Enum Result 

pub enum Result<T, E> {
    Ok(T),
    Err(E),
}

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

See the module documentation for details.

Variants

Ok(T)

Contains the success value

Err(E)

Contains the error value

Implementations

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

pub const fn is_ok(&self) -> bool

Returns true if the result is Ok.

Examples

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);

pub fn is_ok_and<F>(self, f: F) -> bool

where F: FnOnce(T) -> bool, T:, E:,

Returns true if the result is Ok and the value inside of it matches a predicate.

Examples

let x: Result<u32, &str> = Ok(2);
assert_eq!(x.is_ok_and(|x| x > 1), true);

let x: Result<u32, &str> = Ok(0);
assert_eq!(x.is_ok_and(|x| x > 1), false);

let x: Result<u32, &str> = Err("hey");
assert_eq!(x.is_ok_and(|x| x > 1), false);

let x: Result<String, &str> = Ok("ownership".to_string());
assert_eq!(x.as_ref().is_ok_and(|x| x.len() > 1), true);
println!("still alive {:?}", x);

pub const fn is_err(&self) -> bool

Returns true if the result is Err.

Examples

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);

pub fn is_err_and<F>(self, f: F) -> bool

where F: FnOnce(E) -> bool, E:, T:,

Returns true if the result is Err and the value inside of it matches a predicate.

Examples

use std::io::{Error, ErrorKind};

let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), true);

let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);

let x: Result<u32, Error> = Ok(123);
assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);

let x: Result<u32, String> = Err("ownership".to_string());
assert_eq!(x.as_ref().is_err_and(|x| x.len() > 1), true);
println!("still alive {:?}", x);

pub fn ok(self) -> Option<T>

where T:, E:,

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

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

Examples

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);

pub fn err(self) -> Option<E>

where T:, 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

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"));

pub const 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

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"));

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

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

Examples

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);

pub 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(..) => {}
    }
}

pub fn map_or<U, F>(self, default: U, f: F) -> U

where F: FnOnce(T) -> U, T:, E:, U:,

Returns the provided default (if Err), or applies a function to the contained value (if Ok).

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

Examples

let x: Result<_, &str> = Ok("foo");
assert_eq!(x.map_or(42, |v| v.len()), 3);

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

pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U

where D: FnOnce(E) -> U, F: FnOnce(T) -> U,

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

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

Examples

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);

pub const fn map_or_default<U, F>(self, f: F) -> U

where F: FnOnce(T) -> U, U: Default, T:, E:,

🔬This is a nightly-only experimental API. (result_option_map_or_default #138099)

Maps a Result<T, E> to a U by applying function f to the contained value if the result is Ok, otherwise if Err, returns the default value for the type U.

Examples

#![feature(result_option_map_or_default)]

let x: Result<_, &str> = Ok("foo");
let y: Result<&str, _> = Err("bar");

assert_eq!(x.map_or_default(|x| x.len()), 3);
assert_eq!(y.map_or_default(|y| y.len()), 0);

pub 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

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()));

pub fn inspect<F>(self, f: F) -> Self

where F: FnOnce(&T),

Calls a function with a reference to the contained value if Ok.

Returns the original result.

Examples

let x: u8 = "4"
    .parse::<u8>()
    .inspect(|x| println!("original: {x}"))
    .map(|x| x.pow(3))
    .expect("failed to parse number");

pub fn inspect_err<F>(self, f: F) -> Self

where F: FnOnce(&E),

Calls a function with a reference to the contained value if Err.

Returns the original result.

Examples

use std::{fs, io};

fn read() -> io::Result<String> {
    fs::read_to_string("address.txt")
        .inspect_err(|e| eprintln!("failed to read file: {e}"))
}

pub fn as_deref(&self) -> Result<&T::Target, &E>

where T: Deref,

Converts from Result<T, E> (or &Result<T, E>) to Result<&<T as Deref>::Target, &E>.

Coerces the Ok variant of the original Result via Deref and returns the new Result.

Examples

let x: Result<String, u32> = Ok("hello".to_string());
let y: Result<&str, &u32> = Ok("hello");
assert_eq!(x.as_deref(), y);

let x: Result<String, u32> = Err(42);
let y: Result<&str, &u32> = Err(&42);
assert_eq!(x.as_deref(), y);

pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>

where T: DerefMut,

Converts from Result<T, E> (or &mut Result<T, E>) to Result<&mut <T as DerefMut>::Target, &mut E>.

Coerces the Ok variant of the original Result via DerefMut and returns the new Result.

Examples

let mut s = "HELLO".to_string();
let mut x: Result<String, u32> = Ok("hello".to_string());
let y: Result<&mut str, &mut u32> = Ok(&mut s);
assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);

let mut i = 42;
let mut x: Result<String, u32> = Err(42);
let y: Result<&mut str, &mut u32> = Err(&mut i);
assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);

pub fn unwrap_or_default(self) -> T

where T: Default, E:,

Returns the contained Ok 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

Converts 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);

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

where T:, E:, U:,

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

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

Examples

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"));

pub 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

fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
    x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
}

assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));

Often used to chain fallible operations that may return Err.

use std::{io::ErrorKind, path::Path};

// Note: on Windows "/" maps to "C:\"
let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
assert!(root_modified_time.is_ok());

let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
assert!(should_fail.is_err());
assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);

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

where T:, E:, F:,

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

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));

pub 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

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));

pub fn unwrap_or(self, default: T) -> T

where T:, E:,

Returns the contained Ok value or a provided default.

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

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

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

pub fn unwrap_or_else<F>(self, op: F) -> T

where F: FnOnce(E) -> T,

Returns the contained Ok value or computes it from a closure.

Examples

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);

pub unsafe fn unwrap_unchecked(self) -> T

Returns the contained Ok value, consuming the self value, without checking that the value is not an Err.

Safety

Calling this method on an Err is undefined behavior.

Examples

let x: Result<u32, &str> = Ok(2);
assert_eq!(unsafe { x.unwrap_unchecked() }, 2);

let x: Result<u32, &str> = Err("emergency failure");
unsafe { x.unwrap_unchecked() }; // Undefined behavior!

pub unsafe fn unwrap_err_unchecked(self) -> E

Returns the contained Err value, consuming the self value, without checking that the value is not an Ok.

Safety

Calling this method on an Ok is undefined behavior.

Examples

let x: Result<u32, &str> = Ok(2);
unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!

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

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

pub const fn copied(self) -> Result<T, E>

where T: Copy,

Maps a Result<&T, E> to a Result<T, E> by copying the contents of the Ok part.

Examples

let val = 12;
let x: Result<&i32, i32> = Ok(&val);
assert_eq!(x, Ok(&12));
let copied = x.copied();
assert_eq!(copied, Ok(12));

pub fn cloned(self) -> Result<T, E>

where T: Clone,

Maps a Result<&T, E> to a Result<T, E> by cloning the contents of the Ok part.

Examples

let val = 12;
let x: Result<&i32, i32> = Ok(&val);
assert_eq!(x, Ok(&12));
let cloned = x.cloned();
assert_eq!(cloned, Ok(12));

impl<T, E> Result<&mut T, E>

pub const fn copied(self) -> Result<T, E>

where T: Copy,

Maps a Result<&mut T, E> to a Result<T, E> by copying the contents of the Ok part.

Examples

let mut val = 12;
let x: Result<&mut i32, i32> = Ok(&mut val);
assert_eq!(x, Ok(&mut 12));
let copied = x.copied();
assert_eq!(copied, Ok(12));

pub fn cloned(self) -> Result<T, E>

where T: Clone,

Maps a Result<&mut T, E> to a Result<T, E> by cloning the contents of the Ok part.

Examples

let mut val = 12;
let x: Result<&mut i32, i32> = Ok(&mut val);
assert_eq!(x, Ok(&mut 12));
let cloned = x.cloned();
assert_eq!(cloned, Ok(12));

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

pub const fn transpose(self) -> Option<Result<T, E>>

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

#[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, F: From<E>> FromResidual<Result<Infallible, E>> for Result<T, F>

fn from_residual(residual: Result<Infallible, E>) -> Self

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from a compatible Residual type.

impl<T, E> Residual<T> for Result<Infallible, E>

type TryType = Result<T, E>

🔬This is a nightly-only experimental API. (try_trait_v2_residual #91285)

The “return” type of this meta-function.

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

type Output = T

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

The type of the value produced by ? when not short-circuiting.

type Residual = Result<Infallible, E>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

The type of the value passed to FromResidual::from_residual as part of ? when short-circuiting.

fn from_output(output: Self::Output) -> Self

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Constructs the type from its Output type.

fn branch(self) -> ControlFlow<Self::Residual, Self::Output>

🔬This is a nightly-only experimental API. (try_trait_v2 #84277)

Used in ? to decide whether the operator should produce a value (because this returned ControlFlow::Continue) or propagate a value back to the caller (because this returned ControlFlow::Break).