Enum std::result::Result 1.0.0[−][src]
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
Contains the success value
Contains the error value
Implementations
Returns true
if the result is an Ok
value containing the given value.
Examples
#![feature(option_result_contains)]
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.contains(&2), true);
let x: Result<u32, &str> = Ok(3);
assert_eq!(x.contains(&2), false);
let x: Result<u32, &str> = Err("Some error message");
assert_eq!(x.contains(&2), false);
RunReturns true
if the result is an Err
value containing the given value.
Examples
#![feature(result_contains_err)]
let x: Result<u32, &str> = Ok(2);
assert_eq!(x.contains_err(&"Some error message"), false);
let x: Result<u32, &str> = Err("Some error message");
assert_eq!(x.contains_err(&"Some error message"), true);
let x: Result<u32, &str> = Err("Some other error message");
assert_eq!(x.contains_err(&"Some error message"), false);
RunConverts from Result<T, E>
to Option<E>
.
Converts self
into an Option<E>
, consuming self
,
and discarding the success value, if any.
Examples
Basic usage:
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"));
RunConverts from &Result<T, E>
to Result<&T, &E>
.
Produces a new Result
, containing a reference
into the original, leaving the original in place.
Examples
Basic usage:
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"));
RunConverts from &mut Result<T, E>
to Result<&mut T, &mut E>
.
Examples
Basic usage:
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);
RunMaps 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(..) => {}
}
}
RunReturns 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);
Run1.41.0[src]pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U where
F: FnOnce(T) -> U,
D: FnOnce(E) -> U,
pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U where
F: FnOnce(T) -> U,
D: FnOnce(E) -> U,
Maps a Result<T, E>
to U
by applying a fallback function to a
contained Err
value, or a default function to a
contained Ok
value.
This function can be used to unpack a successful result while handling an error.
Examples
Basic usage:
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);
RunMaps 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
Basic usage:
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()));
RunReturns an iterator over the possibly contained value.
The iterator yields one value if the result is Result::Ok
, otherwise none.
Examples
Basic usage:
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);
RunReturns a mutable iterator over the possibly contained value.
The iterator yields one value if the result is Result::Ok
, otherwise none.
Examples
Basic usage:
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);
RunReturns res
if the result is Ok
, otherwise returns the Err
value of self
.
Examples
Basic usage:
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"));
RunCalls 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
Basic usage:
fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
fn err(x: u32) -> Result<u32, u32> { Err(x) }
assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
RunReturns 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
Basic usage:
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));
RunCalls 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
Basic usage:
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));
RunReturns 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
Basic usage:
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);
Run🔬 This is a nightly-only experimental API. (option_result_unwrap_unchecked
#81383)
newly added
🔬 This is a nightly-only experimental API. (option_result_unwrap_unchecked
#81383)
newly added
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
#![feature(option_result_unwrap_unchecked)]
let x: Result<u32, &str> = Ok(2);
assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
Run#![feature(option_result_unwrap_unchecked)]
let x: Result<u32, &str> = Err("emergency failure");
unsafe { x.unwrap_unchecked(); } // Undefined behavior!
Run🔬 This is a nightly-only experimental API. (option_result_unwrap_unchecked
#81383)
newly added
🔬 This is a nightly-only experimental API. (option_result_unwrap_unchecked
#81383)
newly added
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
#![feature(option_result_unwrap_unchecked)]
let x: Result<u32, &str> = Ok(2);
unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
Run#![feature(option_result_unwrap_unchecked)]
let x: Result<u32, &str> = Err("emergency failure");
assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
Run🔬 This is a nightly-only experimental API. (result_copied
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_copied
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_copied
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_copied
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_cloned
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_cloned
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_cloned
#63168)
newly added
🔬 This is a nightly-only experimental API. (result_cloned
#63168)
newly added
Returns the contained Ok
value, consuming the self
value.
Panics
Panics if the value is an Err
, with a panic message including the
passed message, and the content of the Err
.
Examples
Basic usage:
let x: Result<u32, &str> = Err("emergency failure");
x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
RunReturns 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> = Ok(2);
assert_eq!(x.unwrap(), 2);
Runlet x: Result<u32, &str> = Err("emergency failure");
x.unwrap(); // panics with `emergency failure`
RunReturns 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
Basic usage:
let x: Result<u32, &str> = Ok(10);
x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
RunReturns 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`
Runlet x: Result<u32, &str> = Err("emergency failure");
assert_eq!(x.unwrap_err(), "emergency failure");
RunReturns 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);
Run🔬 This is a nightly-only experimental API. (unwrap_infallible
#61695)
newly added
🔬 This is a nightly-only experimental API. (unwrap_infallible
#61695)
newly added
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
Basic usage:
fn only_good_news() -> Result<String, !> {
Ok("this is fine".into())
}
let s: String = only_good_news().into_ok();
println!("{}", s);
Run🔬 This is a nightly-only experimental API. (unwrap_infallible
#61695)
newly added
🔬 This is a nightly-only experimental API. (unwrap_infallible
#61695)
newly added
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
Basic usage:
fn only_bad_news() -> Result<!, String> {
Err("Oops, it failed".into())
}
let error: String = only_bad_news().into_err();
println!("{}", error);
RunConverts 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);
RunConverts 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);
RunTransposes 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);
RunConverts from Result<Result<T, E>, E>
to Result<T, E>
Examples
Basic usage:
#![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());
RunFlattening 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());
Run🔬 This is a nightly-only experimental API. (result_into_ok_or_err
#82223)
newly added
🔬 This is a nightly-only experimental API. (result_into_ok_or_err
#82223)
newly added
Returns the Ok
value if self
is Ok
, and the Err
value if
self
is Err
.
In other words, this function returns the value (the T
) of a
Result<T, T>
, regardless of whether or not that result is Ok
or
Err
.
This can be useful in conjunction with APIs such as
Atomic*::compare_exchange
, or slice::binary_search
, but only in
cases where you don’t care if the result was Ok
or not.
Examples
#![feature(result_into_ok_or_err)]
let ok: Result<u32, u32> = Ok(3);
let err: Result<u32, u32> = Err(4);
assert_eq!(ok.into_ok_or_err(), 3);
assert_eq!(err.into_ok_or_err(), 4);
RunTrait Implementations
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]));
RunHere 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!"));
RunHere 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);
RunSince 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, F> FromResidual<Result<Infallible, E>> for Poll<Option<Result<T, F>>> where
F: From<E>,
impl<T, E, F> FromResidual<Result<Infallible, E>> for Poll<Option<Result<T, F>>> where
F: From<E>,
Returns a consuming iterator over the possibly contained value.
The iterator yields one value if the result is Result::Ok
, otherwise none.
Examples
Basic usage:
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, []);
Runtype Item = T
type Item = T
The type of the elements being iterated over.
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
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 v = vec![1, 2];
let res: Result<i32, &'static str> = v.iter().map(|&x: &i32|
if x < 0 { Err("Negative element found") }
else { Ok(x) }
).sum();
assert_eq!(res, Ok(3));
Runtype Residual = Result<Infallible, E>
type Residual = Result<Infallible, E>
The type of the value passed to FromResidual::from_residual
as part of ?
when short-circuiting. Read more
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
Auto Trait Implementations
impl<T, E> RefUnwindSafe for Result<T, E> where
E: RefUnwindSafe,
T: RefUnwindSafe,
impl<T, E> UnwindSafe for Result<T, E> where
E: UnwindSafe,
T: UnwindSafe,
Blanket Implementations
Mutably borrows from an owned value. Read more