Option implementation, once https://github.com/Microsoft/TypeScript/issues/7294 is fixed

Option-fp.ts

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

type Some<T> = {
  flatMap<U>(f: (value: T) => None): None
  flatMap<U>(f: (value: T) => Some<U>): Some<U>
  flatMap<U>(f: (value: T) => Option<U>): Option<U>
  getOrElse(value: T): T
}

type None = {
  flatMap<U>(f: (value: never) => Option<U>): None
  getOrElse<U>(value: U): U
}

let Some: <T>(value: T): Some<T> = ({
  flatMap: f => f(value),
  getOrElse: () => value
})

let None: None = {
  flatMap: f => None,
  getOrElse: value => value
}

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

let a = Option(2)
  .flatMap(n => Some(5 * n))
  .flatMap(n => Some(5 * n))
  .flatMap(() => None)
  .flatMap(n => Some(5 * n))
  .flatMap(n => Some(5 * n))
  .flatMap(() => None)
  .getOrElse(16)