Multiple error types

The previous examples have always been very convenient; Results interact with other Results and Options interact with other Options.

Sometimes an Option needs to interact with a Result, or a Result<T, Error1> needs to interact with a Result<T, Error2>. In those cases, we want to manage our different error types in a way that makes them composable and easy to interact with.

In the following code, two instances of unwrap generate different error types. Vec::first returns an Option, while parse::<i32> returns a Result<i32, ParseIntError>:

fn double_first(vec: Vec<&str>) -> i32 {
    let first = vec.first().unwrap(); // Generate error 1
    2 * first.parse::<i32>().unwrap() // Generate error 2
}

fn main() {
    let numbers = vec!["42", "93", "18"];
    let empty = vec![];
    let strings = vec!["tofu", "93", "18"];

    println!("The first doubled is {}", double_first(numbers));

    println!("The first doubled is {}", double_first(empty));
    // Error 1: the input vector is empty

    println!("The first doubled is {}", double_first(strings));
    // Error 2: the element doesn't parse to a number
}

Over the next sections, we'll see several strategies for handling these kind of problems.