macro_rules! try { ( $ expr : expr ) => { ... }; }
Helper macro for reducing boilerplate code for matching Result
together with converting downstream errors.
The ?
operator was added to replace try!
and should be used instead.
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
.
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 prefered method of quick returning Errors fn write_to_file_question() -> Result<(), MyError> { let mut file = File::create("my_best_friends.txt")?; Ok(()) } // The previous method of quick returning Errors 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.")); 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)), } Ok(()) }
© 2010 The Rust Project Developers
Licensed under the Apache License, Version 2.0 or the MIT license, at your option.
https://doc.rust-lang.org/std/macro.try.html