Error Handling Patterns

Result Match

Handling Success and Failure

Result represents either Ok(value) or Err(error). Matching both cases makes the success and failure paths explicit.

Program

Play the program to parse selected text and build a label from the Result.

text
result_match.rs
Replay: real traced execution (multi-file project)
fn main() {
    let text = "42";
    let parsed = text.parse::<i32>();
    let label = match parsed {
        Ok(value) => format!("ok:{value}"),
        Err(_) => "error".to_string(),
    };
    println!("{label}");
}

fn main() {
    let text = "7";
    let parsed = text.parse::<i32>();
    let label = match parsed {
        Ok(value) => format!("ok:{value}"),
        Err(_) => "error".to_string(),
    };
    println!("{label}");
}

fn main() {
    let text = "bad";
    let parsed = text.parse::<i32>();
    let label = match parsed {
        Ok(value) => format!("ok:{value}"),
        Err(_) => "error".to_string(),
    };
    println!("{label}");
}

  1. text ← "42", parsed ← Ok(42), label ← "ok:42"

    1fn main() {2    let tex→ "42"t = "42"; //@text="42", "7", "bad"3    let parse→ Ok(42)d = text.parse::<i32>();4    let labe→ "ok:42"l = match parseOk(42)d {5        Ok(value) => format!("ok:{value}"),6        Err(_) => "error".to_string(),7    };8    println!("{label}");9}
    outputok:42

  1. text ← "7", parsed ← Ok(7), label ← "ok:7"

    1fn main() {2    let tex→ "7"t = "7";3    let parse→ Ok(7)d = text.parse::<i32>();4    let labe→ "ok:7"l = match parseOk(7)d {5        Ok(value) => format!("ok:{value}"),6        Err(_) => "error".to_string(),7    };8    println!("{label}");9}
    outputok:7

  1. text ← "bad", parsed ← Err(ParseIntError { kind: InvalidDigit })

    1fn main() {2    let tex→ "bad"t = "bad";3    let parse→ Err(ParseIntError { kind: InvalidDigit })d = text.parse::<i32>();4    let labe→ "error"l = match parseErr(ParseIntError { kind: InvalidDigit })d {5        Ok(value) => format!("ok:{value}"),6        Err(_) => "error".to_string(),7    };8    println!("{label}");9}
    outputerror

Follow the Match

  1. The program tries to parse selected text.
  2. The parse produces one Result.
  3. Ok(value) builds the success label.
  4. Err(_) builds the failure label. | Arm | What the page shows | | --- | --- | | Ok(value) | Use the parsed value. | | Err(_) | Keep the program moving with a stable message. |
Result `Result<T, E>` is either `Ok(T)` or `Err(E)`.
match `match parsed` handles both success and error cases.
error path The `Err(_)` arm ignores the error detail and returns a stable label.

Exercise: result_match.rs

Match a parse Result and produce one label for success and one stable label for failure