final-OptionAndEither.js

/* Option types */
const Option = x => (x === undefined || x === null) ? None : Some(x);

const Some = x => ({
  filter: f => f(x) ? Some(x) : None,
  map: f => Some(f(x)),
  fold: (ifEmpty, f) => f(x),
  getOrElse: defaultValue => x
});

const None = {
  filter: f => None,
  map: f => None,
  fold: (ifEmpty, f) => ifEmpty(),
  getOrElse: defaultValue => defaultValue
};

/* Either Types */
const Left = x => ({
  map: f => Left(x),
  fold: (ifLeft, ifRight) => ifLeft(x)
});

const Right = x => ({
  map: f => Right(f(x)),
  fold: (ifLeft, ifRight) => ifRight(x)
});