At this point, we've learned to explicitly handle errors using combinators and early returns. While we generally want to avoid panicking, explicitly handling all of our errors is cumbersome.
In the next section, we'll introduce ? for the cases where we simply need to unwrap without possibly inducing panic.
Sometimes we just want the simplicity of unwrap without the possibility of a panic. Until now, unwrap has forced us to nest deeper and deeper when what we really wanted was to get the variable out. This is exactly the purpose of ?.
Upon finding an Err, there are two valid actions to take:
1. panic! which we already decided to try to avoid if possible
2. return because an Err means it cannot be handled
? is almost exactly equivalent to an unwrap which returns instead of panicking on Errs. Let's see how we can simplify the earlier example that used combinators:
use std::num::ParseIntError;
fn multiply(first_number_str: &str, second_number_str: &str) -> Result<i32, ParseIntError> {
let first_number = first_number_str.parse::<i32>()?;
let second_number = second_number_str.parse::<i32>()?;
Ok(first_number * second_number)
}
fn print(result: Result<i32, ParseIntError>) {
match result {
Ok(n) => println!("n is {}", n),
Err(e) => println!("Error: {}", e),
}
}
fn main() {
print(multiply("10", "2"));
print(multiply("t", "2"));
}
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Before there was ?, the same functionality was achieved with the try! macro. The ? operator is now recommended, but you may still find try! when looking at older code. The same multiply function from the previous example would look like this using try!:
// To compile and run this example without errors, while using Cargo, change the value
// of the `edition` field, in the `[package]` section of the `Cargo.toml` file, to "2015".
use std::num::ParseIntError;
fn multiply(first_number_str: &str, second_number_str: &str) -> Result<i32, ParseIntError> {
let first_number = try!(first_number_str.parse::<i32>());
let second_number = try!(second_number_str.parse::<i32>());
Ok(first_number * second_number)
}
fn print(result: Result<i32, ParseIntError>) {
match result {
Ok(n) => println!("n is {}", n),
Err(e) => println!("Error: {}", e),
}
}
fn main() {
print(multiply("10", "2"));
print(multiply("t", "2"));
}
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1
See re-enter ? for more details.
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
}
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Over the next sections, we'll see several strategies for handling these kind of problems.