Error handling with the Result
type.
Result<T, E>
is the type used for returning and propagating errors. It is an enum with the variants, Ok(T)
, representing success and containing a value, and Err(E)
, representing error and containing an error value.
enum Result<T, E> { Ok(T), Err(E), }
Functions return Result
whenever errors are expected and recoverable. In the std
crate, Result
is most prominently used for I/O.
A simple function returning Result
might be defined and used like so:
#[derive(Debug)] enum Version { Version1, Version2 } fn parse_version(header: &[u8]) -> Result<Version, &'static str> { match header.get(0) { None => Err("invalid header length"), Some(&1) => Ok(Version::Version1), Some(&2) => Ok(Version::Version2), Some(_) => Err("invalid version"), } } let version = parse_version(&[1, 2, 3, 4]); match version { Ok(v) => println!("working with version: {:?}", v), Err(e) => println!("error parsing header: {:?}", e), }
Pattern matching on Result
s is clear and straightforward for simple cases, but Result
comes with some convenience methods that make working with it more succinct.
let good_result: Result<i32, i32> = Ok(10); let bad_result: Result<i32, i32> = Err(10); // The `is_ok` and `is_err` methods do what they say. assert!(good_result.is_ok() && !good_result.is_err()); assert!(bad_result.is_err() && !bad_result.is_ok()); // `map` consumes the `Result` and produces another. let good_result: Result<i32, i32> = good_result.map(|i| i + 1); let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1); // Use `and_then` to continue the computation. let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11)); // Use `or_else` to handle the error. let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20)); // Consume the result and return the contents with `unwrap`. let final_awesome_result = good_result.unwrap();
A common problem with using return values to indicate errors is that it is easy to ignore the return value, thus failing to handle the error. Result
is annotated with the #[must_use]
attribute, which will cause the compiler to issue a warning when a Result value is ignored. This makes Result
especially useful with functions that may encounter errors but don't otherwise return a useful value.
Consider the write_all
method defined for I/O types by the Write
trait:
use std::io; trait Write { fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>; }
Note: The actual definition of Write
uses io::Result
, which is just a synonym for Result
<T,
io::Error
>
.
This method doesn't produce a value, but the write may fail. It's crucial to handle the error case, and not write something like this:
use std::fs::File; use std::io::prelude::*; let mut file = File::create("valuable_data.txt").unwrap(); // If `write_all` errors, then we'll never know, because the return // value is ignored. file.write_all(b"important message");
If you do write that in Rust, the compiler will give you a warning (by default, controlled by the unused_must_use
lint).
You might instead, if you don't want to handle the error, simply assert success with expect
. This will panic if the write fails, providing a marginally useful message indicating why:
use std::fs::File; use std::io::prelude::*; let mut file = File::create("valuable_data.txt").unwrap(); file.write_all(b"important message").expect("failed to write message");
You might also simply assert success:
assert!(file.write_all(b"important message").is_ok());
Or propagate the error up the call stack with ?
:
fn write_message() -> io::Result<()> { let mut file = File::create("valuable_data.txt")?; file.write_all(b"important message")?; Ok(()) }
?
When writing code that calls many functions that return the Result
type, the error handling can be tedious. The question mark operator, ?
, hides some of the boilerplate of propagating errors up the call stack.
It replaces this:
use std::fs::File; use std::io::prelude::*; use std::io; struct Info { name: String, age: i32, rating: i32, } fn write_info(info: &Info) -> io::Result<()> { // Early return on error let mut file = match File::create("my_best_friends.txt") { Err(e) => return Err(e), Ok(f) => f, }; if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) { return Err(e) } if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) { return Err(e) } if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) { return Err(e) } Ok(()) }
With this:
use std::fs::File; use std::io::prelude::*; use std::io; struct Info { name: String, age: i32, rating: i32, } fn write_info(info: &Info) -> io::Result<()> { let mut file = File::create("my_best_friends.txt")?; // Early return on error file.write_all(format!("name: {}\n", info.name).as_bytes())?; file.write_all(format!("age: {}\n", info.age).as_bytes())?; file.write_all(format!("rating: {}\n", info.rating).as_bytes())?; Ok(()) }
It's much nicer!
Ending the expression with ?
will result in the unwrapped success (Ok
) value, unless the result is Err
, in which case Err
is returned early from the enclosing function.
?
can only be used in functions that return Result
because of the early return of Err
that it provides.
IntoIter | |
Iter | |
IterMut |
An iterator over a mutable reference to the |
Result |
|
© 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/result/index.html