Scala's Option, ported to TypeScript

Opt.ts

/*!
Copyright (c) 2018 Matthew McEachen
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

export type Maybe<T> = T | undefined
export type MaybeNull<T> = Maybe<T> | null | void

// Scala got a Some things right.
// HUR HUR I AM HILLLARIOUS

// "Opt" instead of "Option" due to Option being an html entity already

export type MaybeOpt<T> = Opt<T> | MaybeNull<T>

/**
 * @see http://www.scala-lang.org/api/current/scala/Option.html
 */
export interface Opt<A> {
  /**
   * @return true if the option is an instance of Some, false otherwise
   */
  isDefined: boolean
  /**
   * @return true if the option is None, false otherwise
   */
  isEmpty: boolean
  /**
   * @return the option's value.
   */
  get(): Maybe<A>
  /**
   * @return true if this option is nonempty and the predicate `p` returns true
   * when applied to this Option's value.
   */
  exists(p: (a: A) => boolean): boolean
  /**
   * @return a `Some` containing the result of applying `f` to this `Option`'s value
   * if this `Option` is nonempty.
   */
  map<B>(f: (a: A) => B): Opt<B>
  /**
   * @return the result of applying `f` to this `Option`'s value if this
   * `Option` is nonempty. By supporting `undefined` or `B`, we make caller's
   * lives a little easier--we'll wrap the result in an `Option` for you.
   */
  flatMap<B>(f: (a: A) => MaybeOpt<B>): Opt<B>
  /**
   * @return this `Option` if it is both nonempty
   * and applying the predicate `p` to this `Option`'s value returns true.
   */
  filter(p: (a: A) => boolean): Opt<A>
  /**
   * Apply the given procedure `f` to the `Option`'s value
   * if this `Option` is nonempty.
   */
  forEach(f: (a: A) => void): void
  /**
   * @return this `Option`'s value if this `Option` is nonempty,
   * otherwise return the result of evaluating `f`.
   */
  getOrElse(f: () => A): A
  /**
   * @return this `Option`'s value if this `Option` is nonempty,
   * otherwise return the result of evaluating `f`.
   */
  orElse(f: () => MaybeOpt<A>): Opt<A>

  /**
   * @param f will only be invoked if both `this` and `b` are defined
   */
  zip1<B, T>(b: MaybeOpt<B>, f: (a: A, b: B) => MaybeOpt<T>): Opt<T>

  zip2<B, C, T>(
    b: MaybeOpt<B>,
    c: MaybeOpt<C>,
    f: (a: A, b: B, c: C) => MaybeOpt<T>
  ): Opt<T>

  zip3<B, C, D, T>(
    b: MaybeOpt<B>,
    c: MaybeOpt<C>,
    d: MaybeOpt<D>,
    f: (a: A, b: B, c: C, d: D) => MaybeOpt<T>
  ): Opt<T>
}

namespace NoneImpl {
  export const isDefined = false
  export const isEmpty = true
  export function get(): any {
    return
  }
  export const exists = () => false
  const noop = () => NoneImpl
  export const map = noop
  export const flatMap = noop
  export const filter = noop
  export const forEach = noop
  export function getOrElse<A>(f: () => A): A {
    return f()
  }
  export function orElse<A>(f: () => MaybeOpt<A>): Opt<A> {
    return Opt(f())
  }
  export const zip1 = noop
  export const zip2 = noop
  export const zip3 = noop
}

export const None = NoneImpl as Opt<any>

export class Some<A> implements Opt<A> {
  readonly isDefined = true
  readonly isEmpty = false

  constructor(private readonly a: A) {}

  get(): A {
    return this.a
  }

  exists(f: (a: A) => boolean): boolean {
    return f(this.a)
  }

  map<B>(f: (a: A) => B): Opt<B> {
    return new Some(f(this.a))
  }

  flatMap<B>(f: (a: A) => Opt<B> | MaybeNull<B>): Opt<B> {
    const b = f(this.a)
    return isOpt(b) ? b : Opt(b)
  }

  filter(f: (a: A) => boolean): Opt<A> {
    return Opt(f(this.a) ? this.a : undefined)
  }

  forEach(f: (a: A) => void): void {
    f(this.a)
  }

  getOrElse(_orElse: () => A): A {
    return this.a
  }

  orElse(_orElse: () => Opt<A>): Opt<A> {
    return this
  }

  zip1<B, T>(b: MaybeOpt<B>, f: (a: A, b: B) => MaybeOpt<T>): Opt<T> {
    return Opt(b).flatMap(eb => f(this.a, eb))
  }

  zip2<B, C, T>(
    b: Opt<B>,
    c: Opt<C>,
    f: (a: A, b: B, c: C) => MaybeOpt<T>
  ): Opt<T> {
    return Opt(b).flatMap(eb => Opt(c).flatMap(ec => f(this.a, eb, ec)))
  }

  zip3<B, C, D, T>(
    b: MaybeOpt<B>,
    c: MaybeOpt<C>,
    d: MaybeOpt<D>,
    f: (a: A, b: B, c: C, d: D) => MaybeOpt<T>
  ): Opt<T> {
    return Opt(b).flatMap(eb =>
      Opt(c).flatMap(ec => Opt(d).flatMap(ed => f(this.a, eb, ec, ed)))
    )
  }
}

/**
 * @see http://www.scala-lang.org/api/current/scala/Option.html
 */
export function Opt<A>(a: MaybeOpt<A>): Opt<A> {
  return isOpt(a) ? a : a != null ? new Some(a) : None
}

export function isOpt<A>(a: MaybeOpt<A>): a is Opt<A> {
  return a instanceof Some || a === None
}