std::result::Result - Rust
Description: API documentation for the Rust `Result` enum in crate `std`.
Keywords: rust, rustlang, rust-lang, Result

# Enum std::result::Result 1.0.0 [−] [src]

    #[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>`
[src]

#### `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);Run

#### `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);Run

#### `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);Run

#### `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"));Run

#### `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"));Run

#### `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);Run

#### `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(..) => {}
        }
    }Run

#### `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()));Run

#### `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);Run

#### `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);Run

#### `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"));Run

#### `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));Run

#### `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));Run

#### `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));Run

#### `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);Run

#### `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);Run

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

#### `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);Run

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

#### `fn expect(self, msg: &str) -> T`
1.4.0

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`Run

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

#### `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`Run

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

#### `fn expect_err(self, msg: &str) -> E`
1.17.0

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`Run

### `impl<T, E> Result<T, E> where
T: Default,`
[src]

#### `fn unwrap_or_default(self) -> T`
1.16.0

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);Run

## Trait Implementations

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

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

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

#### `fn hash_slice<H>(data: &[Self], state: &mut H) where
H: Hasher,`
1.3.0

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

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

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

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

#### `type Ok = T`

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

The type of this value when viewed as successful.

#### `type Error = E`

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

The type of this value when viewed as failed.

#### `fn into_result(self) -> Result<T, E>`

🔬 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

#### `fn from_ok(v: T) -> Result<T, E>`

🔬 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

#### `fn from_error(v: E) -> Result<T, E>`

🔬 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<'a, T, E> IntoIterator for &'a mut Result<T, E>`
1.4.0
[src]

#### `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. Read more

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

#### `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, []);Run

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

#### `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. Read more

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

#### `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:

    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]));Run

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

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

Returns a copy of the value. Read more

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

Performs copy-assignment from `source`. Read more

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

#### `fn product<I>(iter: I) -> Result<T, E> where
I: Iterator<Item = Result<U, 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, the product of all elements is returned.

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

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

Formats the value using the given formatter.

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

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

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

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

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

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

This method tests for `!=`.

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

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

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

#### `fn max(self, other: Self) -> Self`

🔬 This is a nightly-only experimental API. (`ord_max_min` #25663)

Compares and returns the maximum of two values. Read more

#### `fn min(self, other: Self) -> Self`

🔬 This is a nightly-only experimental API. (`ord_max_min` #25663)

Compares and returns the minimum of two values. Read more

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

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

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

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

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

#### `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. Read more

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

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

#### `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. Read more

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