Multiple error types - Rust By Example
Description: Rust by Example (RBE) is a collection of runnable examples that illustrate various Rust concepts and standard libraries.

# Multiple error types

The previous examples have always been very convenient; `Result`s interact with other `Result`s and `Option`s interact with other `Option`s.

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.