Box and recursive data · Coding Class

Advanced · Take control← Read the Learn chapter

Apply the chapter in three steps: an exercise, a program project, and a debugging task. Implement first, then consult the hints and completion criteria. Use a separate file or working directory for each task.

1. Exercise · Represent the end of a link

Use prepend from the chapter to connect 5, 10, 20 with total 35. Also run the sum for an empty list.

Completion: Totals are 35 and 0; sketch each next link and the final None.

Hint: Create 20 first, then prepend 10 and 5.

2. Build a program · Build a consuming length function

Write length using the same ownership transfer pattern as sum. Construct separate empty and three-node lists to pass to it.

Completion: Results are 0 and 3; do not pass a list to length after consuming it with sum.

Hint: Some contributes 1 plus the tail length; None contributes 0.

3. Debug · Fix reuse of a moved owner

The program renames a box, then consumes it through the old name. Fix the owner used without creating a duplicate box.

Start with this deliberately broken program.

class_move.xen

fn main() {
    let original = box(42);
    let transferred = original;
    println(original.into_inner());
}

xen check class_move.xen

Diagnostic excerpt:

class_move.stderr

value moved here

Completion: 42 prints once; identify the assignment transferring ownership.

Hint: The new owner is transferred; into_inner also consumes the box.

Verify and record

Use xen check file.xen followed by xen run file.xen for ordinary programs; use xen test file.xen for test-only files. Record the original error, its cause, and the corrected result, then rerun every input in the completion criteria.