Rust by Example

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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 {

let first_number = try!(first_number_str.parse::());

let second_number = try!(second_number_str.parse::());

Ok(first_number * second_number)

}

fn print(result: Result) {

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.

Multiple error types

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 needs to interact with a Result. 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:: returns a Result:

fn double_first(vec: Vec<&str>) -> i32 {

let first = vec.first().unwrap(); // Generate error 1

2 * first.parse::().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));