Error Handling

• std::result::Result

Ways to handle errors in Rust:

• panic: panic!()
• Option: Option<T>, variants: Some(T), None
• Result: Result<T, E>, variants: Ok(T), Err(E)
• Error propagating: ?

fn read() -> Result<String, io::Error> {
    let mut s = String::new();
    File::open("hello.txt")?.read_to_string(&mut s)?;
    Ok(s)
}

panic

When to panic

• In example code or tests.
• When you know Err never happens, let ip: IpAddr = "127.0.0.1".parse().unwrap()

panic!("This is totally wrong!");

panic can be conditionally compiled. Currenty available values are abort and unwind.

Option

Unwrap of Option and Result

unwrap and expect can return the contained Some/Ok value.

• it will panic if value is None/Err

Defaults

• or and or_else can be used to return default value when contained value not matched.
• unwrap_or
• get_or_insert

Combinators

• and
• and_then
• map
• map_or

Error Propagating with ?

fn do_the_thing(i: i32) -> Result<i32, Error> {
    let i = match halves_if_even(i) {
        Ok(i) => i,
        Err(e) => return Err(e),
    };

    // use `i`
}

// can be written with
fn do_the_thing(i: i32) -> Result<i32, Error> {
    let i = halves_if_even(i)?;

    // use `i`
}

Result

The Result type can also be used in main function.