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 empty = vec![];
let strings = vec!["tofu", "93", "18"];
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
}
Using our knowledge of combinators, we can rewrite the above to explicitly
handle errors. Since two different types of errors can occur, we need to
convert them to a common type such as a String
.
To do so, we convert both the Option
and Result
into Result
s, and
then map their errors to the same type:
// Use `String` as our error type type Result<T> = std::result::Result<T, String>; fn double_first(vec: Vec<&str>) -> Result<i32> { vec.first() // Convert the `Option` to a `Result` if there is a value. // Otherwise, provide an `Err` containing this `String`. .ok_or("Please use a vector with at least one element.".to_owned()) .and_then(|s| s.parse::<i32>() // Map any errors that `parse` yields to `String`. .map_err(|e| e.to_string()) // `Result<T, String>` is the new return type, // and we can now double the number inside. .map(|i| 2 * i)) } fn print(result: Result<i32>) { match result { Ok(n) => println!("The first doubled is {}", n), Err(e) => println!("Error: {}", e), } } fn main() { let empty = vec![]; let strings = vec!["tofu", "93", "18"]; print(double_first(empty)); print(double_first(strings)); }
In the next section, we'll see an alternate method of explicitly handling these errors.