std::result - Rust (original) (raw)

Module result

1.0.0 · Source

Expand description

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 Results is clear and straightforward for simple cases, but Result comes with some convenience methods that make working with it more succinct.

// The `is_ok` and `is_err` methods do what they say.
let good_result: Result<i32, i32> = Ok(10);
let bad_result: Result<i32, i32> = Err(10);
assert!(good_result.is_ok() && !good_result.is_err());
assert!(bad_result.is_err() && !bad_result.is_ok());

// `map` and `map_err` consume the `Result` and produce another.
let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
let bad_result: Result<i32, i32> = bad_result.map_err(|i| i - 1);
assert_eq!(good_result, Ok(11));
assert_eq!(bad_result, Err(9));

// Use `and_then` to continue the computation.
let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
assert_eq!(good_result, Ok(true));

// Use `or_else` to handle the error.
let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
assert_eq!(bad_result, Ok(29));

// Consume the result and return the contents with `unwrap`.
let final_awesome_result = good_result.unwrap();
assert!(final_awesome_result)

§Results must be used

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](enum.Result.html "enum std::result::Result")<T, [io::Error](../../std/io/struct.Error.html "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(())
}

§The question mark operator, ?

When writing code that calls many functions that return theResult 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 Ok’s unwrapped value, unless the result is Err, in which case Err is returned early from the enclosing function.

? can be used in functions that return Result because of the early return of Err that it provides.

§Representation

In some cases, Result<T, E> will gain the same size, alignment, and ABI guarantees as Option has. One of either the T or E type must be a type that qualifies for the Option representation guarantees, and the other type must meet all of the following conditions:

For example, NonZeroI32 qualifies for the Option representation guarantees, and () is a zero-sized type with alignment 1, no fields, and it isn’t non_exhaustive. This means that both Result<NonZeroI32, ()> andResult<(), NonZeroI32> have the same size, alignment, and ABI guarantees as Option<NonZeroI32>. The only difference is the implied semantics:

§Method overview

In addition to working with pattern matching, Result provides a wide variety of different methods.

§Querying the variant

The is_ok and is_err methods return true if the Resultis Ok or Err, respectively.

§Adapters for working with references

These methods extract the contained value in a Result<T, E> when it is the Ok variant. If the Result is Err:

The panicking methods expect and unwrap require E to implement the Debug trait.

These methods extract the contained value in a Result<T, E> when it is the Err variant. They require T to implement the Debugtrait. If the Result is Ok:

§Transforming contained values

These methods transform Result to Option:

This method transforms the contained value of the Ok variant:

This method transforms the contained value of the Err variant:

These methods transform a Result<T, E> into a value of a possibly different type U:

§Boolean operators

These methods treat the Result as a boolean value, where Okacts like true and Err acts like false. There are two categories of these methods: ones that take a Result as input, and ones that take a function as input (to be lazily evaluated).

The and and or methods take another Result as input, and produce a Result as output. The and method can produce aResult<U, E> value having a different inner type U thanResult<T, E>. The or method can produce a Result<T, F>value having a different error type F than Result<T, E>.

method self input output
and Err(e) (ignored) Err(e)
and Ok(x) Err(d) Err(d)
and Ok(x) Ok(y) Ok(y)
or Err(e) Err(d) Err(d)
or Err(e) Ok(y) Ok(y)
or Ok(x) (ignored) Ok(x)

The and_then and or_else methods take a function as input, and only evaluate the function when they need to produce a new value. Theand_then method can produce a Result<U, E> value having a different inner type U than Result<T, E>. The or_else method can produce a Result<T, F> value having a different error type Fthan Result<T, E>.

method self function input function result output
and_then Err(e) (not provided) (not evaluated) Err(e)
and_then Ok(x) x Err(d) Err(d)
and_then Ok(x) x Ok(y) Ok(y)
or_else Err(e) e Err(d) Err(d)
or_else Err(e) e Ok(y) Ok(y)
or_else Ok(x) (not provided) (not evaluated) Ok(x)

§Comparison operators

If T and E both implement PartialOrd then Result<T, E> will derive its PartialOrd implementation. With this order, an Okcompares as less than any Err, while two Ok or two Errcompare as their contained values would in T or E respectively. If Tand E both also implement Ord, then so does Result<T, E>.

assert!(Ok(1) < Err(0));
let x: Result<i32, ()> = Ok(0);
let y = Ok(1);
assert!(x < y);
let x: Result<(), i32> = Err(0);
let y = Err(1);
assert!(x < y);

§Iterating over Result

A Result can be iterated over. This can be helpful if you need an iterator that is conditionally empty. The iterator will either produce a single value (when the Result is Ok), or produce no values (when the Result is Err). For example, into_iter acts likeonce(v) if the Result is Ok(v), and like empty() if theResult is Err.

Iterators over Result<T, E> come in three types:

See Iterating over Option for examples of how this can be useful.

You might want to use an iterator chain to do multiple instances of an operation that can fail, but would like to ignore failures while continuing to process the successful results. In this example, we take advantage of the iterable nature of Result to select only theOk values using flatten.

let mut results = vec![];
let mut errs = vec![];
let nums: Vec<_> = ["17", "not a number", "99", "-27", "768"]
   .into_iter()
   .map(u8::from_str)
   // Save clones of the raw `Result` values to inspect
   .inspect(|x| results.push(x.clone()))
   // Challenge: explain how this captures only the `Err` values
   .inspect(|x| errs.extend(x.clone().err()))
   .flatten()
   .collect();
assert_eq!(errs.len(), 3);
assert_eq!(nums, [17, 99]);
println!("results {results:?}");
println!("errs {errs:?}");
println!("nums {nums:?}");

§Collecting into Result

Result implements the FromIterator trait, which allows an iterator over Result values to be collected into aResult of a collection of each contained value of the originalResult values, or Err if any of the elements was Err.

let v = [Ok(2), Ok(4), Err("err!"), Ok(8)];
let res: Result<Vec<_>, &str> = v.into_iter().collect();
assert_eq!(res, Err("err!"));
let v = [Ok(2), Ok(4), Ok(8)];
let res: Result<Vec<_>, &str> = v.into_iter().collect();
assert_eq!(res, Ok(vec![2, 4, 8]));

Result also implements the Product andSum traits, allowing an iterator over Result values to provide the product andsum methods.

let v = [Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
let res: Result<i32, &str> = v.into_iter().sum();
assert_eq!(res, Err("error!"));
let v = [Ok(1), Ok(2), Ok(21)];
let res: Result<i32, &str> = v.into_iter().product();
assert_eq!(res, Ok(42));

IntoIter

An iterator over the value in a Ok variant of a Result.

Iter

An iterator over a reference to the Ok variant of a Result.

IterMut

An iterator over a mutable reference to the Ok variant of a Result.

Result

Result is a type that represents either success (Ok) or failure (Err).