either.ts

export class Either<L, R> {
  constructor(private left: L | null, private right: R | null) {}

  static left<L, R>(value: L): Either<L, R> {
    return new Either<L, R>(value, null);
  }

  static right<L, R>(value: R): Either<L, R> {
    return new Either<L, R>(null, value);
  }

  isLeft(): boolean {
    return this.left !== null;
  }

  isRight(): boolean {
    return this.right !== null;
  }

  getLeft(): L {
    if (this.isLeft()) {
      return this.left as L;
    }
    throw new Error("Attempted to get Left value from a Right");
  }

  getRight(): R {
    if (this.isRight()) {
      return this.right as R;
    }
    throw new Error("Attempted to get Right value from a Left");
  }

  map<T>(f: (right: R) => T): Either<L, T> {
    if (this.isRight()) {
      return Either.right<L, T>(f(this.getRight()));
    } else {
      return Either.left<L, T>(this.getLeft() as L);
    }
  }
}