Day 11: Result & Error Handling

Rust uses Result<T, E> for operations that can fail. No exceptions!

The Result Enum

enum Result<T, E> {
    Ok(T),   // Success with value T
    Err(E),  // Error with value E
}

Handling Results

let result: Result<i32, String> = "42".parse();

match result {
    Ok(num) => println!("Parsed: {}", num),
    Err(e) => println!("Error: {}", e),
}

The ? Operator

Propagate errors automatically:

fn read_number() -> Result<i32, String> {
    let num = "42".parse().map_err(|_| String::from("parse error"))?;
    Ok(num * 2)
}

The ? operator can only be used in functions that return Result or Option!

Common Result Methods

result.unwrap()        // Panics on Err
result.expect("msg")   // Panics with message
result.unwrap_or(0)    // Default value on Err
result.is_ok()         // Returns bool
result.is_err()        // Returns bool

The Task

Implement parse_and_double(s) that:

Parses the string to an i32
Returns Ok(number * 2) on success
Returns Err("Invalid number") on failure

Requirements

parse_and_double("21") → Ok(42)
parse_and_double("abc") → Err("Invalid number")

Hints

fn parse_and_double(s: &str) -> Result<i32, String> {
    match s.parse::<i32>() {
        Ok(n) => Ok(n * 2),
        Err(_) => Err(String::from("Invalid number")),
    }
}