0xC0Lesson 13 of 17

Result propagation and the ? operator

Handle fallible steps clearly with `Result` and `?`.

18 min 5-question quiz 1 code exercise

By the end of this lesson you can

Use ? to propagate parsing and validation errors.

The ? operator short-circuits on Err and unwraps Ok values, making fallible code concise. It works in functions that return Result (or compatible types) and keeps error flow explicit.

main.rs

1fn parse_positive(value: &str) -> Result<i32, String> {
2    let number: i32 = value.trim().parse().map_err(|_| "invalid integer".to_string())?;
3    if number > 0 { Ok(number) } else { Err("must be positive".to_string()) }
4}
5
6fn main() {
7    println!("{:?}", parse_positive("12"));
8}

Output

Ok(12)

Use ? for each fallible step to return early on failure. This keeps the happy path readable while preserving explicit error handling.

Try it

Up to 15 XP

Trace `?` through a fallible workflow

Follow how ? either continues with Ok or returns early with Err.

Message 1 of 3Predicted 0/0

Input text

parse helper

caller handling

Key takeaways

Use ? to propagate parsing and validation errors.
Rust uses types and ownership rules to prevent common bugs.
Small experiments build intuition quickly.

Lesson quiz

5 questions · pass with 4 correct · up to 50 XP

Passing this quiz completes the lesson and keeps your streak going. Questions you miss come back in review sessions later.

Practice: write Rust

These run for real. Write the program, press Run tests, and read what the compiler tells you - learning to read compiler messages is half of learning Rust.

Exercise 1

Divide with propagated errors

+25 XP

Implement safe_divide returning Result<i32, String>. Return an error for division by zero; otherwise return quotient. Print value or error.

Valid division
Division by zero

main.rs

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Rust lessons are authored for modern stable Rust; exercises run on Rust 1.82 via Wandbox, a free public service. Your source code and test input are sent to that service.

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