Result in std::result - Rust
pub enum Result<T, E> {
Ok(T),
Err(E),
}
Expand description
§ 1.0.0
Contains the success value
§ 1.0.0
Contains the error value
1.0.0 (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);
1.0.0 (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);
1.0.0 · Source
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);
1.0.0 · Source
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"));
1.0.0 (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"));
1.0.0 (const: 1.83.0) · 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);
1.0.0 · Source
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);
1.0.0 · Source
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 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");
1.76.0 · Source
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}"))
}
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);
1.0.0 · Source
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);
1.0.0 · Source
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.
1.0.0 · Source
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`
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`
1.0.0 · Source
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}");
1.0.0 · Source
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"));
1.0.0 · Source
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);
1.0.0 · Source
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));
1.0.0 · Source
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));
1.0.0 · Source
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);
1.0.0 · Source
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 (const: 1.83.0) · 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 (const: 1.83.0) · 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: 1.83.0) · 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());
1.0.0 · Source §1.0.0 · Source §1.0.0 · 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, 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
1.0.0 · 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
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?
1.0.0 · Source §1.0.0 · Source §
Tests for self and other values to be equal, and is used by ==.
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
Tests less than (for self and other) and is used by the < operator. Read more
Tests less than or equal to (for self and other) and is used by the
<= operator. Read more
Tests greater than (for self and other) and is used by the >
operator. Read more
Tests greater than or equal to (for self and other) and is used by
the >= operator. Read more
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());
Source §
🔬This is a nightly-only experimental API. ( try_trait_v2_residual #91285)
The “return” type of this meta-function.
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"));
Is called to get the representation of the value as status code. This status code is returned to the operating system.
Source §
🔬This is a nightly-only experimental API. ( try_trait_v2 #84277)
The type of the value produced by ? when not short-circuiting.
Source §
🔬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