Either.js

class Either<T> {
  right: ?T
  left: ?string

  constructor(right: ?T, left: ?string) {
    this.right = right;
    this.left = left;
  }

  static right<U>(data: U) {
    return new Either(data, null);
  }

  static left(error) {
    return new Either(null, error);
  }

  then(bind) {
    return this.left != null ?
      Either.left(this.left) :
      bind(this.right);
  }
}