pub enum Result<T, E> {
Ok(T),
Err(E),
}
Expand description
Result
is a type that represents either success (Ok
) or failure (Err
).
See the module documentation for details.
Variants§
Implementations§
source§impl<T, E> Result<T, E>
impl<T, E> Result<T, E>
1.70.0 · sourcepub fn is_ok_and(self, f: impl FnOnce(T) -> bool) -> bool
pub fn is_ok_and(self, f: impl FnOnce(T) -> bool) -> bool
1.70.0 · sourcepub fn is_err_and(self, f: impl FnOnce(E) -> bool) -> bool
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);
1.0.0 (const: 1.48.0) · sourcepub const fn as_ref(&self) -> Result<&T, &E>
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
1.0.0 (const: unstable) · sourcepub fn as_mut(&mut self) -> Result<&mut T, &mut E>
pub fn as_mut(&mut self) -> Result<&mut T, &mut E>
Converts from &mut Result<T, E>
to Result<&mut T, &mut E>
.
§Examples
1.0.0 · sourcepub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E>
pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E>
1.41.0 · sourcepub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U
pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> 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
1.41.0 · sourcepub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(
self,
default: D,
f: F,
) -> U
pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>( self, default: D, f: F, ) -> U
1.0.0 · sourcepub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F>
pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F>
1.76.0 · sourcepub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self
pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self
1.47.0 · sourcepub fn as_deref(&self) -> Result<&T::Target, &E>where
T: Deref,
pub fn as_deref(&self) -> Result<&T::Target, &E>where
T: Deref,
1.47.0 · sourcepub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>where
T: DerefMut,
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);
1.0.0 · sourcepub fn iter(&self) -> Iter<'_, T> ⓘ
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
1.0.0 · sourcepub fn iter_mut(&mut self) -> IterMut<'_, T> ⓘ
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
1.4.0 · sourcepub fn expect(self, msg: &str) -> Twhere
E: Debug,
pub fn expect(self, msg: &str) -> Twhere
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.
1.0.0 · sourcepub fn unwrap(self) -> Twhere
E: Debug,
pub fn unwrap(self) -> Twhere
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 provided by the
Err
’s value.
§Examples
Basic usage:
let x: Result<u32, &str> = Err("emergency failure");
x.unwrap(); // panics with `emergency failure`
1.16.0 · sourcepub fn unwrap_or_default(self) -> Twhere
T: Default,
pub fn unwrap_or_default(self) -> Twhere
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.
1.17.0 · sourcepub fn expect_err(self, msg: &str) -> Ewhere
T: Debug,
pub fn expect_err(self, msg: &str) -> Ewhere
T: Debug,
1.0.0 · sourcepub fn unwrap_err(self) -> Ewhere
T: Debug,
pub fn unwrap_err(self) -> Ewhere
T: Debug,
sourcepub fn into_ok(self) -> T
🔬This is a nightly-only experimental API. (unwrap_infallible
#61695)
pub fn into_ok(self) -> T
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
sourcepub fn into_err(self) -> E
🔬This is a nightly-only experimental API. (unwrap_infallible
#61695)
pub fn into_err(self) -> E
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
1.0.0 · sourcepub fn and<U>(self, res: Result<U, E>) -> Result<U, E>
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"));
1.0.0 · sourcepub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E>
pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> 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);
1.0.0 · sourcepub fn or<F>(self, res: Result<T, F>) -> Result<T, F>
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));
1.0.0 · sourcepub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F>
pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> 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));
1.0.0 · sourcepub fn unwrap_or(self, default: T) -> T
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
1.0.0 · sourcepub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T
pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T
1.58.0 · sourcepub unsafe fn unwrap_unchecked(self) -> T
pub unsafe fn unwrap_unchecked(self) -> T
1.58.0 · sourcepub unsafe fn unwrap_err_unchecked(self) -> E
pub unsafe fn unwrap_err_unchecked(self) -> E
source§impl<T, E> Result<&T, E>
impl<T, E> Result<&T, E>
source§impl<T, E> Result<&mut T, E>
impl<T, E> Result<&mut T, E>
1.59.0 · sourcepub fn copied(self) -> Result<T, E>where
T: Copy,
pub 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
source§impl<T, E> Result<Option<T>, E>
impl<T, E> Result<Option<T>, E>
source§impl<T, E> Result<Result<T, E>, E>
impl<T, E> Result<Result<T, E>, E>
sourcepub fn flatten(self) -> Result<T, E>
🔬This is a nightly-only experimental API. (result_flattening
#70142)
pub fn flatten(self) -> Result<T, E>
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:
Trait Implementations§
1.0.0 · source§impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E>
impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E>
source§fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E>
fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, 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.
source§impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Poll<Option<Result<T, F>>>
impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Poll<Option<Result<T, F>>>
source§fn from_residual(x: Result<Infallible, E>) -> Self
fn from_residual(x: Result<Infallible, E>) -> Self
try_trait_v2
#84277)Residual
type. Read moresource§impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Poll<Result<T, F>>
impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Poll<Result<T, F>>
source§fn from_residual(x: Result<Infallible, E>) -> Self
fn from_residual(x: Result<Infallible, E>) -> Self
try_trait_v2
#84277)Residual
type. Read moresource§impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Result<T, F>
impl<T, E, F: From<E>> FromResidual<Result<Infallible, E>> for Result<T, F>
source§fn from_residual(residual: Result<Infallible, E>) -> Self
fn from_residual(residual: Result<Infallible, E>) -> Self
try_trait_v2
#84277)Residual
type. Read more1.4.0 · source§impl<'a, T, E> IntoIterator for &'a Result<T, E>
impl<'a, T, E> IntoIterator for &'a Result<T, E>
1.4.0 · source§impl<'a, T, E> IntoIterator for &'a mut Result<T, E>
impl<'a, T, E> IntoIterator for &'a mut Result<T, E>
1.0.0 · source§impl<T, E> IntoIterator for Result<T, E>
impl<T, E> IntoIterator for Result<T, E>
1.0.0 · source§impl<T: Ord, E: Ord> Ord for Result<T, E>
impl<T: Ord, E: Ord> Ord for Result<T, E>
1.21.0 · source§fn max(self, other: Self) -> Selfwhere
Self: Sized,
fn max(self, other: Self) -> Selfwhere
Self: Sized,
1.0.0 · source§impl<T: PartialOrd, E: PartialOrd> PartialOrd for Result<T, E>
impl<T: PartialOrd, E: PartialOrd> PartialOrd for Result<T, E>
1.16.0 · source§impl<T, U, E> Product<Result<U, E>> for Result<T, E>where
T: Product<U>,
impl<T, U, E> Product<Result<U, E>> for Result<T, E>where
T: Product<U>,
source§fn product<I>(iter: I) -> Result<T, E>
fn product<I>(iter: I) -> Result<T, E>
source§impl<T, E> Residual<T> for Result<Infallible, E>
impl<T, E> Residual<T> for Result<Infallible, E>
1.16.0 · source§impl<T, U, E> Sum<Result<U, E>> for Result<T, E>where
T: Sum<U>,
impl<T, U, E> Sum<Result<U, E>> for Result<T, E>where
T: Sum<U>,
source§fn sum<I>(iter: I) -> Result<T, E>
fn sum<I>(iter: I) -> Result<T, E>
source§impl<T, E> Try for Result<T, E>
impl<T, E> Try for Result<T, E>
source§type Output = T
type Output = T
try_trait_v2
#84277)?
when not short-circuiting.source§type Residual = Result<Infallible, E>
type Residual = Result<Infallible, E>
try_trait_v2
#84277)FromResidual::from_residual
as part of ?
when short-circuiting. Read moresource§fn from_output(output: Self::Output) -> Self
fn from_output(output: Self::Output) -> Self
try_trait_v2
#84277)Output
type. Read moresource§fn branch(self) -> ControlFlow<Self::Residual, Self::Output>
fn branch(self) -> ControlFlow<Self::Residual, Self::Output>
try_trait_v2
#84277)?
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