;
  }

  // ----- Pointed Maybe
  static of(x) {
    return new Maybe(x);
  }

  // ----- Functor Maybe
  map(fn) {
    return this.isNothing ? this : Maybe.of(fn(this.$value));
  }

  // ----- Applicative Maybe
  ap(f) {
    return this.isNothing ? this : f.map(this.$value);
  }

  // ----- Monad Maybe
  chain(fn) {
    return this.map(fn).join();
  }

  join() {
    return this.isNothing ? this : this.$value;
  }

  // ----- Traversable Maybe
  sequence(of) {
    return this.traverse(of, identity);
  }

  traverse(of, fn) {
    return this.isNothing ? of(this) : fn(this.$value).map(Maybe.of);
  }
}

Task

class Task {
  constructor(fork) {
    this.fork = fork;
  }

  [util.inspect.custom]() {
    return 'Task(?)';
  }

  static rejected(x) {
    return new Task((reject, _) => reject(x));
  }

  // ----- Pointed (Task a)
  static of(x) {
    return new Task((_, resolve) => resolve(x));
  }

  // ----- Functor (Task a)
  map(fn) {
    return new Task((reject, resolve) => this.fork(reject, compose(resolve, fn)));
  }

  // ----- Applicative (Task a)
  ap(f) {
    return this.chain(fn => f.map(fn));
  }

  // ----- Monad (Task a)
  chain(fn) {
    return new Task((reject, resolve) => this.fork(reject, x => fn(x).fork(reject, resolve)));
  }

  join() {
    return this.chain(identity);
  }
}