diff options
| -rw-r--r-- | src/libcore/macros.rs | 33 |
1 files changed, 26 insertions, 7 deletions
diff --git a/src/libcore/macros.rs b/src/libcore/macros.rs index c916ad930ff..f29a49dd5fe 100644 --- a/src/libcore/macros.rs +++ b/src/libcore/macros.rs @@ -189,10 +189,19 @@ macro_rules! debug_assert_eq { ($($arg:tt)*) => (if cfg!(debug_assertions) { assert_eq!($($arg)*); }) } -/// Helper macro for unwrapping `Result` values while returning early with an -/// error if the value of the expression is `Err`. Can only be used in -/// functions that return `Result` because of the early return of `Err` that -/// it provides. +/// Helper macro for reducing boilerplate code for matching `Result` together +/// with converting downstream errors. +/// +/// `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 /// @@ -201,18 +210,28 @@ macro_rules! debug_assert_eq { /// use std::fs::File; /// use std::io::prelude::*; /// -/// fn write_to_file_using_try() -> Result<(), io::Error> { +/// 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<(), io::Error> { +/// 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(e), +/// Err(e) => return Err(From::from(e)), /// } /// println!("I wrote to the file"); /// Ok(()) |