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.