Macro std::try

macro_rules! try {
    ( $ expr : expr ) => { ... };
}

Helper macro for reducing boilerplate code for matching Result together with converting downstream errors.

Prefer using ? syntax to try!. ? is built in to the language and is more succinct than try!. It is the standard method for error propagation.

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
    }
}

fn write_to_file_using_try() -> Result<(), MyError> {
    let mut file = try!(File::create("my_best_friends.txt"));
    try!(file.write_all(b"This is a list of my best friends."));
    println!("I wrote to the file");
    Ok(())
}
// This is equivalent to:
fn write_to_file_using_match() -> Result<(), MyError> {
    let mut file = 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)),
    }
    println!("I wrote to the file");
    Ok(())
}