Lampiran B: Dukungan Struktur Aljabar

Dalam lampiran ini, Anda akan menemukan beberapa implementasi JavaScript dasar dari berbagai struktur aljabar yang dijelaskan dalam buku ini.

Ingatlah bahwa implementasi ini mungkin bukan implementasi tercepat atau paling efisien di luar sana; mereka dibuat hanya untuk tujuan pendidikan.

Untuk menemukan struktur yang lebih siap produksi, intip folktale atau fantasy-land.

Perhatikan bahwa beberapa metode juga merujuk ke fungsi yang didefinisikan dalam Lampiran 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

Note that Maybe could also be defined in a similar fashion as we did for Either with two child classes Just and Nothing. This is simply a different flavor.

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);
  }
}