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/Rust

Enum std::result::Result

#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
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

Contains the success value

Contains the error value

Methods

impl<T, E> Result<T, E> [src]

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);

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);

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);

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"));

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"));

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);

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(..) => {}
    }
}

🔬 This is a nightly-only experimental API. (result_map_or_else #53268)

Maps a Result<T, E> to U by applying a function to a contained Ok value, or a fallback function to a contained Err value.

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

Examples

Basic usage:

#![feature(result_map_or_else)]
let k = 21;

let x : Result<_, &str> = Ok("foo");
assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);

let x : Result<&str, _> = Err("bar");
assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);

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()));

ⓘImportant traits for Iter<'a, T>
impl<'a, T> Iterator for Iter<'a, T>
    type Item = &'a T;

Returns an iterator over the possibly contained value.

The iterator yields one value if the result is Result::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);

ⓘImportant traits for IterMut<'a, T>
impl<'a, T> Iterator for IterMut<'a, T>
    type Item = &'a mut T;

Returns a mutable iterator over the possibly contained value.

The iterator yields one value if the result is Result::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);

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"));

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));

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

Arguments passed to or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use or_else, which is lazily evaluated.

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));

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));

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

Arguments passed to unwrap_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use unwrap_or_else, which is lazily evaluated.

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);

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
[src]

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`

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
[src]

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");

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
[src]

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.

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);

impl<T, E> Result<T, E> where
    T: Deref
[src]

🔬 This is a nightly-only experimental API. (inner_deref #50264)newly added

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

Leaves the original Result in-place, creating a new one with a reference to the original one, additionally coercing the Ok arm of the Result via Deref.

impl<T, E> Result<T, E> where
    E: Deref
[src]

🔬 This is a nightly-only experimental API. (inner_deref #50264)newly added

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

Leaves the original Result in-place, creating a new one with a reference to the original one, additionally coercing the Err arm of the Result via Deref.

impl<T, E> Result<T, E> where
    E: Deref,
    T: Deref
[src]

🔬 This is a nightly-only experimental API. (inner_deref #50264)newly added

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

Leaves the original Result in-place, creating a new one with a reference to the original one, additionally coercing both the Ok and Err arms of the Result via Deref.

impl<T, E> Result<Option<T>, E> [src]

🔬 This is a nightly-only experimental API. (transpose_result #47338)

Transposes a Result of an Option into an Option of a Result.

Ok(None) will be mapped to None. Ok(Some(_)) and Err(_) will be mapped to Some(Ok(_)) and Some(Err(_)).

Examples

#![feature(transpose_result)]

#[derive(Debug, Eq, PartialEq)]
struct SomeErr;

let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
assert_eq!(x.transpose(), y);

Trait Implementations

impl<T, E> IntoIterator for Result<T, E> [src]

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

ⓘImportant traits for IntoIter<T>
impl<T> Iterator for IntoIter<T>
    type Item = T;

Returns a consuming iterator over the possibly contained value.

The iterator yields one value if the result is Result::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 mut Result<T, E> [src]
1.4.0

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

ⓘImportant traits for IterMut<'a, T>
impl<'a, T> Iterator for IterMut<'a, T>
    type Item = &'a mut T;

Creates an iterator from a value. Read more

impl<'a, T, E> IntoIterator for &'a Result<T, E> [src]
1.4.0

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

ⓘImportant traits for Iter<'a, T>
impl<'a, T> Iterator for Iter<'a, T>
    type Item = &'a T;

Creates an iterator from a value. Read more

impl<T, E> Hash for Result<T, E> where
    E: Hash,
    T: Hash
[src]

Feeds this value into the given [Hasher]. Read more

Feeds a slice of this type into the given [Hasher]. Read more

impl<T, E> Clone for Result<T, E> where
    E: Clone,
    T: Clone
[src]

Returns a copy of the value. Read more

Performs copy-assignment from source. Read more

impl<T, E> PartialOrd<Result<T, E>> for Result<T, E> where
    E: PartialOrd<E>,
    T: PartialOrd<T>, 
[src]

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

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

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

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

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

impl<T, E> Ord for Result<T, E> where
    E: Ord,
    T: Ord
[src]

This method returns an Ordering between self and other. Read more

Compares and returns the maximum of two values. Read more

Compares and returns the minimum of two values. Read more

impl<T, U, E> Sum<Result<U, E>> for Result<T, E> where
    T: Sum<U>, 
[src]
1.16.0

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, the sum of all elements is returned.

Examples

This sums up every integer in a vector, rejecting the sum if a negative element is encountered:

let v = vec![1, 2];
let res: Result<i32, &'static str> = v.iter().map(|&x: &i32|
    if x < 0 { Err("Negative element found") }
    else { Ok(x) }
).sum();
assert_eq!(res, Ok(3));

impl<T, E> Debug for Result<T, E> where
    E: Debug,
    T: Debug
[src]

Formats the value using the given formatter. Read more

impl<T, E> Eq for Result<T, E> where
    E: Eq,
    T: Eq
[src]

impl<T, E> PartialEq<Result<T, E>> for Result<T, E> where
    E: PartialEq<E>,
    T: PartialEq<T>, 
[src]

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

This method tests for !=.

impl<T, E> Copy for Result<T, E> where
    E: Copy,
    T: Copy
[src]

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

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:

let v = vec![1, 2];
let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
    x.checked_add(1).ok_or("Overflow!")
).collect();
assert!(res == Ok(vec![2, 3]));

impl<T, E> Try for Result<T, E> [src]

🔬 This is a nightly-only experimental API. (try_trait #42327)

The type of this value when viewed as successful.

🔬 This is a nightly-only experimental API. (try_trait #42327)

The type of this value when viewed as failed.

🔬 This is a nightly-only experimental API. (try_trait #42327)

Applies the "?" operator. A return of Ok(t) means that the execution should continue normally, and the result of ? is the value t. A return of Err(e) means that execution should branch to the innermost enclosing catch, or return from the function. Read more

🔬 This is a nightly-only experimental API. (try_trait #42327)

Wrap an OK value to construct the composite result. For example, Result::Ok(x) and Result::from_ok(x) are equivalent. Read more

🔬 This is a nightly-only experimental API. (try_trait #42327)

Wrap an error value to construct the composite result. For example, Result::Err(x) and Result::from_error(x) are equivalent. Read more

impl<T, U, E> Product<Result<U, E>> for Result<T, E> where
    T: Product<U>, 
[src]
1.16.0

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, the product of all elements is returned.

impl<E: Debug> Termination for Result<(), E> [src]

🔬 This is a nightly-only experimental API. (termination_trait_lib #43301)

Is called to get the representation of the value as status code. This status code is returned to the operating system. Read more

impl<E: Debug> Termination for Result<!, E> [src]

🔬 This is a nightly-only experimental API. (termination_trait_lib #43301)

Is called to get the representation of the value as status code. This status code is returned to the operating system. Read more

Auto Trait Implementations

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

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

Blanket Implementations

impl<T, U> TryFrom for T where
    T: From<U>, 
[src]

🔬 This is a nightly-only experimental API. (try_from #33417)

The type returned in the event of a conversion error.

🔬 This is a nightly-only experimental API. (try_from #33417)

Performs the conversion.

impl<T> From for T [src]

Performs the conversion.

impl<I> IntoIterator for I where
    I: Iterator
[src]

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

Creates an iterator from a value. Read more

impl<T, U> TryInto for T where
    U: TryFrom<T>, 
[src]

🔬 This is a nightly-only experimental API. (try_from #33417)

The type returned in the event of a conversion error.

🔬 This is a nightly-only experimental API. (try_from #33417)

Performs the conversion.

impl<T, U> Into for T where
    U: From<T>, 
[src]

Performs the conversion.

impl<T> Borrow for T where
    T: ?Sized
[src]

ⓘImportant traits for &'a mut I
impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<'a, R: Read + ?Sized> Read for &'a mut R
impl<'a, W: Write + ?Sized> Write for &'a mut W

Immutably borrows from an owned value. Read more

impl<T> BorrowMut for T where
    T: ?Sized
[src]

ⓘImportant traits for &'a mut I
impl<'a, I> Iterator for &'a mut I where
    I: Iterator + ?Sized, 
    type Item = <I as Iterator>::Item;
impl<'a, R: Read + ?Sized> Read for &'a mut R
impl<'a, W: Write + ?Sized> Write for &'a mut W

Mutably borrows from an owned value. Read more

impl<T> Any for T where
    T: 'static + ?Sized
[src]

🔬 This is a nightly-only experimental API. (get_type_id #27745)this method will likely be replaced by an associated static

Gets the TypeId of self. Read more

impl<T> ToOwned for T where
    T: Clone
[src]

Creates owned data from borrowed data, usually by cloning. Read more

🔬 This is a nightly-only experimental API. (toowned_clone_into #41263)recently added

Uses borrowed data to replace owned data, usually by cloning. Read more

© 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/enum.Result.html