Error Handling Patterns

Option to Result

Adding an Error Message

Option says a value may be absent. ok_or turns absence into a Result with an explicit error value.

Program

Play the program to look up a selected setting and label missing keys.

option_to_result.rs 
fn lookup(key: &str) -> Result<&'static str, &'static str> {
    let value = match key {
        "host" => Some("local"),
        "port" => Some("8080"),
        _ => None,
    };
    value.ok_or("missing")
}

fn main() {
    let key = ;
    let result = lookup(key);
    let label = match result {
        Ok(value) => format!("{key}={value}"),
        Err(error) => format!("{key}:{error}"),
    };
    println!("{label}");
}

fn lookup(key: &str) -> Result<&'static str, &'static str> {
    let value = match key {
        "host" => Some("local"),
        "port" => Some("8080"),
        _ => None,
    };
    value.ok_or("missing")
}

fn main() {
    let key = ;
    let result = lookup(key);
    let label = match result {
        Ok(value) => format!("{key}={value}"),
        Err(error) => format!("{key}:{error}"),
    };
    println!("{label}");
}

fn lookup(key: &str) -> Result<&'static str, &'static str> {
    let value = match key {
        "host" => Some("local"),
        "port" => Some("8080"),
        _ => None,
    };
    value.ok_or("missing")
}

fn main() {
    let key = ;
    let result = lookup(key);
    let label = match result {
        Ok(value) => format!("{key}={value}"),
        Err(error) => format!("{key}:{error}"),
    };
    println!("{label}");
}
Option `Option<T>` is either `Some(T)` or `None`.
ok_or `ok_or(error)` turns `Some` into `Ok` and `None` into `Err(error)`.
lookup result The caller gets a `Result` with a clear missing-key label.