Runtime borrow checker for JavaScript

borrow.ts

export interface Borrowed<X> {
  unsafeBorrow<Result>(use: (x: X) => Result): [Borrowed<X>, Result];
  get isBorrowed(): boolean;
}

export class AlreadyBorrowedError extends Error {
  override name = "AlreadyBorrowedError";
}

export function unsafeBorrowed<X>(x: X): Borrowed<X> {
  let alreadyBorrowed = false;
  const b = {
    // This is unsafe because it cannot prevent the `x` object from escaping from the scope of `use`.
    // So you should use this with a proper wrapper function.
    unsafeBorrow<Result>(use: (x: X) => Result): [Borrowed<X>, Result] {
      if (alreadyBorrowed) {
        throw new AlreadyBorrowedError();
      }
      const r = use(x);
      alreadyBorrowed = true;
      return [unsafeBorrowed(x), r];
    },
    get isBorrowed(): boolean {
      return alreadyBorrowed;
    },
  };
  return b;
}

export function borrowed<X>(x: X): Borrowed<X> {
  return unsafeBorrowed(structuredClone(x));
}