Scope modes · 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 · Enable references where needed

Run two versions of scope_modes.xen from the chapter: annotate all of main, then annotate only the call block.

Completion: Both print [1, 2, 3] and 10; identify the extent of reference permission in each.

Hint: Use #![explc] for a function and #scope[explc] for a block.

2. Build a program · Compare optimization modes

Run the same program with —opt=off and —opt=basic and compare stdout. Also run basic with —opt-report, distinguishing the report from program output.

Completion: Outputs agree; the report appears on stderr. Output agreement alone is not evidence of a speed increase.

Hint: Use the chapter’s commands; do not make internal report identifiers part of expected output.

3. Debug · Fix a missing caller annotation

Check the program below and fix it by enclosing only the reference-creating call in main in a block. Explain why —opt cannot fix it.

Start with this deliberately broken program.

class_scope.xen

#![explc]
fn append(values: &mut Vec<Int>) { values.push(3); }
fn main() {
    let mut values = [1, 2];
    append(&mut values);
    println(values);
}

xen check class_scope.xen

Diagnostic excerpt:

class_scope.stderr

passing references requires #![explc]

Completion: Output is [1, 2, 3]. Check the callee’s declaration and the caller’s scope separately.

Hint: A callee’s explc setting does not propagate to its caller.

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.