Unwraps a result or propagates its error.

The ? operator was added to replace try! and should be used instead. Furthermore, try is a reserved word in Rust 2018, so if you must use it, you will need to use the raw-identifier syntax: r#try.

try! matches the given Result. In case of the Ok variant, the expression has the value of the wrapped value.

In case of the Err variant, it retrieves the inner error. try! then performs conversion using From. This provides automatic conversion between specialized errors and more general ones. The resulting error is then immediately returned.

Because of the early return, try! can only be used in functions that return Result.

Examples

use std::io;
use std::fs::File;
use std::io::prelude::*;

enum MyError {
    FileWriteError
}

impl From<io::Error> for MyError {
    fn from(e: io::Error) -> MyError {
        MyError::FileWriteError
    }
}

// The preferred method of quick returning Errors
fn write_to_file_question() -> Result<(), MyError> {
    let mut file = File::create("my_best_friends.txt")?;
    file.write_all(b"This is a list of my best friends.")?;
    Ok(())
}

// The previous method of quick returning Errors
fn write_to_file_using_try() -> Result<(), MyError> {
    let mut file = r#try!(File::create("my_best_friends.txt"));
    r#try!(file.write_all(b"This is a list of my best friends."));
    Ok(())
}

// This is equivalent to:
fn write_to_file_using_match() -> Result<(), MyError> {
    let mut file = r#try!(File::create("my_best_friends.txt"));
    match file.write_all(b"This is a list of my best friends.") {
        Ok(v) => v,
        Err(e) => return Err(From::from(e)),
    }
    Ok(())
}