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.