Enum Result

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

Available on crate feature std only.

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(self, f: impl FnOnce(T) -> bool) -> bool

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);

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(self, f: impl FnOnce(E) -> bool) -> bool

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);

pub 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

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>

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,

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 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) -> Result<T, E>

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) -> Result<T, E>

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 as Deref>::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 as Deref>::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 iter(&self) -> Iter<'_, T>

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);

pub 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 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);

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

where E: Debug,

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” in the std::error module docs.

pub fn unwrap(self) -> T

where E: Debug,

Returns the contained Ok value, consuming the self value.

Because this function may panic, its use is generally discouraged. Panics are meant for unrecoverable errors, and may abort the entire program.

Instead, prefer to use the ? (try) operator, or pattern matching to 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 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`

pub fn unwrap_or_default(self) -> T

where T: Default,

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

where T: Debug,

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`

pub fn unwrap_err(self) -> E

where T: Debug,

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");

pub fn into_ok(self) -> T

where E: Into<!>,

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

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}");

pub fn into_err(self) -> E

where T: Into<!>,

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

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}");

pub 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.

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>

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

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);

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

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

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

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());

Trait Implementations

impl<T, U> Archive for Result<T, U>

where T: Archive, U: Archive,

type Archived = ArchivedResult<<T as Archive>::Archived, <U as Archive>::Archived>

The archived representation of this type.

type Resolver = Result<<T as Archive>::Resolver, <U as Archive>::Resolver>

The resolver for this type. It must contain all the additional information from serializing needed to make the archived type from the normal type.

fn resolve( &self, resolver: <Result<T, U> as Archive>::Resolver, out: Place<<Result<T, U> as Archive>::Archived>, )

Creates the archived version of this value at the given position and writes it to the given output.

const COPY_OPTIMIZATION: CopyOptimization<Self> = _

An optimization flag that allows the bytes of this type to be copied directly to a writer instead of calling serialize.

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

where T: Clone, E: Clone,

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

Returns a copy of the value.

fn clone_from(&mut self, source: &Result<T, E>)

Performs copy-assignment from source.

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

where T: Debug, E: Debug,

fn fmt(&self, f: &mut Formatter<'_>) -> Result<(), Error>

Formats the value using the given formatter.

impl<'de, T, E> Deserialize<'de> for Result<T, E>

where T: Deserialize<'de>, E: Deserialize<'de>,

fn deserialize<D>( deserializer: D, ) -> Result<Result<T, E>, <D as Deserializer<'de>>::Error>

where D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer.

impl<T, U, D> Deserialize<Result<T, U>, D> for ArchivedResult<<T as Archive>::Archived, <U as Archive>::Archived>

where T: Archive, U: Archive, D: Fallible + ?Sized, <T as Archive>::Archived: Deserialize<T, D>, <U as Archive>::Archived: Deserialize<U, D>,

fn deserialize( &self, deserializer: &mut D, ) -> Result<Result<T, U>, <D as Fallible>::Error>

Deserializes using the given deserializer

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

fn contains<U: PartialEq<T>>(&self, x: &U) -> bool

Returns true if the result is an Ok value containing the given value.

fn contains_err<F: PartialEq<E>>(&self, f: &F) -> bool

Returns true if the result is an Err value containing the given value.

impl From<&StreamResult> for Result<MZStatus, MZError>

fn from(res: &StreamResult) -> Result<MZStatus, MZError>

Converts to this type from the input type.

impl<L, R> From<Result<R, L>> for Either<L, R>

Convert from Result to Either with Ok => Right and Err => Left.

fn from(r: Result<R, L>) -> Either<L, R>

Converts to this type from the input type.

impl From<StreamResult> for Result<MZStatus, MZError>

fn from(res: StreamResult) -> Result<MZStatus, MZError>

Converts to this type from the input type.

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:

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.

impl<T, E, V> FromIteratorIn<Result<T, E>> for Result<V, E>

where V: FromIteratorIn<T>,

fn from_iter_in<I>( iter: I, alloc: <Result<V, E> as FromIteratorIn<Result<T, E>>>::Alloc, ) -> Result<V, E>

where I: IntoIterator<Item = Result<T, 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 bump = Bump::new();

let v = vec![1, 2, u32::MAX];
let res: Result<Vec<u32>, &'static str> = v.iter().take(2).map(|x: &u32|
    x.checked_add(1).ok_or("Overflow!")
).collect_in(&bump);
assert_eq!(res, Ok(bumpalo::vec![in &bump; 2, 3]));

let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
    x.checked_add(1).ok_or("Overflow!")
).collect_in(&bump);
assert_eq!(res, Err("Overflow!"));

type Alloc = <V as FromIteratorIn<T>>::Alloc

The allocator type

impl<C, T, E> FromParallelIterator<Result<T, E>> for Result<C, E>

where C: FromParallelIterator<T>, T: Send, E: Send,

Collect an arbitrary Result-wrapped collection.

If any item is Err, then all previous Ok items collected are discarded, and it returns that error. If there are multiple errors, the one returned is not deterministic.

fn from_par_iter<I>(par_iter: I) -> Result<C, E>

where I: IntoParallelIterator<Item = Result<T, E>>,

Creates an instance of the collection from the parallel iterator par_iter.

impl<T, E, F> FromResidual<Result<Infallible, E>> for Poll<Option<Result<T, F>>>

where F: From<E>,

fn from_residual(x: Result<Infallible, E>) -> Poll<Option<Result<T, F>>>

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

Constructs the type from a compatible Residual type.

impl<T, E, F> FromResidual<Result<Infallible, E>> for Poll<Result<T, F>>

where F: From<E>,

fn from_residual(x: Result<Infallible, E>) -> Poll<Result<T, F>>

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

Constructs the type from a compatible Residual type.

impl<T, E, F> FromResidual<Result<Infallible, E>> for Result<T, F>

where F: From<E>,

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

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

Constructs the type from a compatible Residual type.

impl<T, E, F> FromResidual<Yeet<E>> for Result<T, F>

where F: From<E>,

fn from_residual(_: Yeet<E>) -> Result<T, F>

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

Constructs the type from a compatible Residual type.

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

where T: Hash, E: Hash,

fn hash<__H>(&self, state: &mut __H)

where __H: Hasher,

Feeds this value into the given Hasher.

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

where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher.

impl<L, R> Into<Result<R, L>> for Either<L, R>

Convert from Either to Result with Right => Ok and Left => Err.

fn into(self) -> Result<R, L>

Converts this type into the (usually inferred) input type.

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

fn into_iter(self) -> IntoIter<T>

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, []);

type Item = T

The type of the elements being iterated over.

type IntoIter = IntoIter<T>

Which kind of iterator are we turning this into?

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

where T: Sync,

type Item = &'a T

The type of item that the parallel iterator will produce.

type Iter = Iter<'a, T>

The parallel iterator type that will be created.

fn into_par_iter(self) -> <&'a Result<T, E> as IntoParallelIterator>::Iter

Converts self into a parallel iterator.

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

where T: Send,

type Item = &'a mut T

The type of item that the parallel iterator will produce.

type Iter = IterMut<'a, T>

The parallel iterator type that will be created.

fn into_par_iter(self) -> <&'a mut Result<T, E> as IntoParallelIterator>::Iter

Converts self into a parallel iterator.

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

where T: Send,

type Item = T

The type of item that the parallel iterator will produce.

type Iter = IntoIter<T>

The parallel iterator type that will be created.

fn into_par_iter(self) -> <Result<T, E> as IntoParallelIterator>::Iter

Converts self into a parallel iterator.

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

where T: Ord, E: Ord,

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

This method returns an Ordering between self and other.

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

where Self: Sized,

Compares and returns the maximum of two values.

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

where Self: Sized,

Compares and returns the minimum of two values.

fn clamp(self, min: Self, max: Self) -> Self

where Self: Sized,

Restrict a value to a certain interval.

impl<T, U, E, F> PartialEq<Result<T, E>> for ArchivedResult<U, F>

where U: PartialEq<T>, F: PartialEq<E>,

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

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

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

where T: PartialEq, E: PartialEq,

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

Tests for self and other values to be equal, and is used by ==.

fn ne(&self, other: &Rhs) -> bool

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

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

where T: PartialOrd, E: PartialOrd,

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

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

fn lt(&self, other: &Rhs) -> bool

Tests less than (for self and other) and is used by the < operator.

fn le(&self, other: &Rhs) -> bool

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

fn gt(&self, other: &Rhs) -> bool

Tests greater than (for self and other) and is used by the > operator.

fn ge(&self, other: &Rhs) -> bool

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

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>>,

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());

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)

The “return” type of this meta-function.

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

fn into_error<U>(self) -> Result<T, U>

where U: Source, E: Error + Send + Sync + 'static,

Returns a Result with this error type converted to U.

fn into_trace<U, R>(self, trace: R) -> Result<T, U>

where U: Source, R: Debug + Display + Send + Sync + 'static, E: Error + Send + Sync + 'static,

Returns a Result with this error type converted to U and with an additional trace message added.

fn into_with_trace<U, R, F>(self, f: F) -> Result<T, U>

where U: Source, R: Debug + Display + Send + Sync + 'static, F: FnOnce() -> R, E: Error + Send + Sync + 'static,

Returns a Result with this error type converted to U and with an additional trace message added by evaluating the given function f. The function is evaluated only if an error occurred.

fn trace<R>(self, trace: R) -> Result<T, E>

where R: Debug + Display + Send + Sync + 'static, E: Trace,

Adds an additional trace message to the error value of this type.

fn with_trace<R, F>(self, f: F) -> Result<T, E>

where R: Debug + Display + Send + Sync + 'static, F: FnOnce() -> R, E: Trace,

Adds an additional trace message to the error value of this type by evaluating the given function f. The function is evaluated only if an error occurred.

fn always_ok(self) -> T

where E: Never,

Safely unwraps a result that is always Ok.

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

where T: IntoWasmAbi, E: Into<JsValue>, <T as IntoWasmAbi>::Abi: WasmAbi<Prim3 = (), Prim4 = ()>,

type Abi = Result<<T as IntoWasmAbi>::Abi, u32>

Same as IntoWasmAbi::Abi

fn return_abi(self) -> <Result<T, E> as ReturnWasmAbi>::Abi

Same as IntoWasmAbi::into_abi, except that it may throw and never return in the case of Err.

impl<T, U, S> Serialize<S> for Result<T, U>

where T: Serialize<S>, U: Serialize<S>, S: Fallible + ?Sized,

fn serialize( &self, serializer: &mut S, ) -> Result<<Result<T, U> as Archive>::Resolver, <S as Fallible>::Error>

Writes the dependencies for the object and returns a resolver that can create the archived type.

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

where T: Serialize, E: Serialize,

fn serialize<S>( &self, serializer: S, ) -> Result<<S as Serializer>::Ok, <S as Serializer>::Error>

where S: Serializer,

Serialize this value into the given Serde serializer.

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>>,

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"));

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

where T: Termination, E: Debug,

fn report(self) -> ExitCode

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

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

type Output = T

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

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)

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

fn from_output(output: <Result<T, E> as Try>::Output) -> Result<T, E>

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

Constructs the type from its Output type.

fn branch( self, ) -> ControlFlow<<Result<T, E> as Try>::Residual, <Result<T, E> as Try>::Output>

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

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).

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

where E: Debug,

fn unwrap_throw(self) -> T

Unwrap this Option or Result, but instead of panicking on failure, throw an exception to JavaScript.

fn expect_throw(self, message: &str) -> T

Unwrap this container’s T value, or throw an error to JS with the given message if the T value is unavailable (e.g. an Option<T> is None).

impl<T> WasmAbi for Result<T, u32>

where T: WasmAbi<Prim3 = (), Prim4 = ()>,

type Prim3 = u32

If this Result is an Err, the error value.

type Prim4 = u32

Whether this Result is an Err.

type Prim1 = <T as WasmAbi>::Prim1

type Prim2 = <T as WasmAbi>::Prim2

fn split(self) -> (<T as WasmAbi>::Prim1, <T as WasmAbi>::Prim2, u32, u32)

Splits this type up into primitives to be sent over the ABI.

fn join( prim1: <T as WasmAbi>::Prim1, prim2: <T as WasmAbi>::Prim2, err: u32, is_err: u32, ) -> Result<T, u32>

Reconstructs this type from primitives received over the ABI.

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

where T: Copy, E: Copy,

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

where T: Eq, E: Eq,

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