Options in TypeScript

options-in-typescript.ts

type None = {
  flatMap<U>(f: (value: null) => Option<U>): None
  getOrElse<U>(def: U): U
  isEmpty(): true
  map<U>(f: (value: null) => U): None
  nonEmpty(): false
  orElse<U>(alternative: Option<U>): Option<U>
}

type Some<T> = {
  flatMap<U>(f: (value: T) => Some<U>): Some<U>
  flatMap<U>(f: (value: T) => None): None
  flatMap<U>(f: (value: T) => Option<U>): Option<U>
  get(): T
  getOrElse<U extends T>(def: U): T | U
  isEmpty(): false
  map(f: (value: T) => null): None
  map<U>(f: (value: T) => U): Some<U>
  map<U>(f: (value: T) => U): Option<U>
  nonEmpty(): true
  orElse<U extends T>(alternative: Option<U>): Option<T> | Option<U>
}

type Option<T> = Some<T> | None

let None: None = {
  flatMap: <T>(_f: (value: never) => Option<T>) => None,
  getOrElse: <T>(def: T) => def,
  isEmpty: () => true,
  map: <T>(_f: (value: never) => T) => None,
  nonEmpty: () => false,
  orElse: <U>(alternative: Option<U>): Option<U> => alternative
}

function Some<T>(value: T): Some<T> {
  return {
    flatMap: <U>(f: (value: T) => Option<U>) => f(value) as any, // TODO
    get: () => value,
    getOrElse: <U extends T>(def: U): T | U  => value || def,
    isEmpty: () => false,
    map: <U>(f: (value: T) => U) => Option(f(value)) as any, // TODO
    nonEmpty: () => true,
    orElse: <U>(_alternative: Option<U>): Option<T> | Option<U> => Some(value)
  }
}

function Option<T>(value: T): Some<T>
function Option<T>(value: null): None
function Option<T>(value: T | null) {
  if (value === null) {
    return None
  }
  return Some(value)
}

let y = Option(3)
  .map(() => 4)
  .flatMap(() => Option(5))
  .get()