Module std::result

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.

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();

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<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(())
}

The question mark operator, ?

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.

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 Result is Ok or Err, respectively.

Adapters for working with references

• as_ref converts from &Result<T, E> to Result<&T, &E>
• as_mut converts from &mut Result<T, E> to Result<&mut T, &mut E>
• as_deref converts from &Result<T, E> to Result<&T::Target, &E>
• as_deref_mut converts from &mut Result<T, E> to Result<&mut T::Target, &mut E>

Extracting contained values

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

• expect panics with a provided custom message
• unwrap panics with a generic message
• unwrap_or returns the provided default value
• unwrap_or_default returns the default value of the type T (which must implement the Default trait)
• unwrap_or_else returns the result of evaluating the provided function

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 Debug trait. If the Result is Ok:

• expect_err panics with a provided custom message
• unwrap_err panics with a generic message

Transforming contained values

These methods transform Result to Option:

• err transforms Result<T, E> into Option<E>, mapping Err(e) to Some(e) and Ok(v) to None
• ok transforms Result<T, E> into Option<T>, mapping Ok(v) to Some(v) and Err(e) to None
• transpose transposes a Result of an Option into an Option of a Result

This method transforms the contained value of the Ok variant:

• map transforms Result<T, E> into Result<U, E> by applying the provided function to the contained value of Ok and leaving Err values unchanged

This method transforms the contained value of the Err variant:

• map_err transforms Result<T, E> into Result<T, F> by applying the provided function to the contained value of Err and leaving Ok values unchanged

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

• map_or applies the provided function to the contained value of Ok, or returns the provided default value if the Result is Err
• map_or_else applies the provided function to the contained value of Ok, or applies the provided fallback function to the contained value of Err

Boolean operators

These methods treat the Result as a boolean value, where Ok acts 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 a Result<U, E> value having a different inner type U than Result<T, E>. The or method can produce a Result<T, F> value having a different error type F than Result<T, E>.

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. The and_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 F than Result<T, E>.

Comparison operators

If T and E both implement PartialOrd then Result<T, E> will derive its PartialOrd implementation. With this order, an Ok compares as less than any Err, while two Ok or two Err compare as their contained values would in T or E respectively. If T and 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 like once(v) if the Result is Ok(v), and like empty() if the Result is Err.

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

• into_iter consumes the Result and produces the contained value
• iter produces an immutable reference of type &T to the contained value
• iter_mut produces a mutable reference of type &mut T to the contained value

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 the Ok values using flatten.

let mut results = vec![];
let mut errs = vec![];
let nums: Vec<_> = 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 a Result of a collection of each contained value of the original Result values, or Err if any of the elements was Err.

let v = vec![Ok(2), Ok(4), Err("err!"), Ok(8)];
let res: Result<Vec<_>, &str> = v.into_iter().collect();
assert_eq!(res, Err("err!"));
let v = vec![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 and Sum traits, allowing an iterator over Result values to provide the product and sum methods.

let v = vec![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: Vec<Result<i32, &str>> = vec![Ok(1), Ok(2), Ok(21)];
let res: Result<i32, &str> = v.into_iter().product();
assert_eq!(res, Ok(42));

Structs

Enums