7. Borrowing

Zryna 0.2.1 at 841c8aee901782c9f7bf442bfe8eb2fe6b7f6446 publishes bounded lexical borrowing through data-ownership-v1. See M3_BORROWING_SEMANTICS.md.

function update(): i32 {
  let score: i32 = 7;
  {
    const loan: BorrowMut<i32> = borrowMut(score);
    loan = 12;
  }
  return score;
}

export function score(): i32 {
  return update();
}

borrowMut(score) creates exclusive access without moving score. The lexical block marks the loan’s lifetime. After the closing brace, the owner can be read again and contains the updated value.

& $zryna run src/borrowing.zry --project-root "$workspace\tutorial-ownership" --profile data-ownership-v1 --target javascript --name borrow-js --export score
& $zryna run src/borrowing.zry --project-root "$workspace\tutorial-ownership" --profile data-ownership-v1 --target webassembly --name borrow-wasm --export score

Expected outputs are javascript: i32 12 and webassembly: i32 12.

Borrow<T> is shared read access; BorrowMut<T> is exclusive mutable access. Borrow aliases cannot escape their admitted lexical scope.

Common error: conflicting access

Creating Borrow<i32> while an overlapping BorrowMut<i32> is active is rejected with ZRYNA-M3017. End the first borrow’s block before creating incompatible access. Stored or returned borrows, implicit lifetime shortening, and unrestricted reference syntax are outside this preview.

Exercise

Explain why the final return score is legal but reading score inside the loan’s block would conflict with exclusive access. Then rerun the verified example with a fresh output name.

← Ownership · Continue to the project →