match and Result

Handle both arms of a Result with match.

match and Result

match is used to handle the error on the spot. By adding guards to the arms, successful values can be classified further:

match "8".parse::<i32>() {
    Ok(n) if n >= 0 => println!("kare: {}", n * n),
    Ok(n) => println!("negatif: {n}"),
    Err(e) => println!("hata: {e}"),
}

In the Err(e) arm, printing e with {} gives a user-friendly message (Display), while {:?} gives developer-oriented detail (Debug).

Result helpers

As with Option, Result has methods that work without writing a match:

let r: Result<i32, String> = Ok(3);
r.is_ok()                       // true
r.ok()                          // Some(3): Result → Option
r.map(|n| n + 1)                // Ok(4)
r.and_then(|n| if n > 5 { Ok(n) } else { Err("küçük".into()) })   // Err("küçük")

and_then chains the success value into a next step that may fail.

Exercise Classify inputs

Loop over the inputs ["8", "-3", "abc"] and handle the result of parse::<i32>() with match: print kare: N for a non-negative number, negatif: N for a negative one and hata: MESAJ for an error.

Show example solution

fn main() {
    for g in ["8", "-3", "abc"] {
        match g.parse::<i32>() {
            Ok(n) if n >= 0 => println!("kare: {}", n * n),
            Ok(n) => println!("negatif: {}", n),
            Err(e) => println!("hata: {}", e),
        }
    }
}

Test yourself

What appears when the error of <code>"".parse::<i32>()</code> is printed with <code>{}</code>?
- None
- cannot parse integer from empty string
- invalid digit found in string
- ParseIntError { kind: Empty }
Show answer

cannot parse integer from empty string — The Display message is for humans; the Debug form would be ParseIntError { kind: Empty }.

The error arm

match "x".parse::<i32>() {
    Ok(n) => println!("{n}"),
    ____(e) => println!("hata: {e}"),
}

Show answer

match "x".parse::<i32>() {
    Ok(n) => println!("{n}"),
    Err(e) => println!("hata: {e}"),
}

Method and result on Ok(3)

is_ok()

true
ok()

Some(3)
map(|n| n + 1)

Ok(4)
is_err()

false

Next lesson: unwrap and expect

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