Result: a richer version of Option

Result is a richer version of the Option type that describes possible error instead of possible absence.

That is, Result<T, E> could have one of two outcomes:

• Ok(T): An element T was found
• Err(E): An error was found with element E

By convention, the expected outcome is Ok while the unexpected outcome is Err.

Like Option, Result has many methods associated with it:

• unwrap(), for example, either yields the element T or panics.
• For case handling, there are many combinators between Result and Option that overlap.

In working with Rust, you will likely encounter methods that return the Result type, such as the parse() method.

parse & unwrap: raise Exception

It might not always be possible to parse a string into the other type, so parse() returns a Result indicating possible failure.

fn multiply(first_number_str: &str, second_number_str: &str) -> i32 {
    // Let's try using `unwrap()` to get the number out. Will it bite us?
    let first_number = first_number_str.parse::<i32>().unwrap();
    let second_number = second_number_str.parse::<i32>().unwrap();
    first_number * second_number
}

fn main() {
    let twenty = multiply("10", "2");
    println!("double is {}", twenty);

    let tt = multiply("t", "2");
    println!("double is {}", tt);
}

1. In the unsuccessful case, parse() leaves us with an error for unwrap() to panic on.
2. Additionally, the panic exits our program and provides an unpleasant error message.

Using Result in main

To improve the quality of our error message, we should be more specific about the return type and consider explicitly handling the error.

The Result type can also be the return type of the main function if specified explicitly.

Typically the main function will be of the form:

fn main() {
    println!("Hello World!");
}

However main is also able to have a return type of Result:

1. If an error occurs within the main function it will return an error code and print a debug representation of the error (using the Debug trait).

use std::num::ParseIntError;

fn main() -> Result<(), ParseIntError> {
    let number_str = "10";
    let number = match number_str.parse::<i32>() {
        Ok(number)  => number,
        Err(e) => return Err(e),
    };
    println!("{}", number);
    Ok(())
}