Enum std::result::Result

#[must_use]
pub enum Result<T, E> {
    Ok(T),
    Err(E),
}

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

See the std::result module documentation for details.

Variants

Ok(T)

Contains the success value

Err(E)

Contains the error value

Methods

impl<T, E> Result<T, E>

fn is_ok(&self) -> bool

Returns true if the result is Ok.

Examples

Basic usage:

let x: Result<i32, &str> = Ok(-3);
assert_eq!(x.is_ok(), true);

let x: Result<i32, &str> = Err("Some error message");
assert_eq!(x.is_ok(), false);

fn is_err(&self) -> bool

Returns true if the result is Err.

Examples

Basic usage:

let x: Result<i32, &str> = Ok(-3);
assert_eq!(x.is_err(), false);

let x: Result<i32, &str> = Err("Some error message");
assert_eq!(x.is_err(), true);

fn ok(self) -> Option<T>

Converts from Result<T, E> to Option<T>.

Converts self into an Option<T>, consuming self, and discarding the error, if any.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(2);
assert_eq!(x.ok(), Some(2));

let x: Result<u32, &str> = Err("Nothing here");
assert_eq!(x.ok(), None);

fn err(self) -> Option<E>

Converts from Result<T, E> to Option<E>.

Converts self into an Option<E>, consuming self, and discarding the success value, if any.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(2);
assert_eq!(x.err(), None);

let x: Result<u32, &str> = Err("Nothing here");
assert_eq!(x.err(), Some("Nothing here"));

fn as_ref(&self) -> Result<&T, &E>

Converts from Result<T, E> to Result<&T, &E>.

Produces a new Result, containing a reference into the original, leaving the original in place.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(2);
assert_eq!(x.as_ref(), Ok(&2));

let x: Result<u32, &str> = Err("Error");
assert_eq!(x.as_ref(), Err(&"Error"));

fn as_mut(&mut self) -> Result<&mut T, &mut E>

Converts from Result<T, E> to Result<&mut T, &mut E>.

Examples

Basic usage:

fn mutate(r: &mut Result<i32, i32>) {
    match r.as_mut() {
        Ok(v) => *v = 42,
        Err(e) => *e = 0,
    }
}

let mut x: Result<i32, i32> = Ok(2);
mutate(&mut x);
assert_eq!(x.unwrap(), 42);

let mut x: Result<i32, i32> = Err(13);
mutate(&mut x);
assert_eq!(x.unwrap_err(), 0);

fn map<U, F>(self, op: F) -> Result<U, E> where F: FnOnce(T) -> U

Maps a Result<T, E> to Result<U, E> by applying a function to a contained Ok value, leaving an Err value untouched.

This function can be used to compose the results of two functions.

Examples

Print the numbers on each line of a string multiplied by two.

let line = "1\n2\n3\n4\n";

for num in line.lines() {
    match num.parse::<i32>().map(|i| i * 2) {
        Ok(n) => println!("{}", n),
        Err(..) => {}
    }
}

fn map_err<F, O>(self, op: O) -> Result<T, F> where O: FnOnce(E) -> F

Maps a Result<T, E> to Result<T, F> by applying a function to a contained Err value, leaving an Ok value untouched.

This function can be used to pass through a successful result while handling an error.

Examples

Basic usage:

fn stringify(x: u32) -> String { format!("error code: {}", x) }

let x: Result<u32, u32> = Ok(2);
assert_eq!(x.map_err(stringify), Ok(2));

let x: Result<u32, u32> = Err(13);
assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));

fn iter(&self) -> Iter<T>

Returns an iterator over the possibly contained value.

The iterator yields one value if the result is Ok, otherwise none.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(7);
assert_eq!(x.iter().next(), Some(&7));

let x: Result<u32, &str> = Err("nothing!");
assert_eq!(x.iter().next(), None);

fn iter_mut(&mut self) -> IterMut<T>

Returns a mutable iterator over the possibly contained value.

The iterator yields one value if the result is Ok, otherwise none.

Examples

Basic usage:

let mut x: Result<u32, &str> = Ok(7);
match x.iter_mut().next() {
    Some(v) => *v = 40,
    None => {},
}
assert_eq!(x, Ok(40));

let mut x: Result<u32, &str> = Err("nothing!");
assert_eq!(x.iter_mut().next(), None);

fn and<U>(self, res: Result<U, E>) -> Result<U, E>

Returns res if the result is Ok, otherwise returns the Err value of self.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(2);
let y: Result<&str, &str> = Err("late error");
assert_eq!(x.and(y), Err("late error"));

let x: Result<u32, &str> = Err("early error");
let y: Result<&str, &str> = Ok("foo");
assert_eq!(x.and(y), Err("early error"));

let x: Result<u32, &str> = Err("not a 2");
let y: Result<&str, &str> = Err("late error");
assert_eq!(x.and(y), Err("not a 2"));

let x: Result<u32, &str> = Ok(2);
let y: Result<&str, &str> = Ok("different result type");
assert_eq!(x.and(y), Ok("different result type"));

fn and_then<U, F>(self, op: F) -> Result<U, E> where F: FnOnce(T) -> Result<U, E>

Calls op if the result is Ok, otherwise returns the Err value of self.

This function can be used for control flow based on Result values.

Examples

Basic usage:

fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
fn err(x: u32) -> Result<u32, u32> { Err(x) }

assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));

fn or<F>(self, res: Result<T, F>) -> Result<T, F>

Returns res if the result is Err, otherwise returns the Ok value of self.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(2);
let y: Result<u32, &str> = Err("late error");
assert_eq!(x.or(y), Ok(2));

let x: Result<u32, &str> = Err("early error");
let y: Result<u32, &str> = Ok(2);
assert_eq!(x.or(y), Ok(2));

let x: Result<u32, &str> = Err("not a 2");
let y: Result<u32, &str> = Err("late error");
assert_eq!(x.or(y), Err("late error"));

let x: Result<u32, &str> = Ok(2);
let y: Result<u32, &str> = Ok(100);
assert_eq!(x.or(y), Ok(2));

fn or_else<F, O>(self, op: O) -> Result<T, F> where O: FnOnce(E) -> Result<T, F>

Calls op if the result is Err, otherwise returns the Ok value of self.

This function can be used for control flow based on result values.

Examples

Basic usage:

fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
fn err(x: u32) -> Result<u32, u32> { Err(x) }

assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
assert_eq!(Err(3).or_else(err).or_else(err), Err(3));

fn unwrap_or(self, optb: T) -> T

Unwraps a result, yielding the content of an Ok. Else, it returns optb.

Examples

Basic usage:

let optb = 2;
let x: Result<u32, &str> = Ok(9);
assert_eq!(x.unwrap_or(optb), 9);

let x: Result<u32, &str> = Err("error");
assert_eq!(x.unwrap_or(optb), optb);

fn unwrap_or_else<F>(self, op: F) -> T where F: FnOnce(E) -> T

Unwraps a result, yielding the content of an Ok. If the value is an Err then it calls op with its value.

Examples

Basic usage:

fn count(x: &str) -> usize { x.len() }

assert_eq!(Ok(2).unwrap_or_else(count), 2);
assert_eq!(Err("foo").unwrap_or_else(count), 3);

impl<T, E> Result<T, E> where E: Debug

fn unwrap(self) -> T

Unwraps a result, yielding the content of an Ok.

Panics

Panics if the value is an Err, with a panic message provided by the Err's value.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(2);
assert_eq!(x.unwrap(), 2);

let x: Result<u32, &str> = Err("emergency failure");
x.unwrap(); // panics with `emergency failure`

fn expect(self, msg: &str) -> T

Unwraps a result, yielding the content of an Ok.

Panics

Panics if the value is an Err, with a panic message including the passed message, and the content of the Err.

Examples

Basic usage:

let x: Result<u32, &str> = Err("emergency failure");
x.expect("Testing expect"); // panics with `Testing expect: emergency failure`

impl<T, E> Result<T, E> where T: Debug

fn unwrap_err(self) -> E

Unwraps a result, yielding the content of an Err.

Panics

Panics if the value is an Ok, with a custom panic message provided by the Ok's value.

Examples

let x: Result<u32, &str> = Ok(2);
x.unwrap_err(); // panics with `2`

let x: Result<u32, &str> = Err("emergency failure");
assert_eq!(x.unwrap_err(), "emergency failure");

fn expect_err(self, msg: &str) -> E

🔬 This is a nightly-only experimental API.   (result_expect_err)

Unwraps a result, yielding the content of an Err.

Panics

Panics if the value is an Ok, with a panic message including the passed message, and the content of the Ok.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(10);
x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`

impl<T, E> Result<T, E> where T: Default

fn unwrap_or_default(self) -> T

🔬 This is a nightly-only experimental API.   (result_unwrap_or_default)

Returns the contained value or a default

Consumes the self argument then, if Ok, returns the contained value, otherwise if Err, returns the default value for that type.

Examples

Convert a string to an integer, turning poorly-formed strings into 0 (the default value for integers). parse converts a string to any other type that implements FromStr, returning an Err on error.

#![feature(result_unwrap_or_default)]

let good_year_from_input = "1909";
let bad_year_from_input = "190blarg";
let good_year = good_year_from_input.parse().unwrap_or_default();
let bad_year = bad_year_from_input.parse().unwrap_or_default();

assert_eq!(1909, good_year);
assert_eq!(0, bad_year);

Trait Implementations

impl<T, E> Ord for Result<T, E> where E: Ord, T: Ord

fn cmp(&self, __arg_0: &Result<T, E>) -> Ordering

This method returns an Ordering between self and other.

impl<T, E> Hash for Result<T, E> where E: Hash, T: Hash

fn hash<__HTE>(&self, __arg_0: &mut __HTE) where __HTE: Hasher

Feeds this value into the state given, updating the hasher as necessary.

fn hash_slice<H>(data: &[Self], state: &mut H) where H: Hasher

Feeds a slice of this type into the state provided.

impl<T, E> PartialOrd<Result<T, E>> for Result<T, E> where E: PartialOrd<E>,
        T: PartialOrd<T>

fn partial_cmp(&self, __arg_0: &Result<T, E>) -> Option<Ordering>

This method returns an ordering between self and other values if one exists.

fn lt(&self, __arg_0: &Result<T, E>) -> bool

This method tests less than (for self and other) and is used by the < operator.

fn le(&self, __arg_0: &Result<T, E>) -> bool

This method tests less than or equal to (for self and other) and is used by the <= operator.

fn gt(&self, __arg_0: &Result<T, E>) -> bool

This method tests greater than (for self and other) and is used by the > operator.

fn ge(&self, __arg_0: &Result<T, E>) -> bool

This method tests greater than or equal to (for self and other) and is used by the >= operator.

impl<T, E> PartialEq<Result<T, E>> for Result<T, E> where E: PartialEq<E>,
        T: PartialEq<T>

fn eq(&self, __arg_0: &Result<T, E>) -> bool

This method tests for self and other values to be equal, and is used by ==.

fn ne(&self, __arg_0: &Result<T, E>) -> bool

This method tests for !=.

impl<T, E> IntoIterator for Result<T, E>

type Item = T

The type of the elements being iterated over.

type IntoIter = IntoIter<T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> IntoIter<T>

Returns a consuming iterator over the possibly contained value.

The iterator yields one value if the result is Ok, otherwise none.

Examples

Basic usage:

let x: Result<u32, &str> = Ok(5);
let v: Vec<u32> = x.into_iter().collect();
assert_eq!(v, [5]);

let x: Result<u32, &str> = Err("nothing!");
let v: Vec<u32> = x.into_iter().collect();
assert_eq!(v, []);

impl<'a, T, E> IntoIterator for &'a Result<T, E>

type Item = &'a T

The type of the elements being iterated over.

type IntoIter = Iter<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> Iter<'a, T>

Creates an iterator from a value.

impl<'a, T, E> IntoIterator for &'a mut Result<T, E>

type Item = &'a mut T

The type of the elements being iterated over.

type IntoIter = IterMut<'a, T>

Which kind of iterator are we turning this into?

fn into_iter(self) -> IterMut<'a, T>

Creates an iterator from a value.

impl<U, V> Carrier for Result<U, V>

type Success = U

🔬 This is a nightly-only experimental API.   (question_mark_carrier)

The type of the value when computation succeeds.

type Error = V

🔬 This is a nightly-only experimental API.   (question_mark_carrier)

The type of the value when computation errors out.

fn from_success(u: U) -> Result<U, V>

🔬 This is a nightly-only experimental API.   (question_mark_carrier)

Create a Carrier from a success value.

fn from_error(e: V) -> Result<U, V>

🔬 This is a nightly-only experimental API.   (question_mark_carrier)

Create a Carrier from an error value.

fn translate<T>(self) -> T where T: Carrier<Success=U, Error=V>

🔬 This is a nightly-only experimental API.   (question_mark_carrier)

Translate this Carrier to another implementation of Carrier with the same associated types.

impl<T, E> Clone for Result<T, E> where E: Clone, T: Clone

fn clone(&self) -> Result<T, E>

Returns a copy of the value.

fn clone_from(&mut self, source: &Self)

Performs copy-assignment from source.

impl<A, E, V> FromIterator<Result<A, E>> for Result<V, E> where V: FromIterator<A>

fn from_iter<I>(iter: I) -> Result<V, E> where I: IntoIterator<Item=Result<A, E>>

Takes each element in the Iterator: if it is an Err, no further elements are taken, and the Err is returned. Should no Err occur, a container with the values of each Result is returned.

Here is an example which increments every integer in a vector, checking for overflow:

use std::u32;

let v = vec![1, 2];
let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32|
    if x == u32::MAX { Err("Overflow!") }
    else { Ok(x + 1) }
).collect();
assert!(res == Ok(vec![2, 3]));

impl<T, U, E> Product<Result<U, E>> for Result<T, E> where T: Product<U>

fn product<I>(iter: I) -> Result<T, E> where I: Iterator<Item=Result<U, E>>

Method which takes an iterator and generates Self from the elements by multiplying the items.

impl<T, E> Eq for Result<T, E> where E: Eq, T: Eq

impl<T, U, E> Sum<Result<U, E>> for Result<T, E> where T: Sum<U>

fn sum<I>(iter: I) -> Result<T, E> where I: Iterator<Item=Result<U, E>>

Method which takes an iterator and generates Self from the elements by "summing up" the items.

impl<T, E> Copy for Result<T, E> where E: Copy, T: Copy

impl<T, E> Debug for Result<T, E> where E: Debug, T: Debug

fn fmt(&self, __arg_0: &mut Formatter) -> Result<(), Error>

Formats the value using the given formatter.