Result in std::result - Rust

const: 1.48.0 · source

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);

1.70.0 · source

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);

const: 1.48.0 · source

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);

1.70.0 · source

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);

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);

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"));

const: 1.48.0 · source

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"));

const: unstable · source

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);

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(..) => {}
    }
}

1.41.0 · source

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);

1.41.0 · source

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);

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()));

1.76.0 · source

Calls the provided closure with a reference to the contained value (if Ok).

§ Examples

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

1.76.0 · source

Calls the provided closure with a reference to the contained error (if Err).

§ 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}"))
}

1.47.0 · source

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);

1.47.0 · source

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);

Returns an iterator over the possibly contained value.

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

§ Examples

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);

Returns a mutable iterator over the possibly contained value.

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

§ Examples

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);

1.4.0 · source

Returns the contained Ok value, consuming the self value.

Because this function may panic, its use is generally discouraged. Instead, prefer to use pattern matching and handle the Err case explicitly, or call unwrap_or, unwrap_or_else, or unwrap_or_default.

§ Panics

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

§ Examples

ⓘ

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

§ Recommended Message Style

We recommend that expect messages are used to describe the reason you expect the Result should be Ok.

ⓘ

let path = std::env::var("IMPORTANT_PATH")
    .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");

Hint: If you’re having trouble remembering how to phrase expect error messages remember to focus on the word “should” as in “env variable should be set by blah” or “the given binary should be available and executable by the current user”.

For more detail on expect message styles and the reasoning behind our recommendation please refer to the section on [“Common Message
Styles”](https://doc.rust-lang.org/std/error/index.html#common-message-styles) in the std::error module docs.

Returns the contained Ok value, consuming the self value.

§ 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`

1.16.0 · source

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);

1.17.0 · source

Returns the contained Err value, consuming the self value.

§ Panics

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

§ Examples

ⓘ

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

Returns the contained Err value, consuming the self value.

§ 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");

source 🔬This is a nightly-only experimental API. ( unwrap_infallible #61695)

Returns the contained Ok value, but never panics.

Unlike unwrap, this method is known to never panic on the result types it is implemented for. Therefore, it can be used instead of unwrap as a maintainability safeguard that will fail to compile if the error type of the Result is later changed to an error that can actually occur.

§ Examples


fn only_good_news() -> Result<String, !> {
    Ok("this is fine".into())
}

let s: String = only_good_news().into_ok();
println!("{s}");

source 🔬This is a nightly-only experimental API. ( unwrap_infallible #61695)

Returns the contained Err value, but never panics.

Unlike unwrap_err, this method is known to never panic on the result types it is implemented for. Therefore, it can be used instead of unwrap_err as a maintainability safeguard that will fail to compile if the ok type of the Result is later changed to a type that can actually occur.

§ Examples


fn only_bad_news() -> Result<!, String> {
    Err("Oops, it failed".into())
}

let error: String = only_bad_news().into_err();
println!("{error}");

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"));

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);

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));

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));

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);

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);

1.58.0 · source

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!

1.58.0 · source

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");

1.59.0 · source

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));

1.59.0 · source

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));

1.59.0 · source

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));

1.59.0 · source

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));

1.33.0 (const: unstable) · source

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);

source 🔬This is a nightly-only experimental API. ( result_flattening #70142)

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

§ Examples

#![feature(result_flattening)]
let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
assert_eq!(Ok("hello"), x.flatten());

let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
assert_eq!(Err(6), x.flatten());

let x: Result<Result<&'static str, u32>, u32> = Err(6);
assert_eq!(Err(6), x.flatten());

Flattening only removes one level of nesting at a time:

#![feature(result_flattening)]
let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
assert_eq!(Ok(Ok("hello")), x.flatten());
assert_eq!(Ok("hello"), x.flatten().flatten());

source §source §source §

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_eq!(res, Ok(vec![2, 3]));

Here is another example that tries to subtract one from another list of integers, this time checking for underflow:

let v = vec![1, 2, 0];
let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
    x.checked_sub(1).ok_or("Underflow!")
).collect();
assert_eq!(res, Err("Underflow!"));

Here is a variation on the previous example, showing that no further elements are taken from iter after the first Err.

let v = vec![3, 2, 1, 10];
let mut shared = 0;
let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
    shared += x;
    x.checked_sub(2).ok_or("Underflow!")
}).collect();
assert_eq!(res, Err("Underflow!"));
assert_eq!(shared, 6);

Since the third element caused an underflow, no further elements were taken, so the final value of shared is 6 (= 3 + 2 + 1), not 16.

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

Constructs the type from a compatible Residual type. Read more

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

Constructs the type from a compatible Residual type. Read more

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

Constructs the type from a compatible Residual type. Read more

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

Constructs the type from a compatible Residual type. Read more

source §1.4.0 · source §

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

1.4.0 · source §

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

Returns a consuming iterator over the possibly contained value.

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

§ Examples

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, []);

The type of the elements being iterated over.

Which kind of iterator are we turning this into?

source §source §

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

This method tests for !=. The default implementation is almost always sufficient, and should not be overridden without very good reason.

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

1.16.0 · source §

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.

§ Examples

This multiplies each number in a vector of strings, if a string could not be parsed the operation returns Err:

let nums = vec!["5", "10", "1", "2"];
let total: Result<usize, _> = nums.iter().map(|w| w.parse::<usize>()).product();
assert_eq!(total, Ok(100));
let nums = vec!["5", "10", "one", "2"];
let total: Result<usize, _> = nums.iter().map(|w| w.parse::<usize>()).product();
assert!(total.is_err());

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

The “return” type of this meta-function.

1.16.0 · source §

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

1.61.0 · source §

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

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

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

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

Constructs the type from its Output type. Read more

source § 🔬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). Read more

source §source §source §

source §source §source §source §

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The resulting type after obtaining ownership.

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