Missing values and errors · Coding Class

Basics · Build a program← 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 · Handle a missing value

Write a function taking Option<Int> and returning the number as a string for Some, or missing for None.

Completion: Some(0) gives 0, Some(42) gives 42, and a typed None gives missing.

Hint: Zero is a present value, distinct from None.

2. Build a program · Build a number converter

Use double_number from the chapter to process [“21”, “hello”, “-2”] in order. Print invalid on invalid input and continue.

Completion: Output is 42, invalid, -4, and the program exits successfully.

Hint: Match each Result in the loop; handle invalid input without ending main.

3. Debug · Fix ? in main

The main below does not return Result. Replace the use of ? with a match handling success and failure in main.

Start with this deliberately broken program.

class_question.xen

use std.convert as convert;
fn main() {
    let value = convert.parse_int("21")?;
    println(value);
}

xen check class_question.xen

Diagnostic excerpt:

class_question.stderr

current function return type must be Result<T,E>

Completion: Input 21 prints 21; hello prints invalid. Keep main’s return type unchanged.

Hint: ? propagates errors only inside a Result-returning function with the same error type.

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.