Enum core::result::Result
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[src]
#[must_use]
pub enum Result<T, E> {
Ok(T),
Err(E),
}1.0.0Result is a type that represents either success (Ok) or failure (Err).
See the std::result module documentation for details.
Variants
Ok | Contains the success value | |
Err | Contains the error value |
Methods
impl<T, E> Result<T, E>
fn is_ok(&self) -> bool
Returns true if the result is Ok
Examples
Basic usage:
fn main() { 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); }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);
fn is_err(&self) -> bool
Returns true if the result is Err
Examples
Basic usage:
fn main() { 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); }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);
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
Basic usage:
fn main() { 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); }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);
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
Basic usage:
fn main() { 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")); }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"));
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
Basic usage:
fn main() { 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")); }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"));
fn as_mut(&mut self) -> Result<&mut T, &mut E>
Converts from Result<T, E> to Result<&mut T, &mut E>
Examples
Basic usage:
fn main() { fn mutate(r: &mut Result<i32, i32>) { match r.as_mut() { Ok(&mut ref mut v) => *v = 42, Err(&mut ref mut 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); }fn mutate(r: &mut Result<i32, i32>) { match r.as_mut() { Ok(&mut ref mut v) => *v = 42, Err(&mut ref mut 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);
fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E>
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.
fn main() { 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(..) => {} } } }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(..) => {} } }
fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, 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
Basic usage:
fn main() { 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())); }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()));
fn iter(&self) -> Iter<T>
Returns an iterator over the possibly contained value.
Examples
Basic usage:
fn main() { 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); }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);
fn iter_mut(&mut self) -> IterMut<T>
Returns a mutable iterator over the possibly contained value.
Examples
Basic usage:
fn main() { 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); }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);
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.
Examples
Basic usage:
fn main() { 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")); }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"));
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
Basic usage:
fn main() { 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)); }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));
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.
Examples
Basic usage:
fn main() { 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)); }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));
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
Basic usage:
fn main() { 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)); }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));
fn unwrap_or(self, optb: T) -> T
Unwraps a result, yielding the content of an Ok.
Else it returns optb.
Examples
Basic usage:
fn main() { let optb = 2; let x: Result<u32, &str> = Ok(9); assert_eq!(x.unwrap_or(optb), 9); let x: Result<u32, &str> = Err("error"); assert_eq!(x.unwrap_or(optb), optb); }let optb = 2; let x: Result<u32, &str> = Ok(9); assert_eq!(x.unwrap_or(optb), 9); let x: Result<u32, &str> = Err("error"); assert_eq!(x.unwrap_or(optb), optb);
fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T
Unwraps a result, yielding the content of an Ok.
If the value is an Err then it calls op with its value.
Examples
Basic usage:
fn main() { 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); }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);
impl<T, E: Debug> Result<T, E>
fn unwrap(self) -> T
Unwraps a result, yielding the content of an Ok.
Panics
Panics if the value is an Err, with a panic message provided by the
Err's value.
Examples
Basic usage:
fn main() { let x: Result<u32, &str> = Ok(2); assert_eq!(x.unwrap(), 2); }let x: Result<u32, &str> = Ok(2); assert_eq!(x.unwrap(), 2);fn main() { let x: Result<u32, &str> = Err("emergency failure"); x.unwrap(); // panics with `emergency failure` }
let x: Result<u32, &str> = Err("emergency failure"); x.unwrap(); // panics with `emergency failure`
fn expect(self, msg: &str) -> T1.4.0
Unwraps a result, yielding the content of an Ok.
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:
fn main() { let x: Result<u32, &str> = Err("emergency failure"); x.expect("Testing expect"); // panics with `Testing expect: emergency failure` }let x: Result<u32, &str> = Err("emergency failure"); x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
impl<T: Debug, E> Result<T, E>
fn unwrap_err(self) -> E
Unwraps a result, yielding the content of an Err.
Panics
Panics if the value is an Ok, with a custom panic message provided
by the Ok's value.
Examples
fn main() { let x: Result<u32, &str> = Ok(2); x.unwrap_err(); // panics with `2` }let x: Result<u32, &str> = Ok(2); x.unwrap_err(); // panics with `2`fn main() { let x: Result<u32, &str> = Err("emergency failure"); assert_eq!(x.unwrap_err(), "emergency failure"); }
let x: Result<u32, &str> = Err("emergency failure"); assert_eq!(x.unwrap_err(), "emergency failure");
Trait Implementations
impl<T, E> IntoIterator for Result<T, E>
type Item = T
type IntoIter = IntoIter<T>
fn into_iter(self) -> IntoIter<T>
Returns a consuming iterator over the possibly contained value.
Examples
Basic usage:
fn main() { 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, []); }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, []);
impl<'a, T, E> IntoIterator for &'a Result<T, E>1.4.0
impl<'a, T, E> IntoIterator for &'a mut Result<T, E>1.4.0
impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for 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:
fn main() { use std::u32; let v = vec!(1, 2); let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32| if x == u32::MAX { Err("Overflow!") } else { Ok(x + 1) } ).collect(); assert!(res == Ok(vec!(2, 3))); }use std::u32; let v = vec!(1, 2); let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32| if x == u32::MAX { Err("Overflow!") } else { Ok(x + 1) } ).collect(); assert!(res == Ok(vec!(2, 3)));