Introducing ?
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:
panic!
which we already decided to try to avoid if possiblereturn
because anErr
means it cannot be handled
?
is almost1 exactly equivalent to an unwrap
which return
s
instead of panic
king on Err
s. 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")); }
The try!
macro
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")); }
1
See re-enter ? for more details.