Enum enum_primitive::OptionStable

pub enum Option<T> {
    None,
    Some(T),
}

The Option type. See the module level documentation for more.

Variants

None	No value
Some	Some value T

Methods

impl<T> Option<T>

fn is_some(&self) -> bool

Returns true if the option is a Some value

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_some(), true);

let x: Option<u32> = None;
assert_eq!(x.is_some(), false);

fn is_none(&self) -> bool

Returns true if the option is a None value

Examples

let x: Option<u32> = Some(2);
assert_eq!(x.is_none(), false);

let x: Option<u32> = None;
assert_eq!(x.is_none(), true);

fn as_ref(&'r self) -> Option<&'r T>

Convert from Option<T> to Option<&T>

Examples

Convert an Option<String> into an Option<usize>, preserving the original. The map method takes the self argument by value, consuming the original, so this technique uses as_ref to first take an Option to a reference to the value inside the original.

let num_as_str: Option<String> = Some("10".to_string());
// First, cast `Option<String>` to `Option<&String>` with `as_ref`,
// then consume *that* with `map`, leaving `num_as_str` on the stack.
let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len());
println!("still can print num_as_str: {:?}", num_as_str);

fn as_mut(&'r mut self) -> Option<&'r mut T>

Convert from Option<T> to Option<&mut T>

Examples

let mut x = Some(2);
match x.as_mut() {
    Some(v) => *v = 42,
    None => {},
}
assert_eq!(x, Some(42));

fn as_mut_slice(&'r mut self) -> &'r mut [T]

Convert from Option<T> to &mut [T] (without copying)

Examples

let mut x = Some("Diamonds");
{
    let v = x.as_mut_slice();
    assert!(v == ["Diamonds"]);
    v[0] = "Dirt";
    assert!(v == ["Dirt"]);
}
assert_eq!(x, Some("Dirt"));

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

Unwraps an option, yielding the content of a Some

Panics

Panics if the value is a None with a custom panic message provided by msg.

Examples

let x = Some("value");
assert_eq!(x.expect("the world is ending"), "value");

let x: Option<&str> = None;
x.expect("the world is ending"); // panics with `world is ending`

fn unwrap(self) -> T

Moves the value v out of the Option<T> if it is Some(v).

Panics

Panics if the self value equals None.

Safety note

In general, because this function may panic, its use is discouraged. Instead, prefer to use pattern matching and handle the None case explicitly.

Examples

let x = Some("air");
assert_eq!(x.unwrap(), "air");

let x: Option<&str> = None;
assert_eq!(x.unwrap(), "air"); // fails

fn unwrap_or(self, def: T) -> T

Returns the contained value or a default.

Examples

assert_eq!(Some("car").unwrap_or("bike"), "car");
assert_eq!(None.unwrap_or("bike"), "bike");

fn unwrap_or_else<F>(self, f: F) -> T where F: FnOnce(), <F as FnOnce()>::Output == T

Returns the contained value or computes it from a closure.

Examples

let k = 10;
assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
assert_eq!(None.unwrap_or_else(|| 2 * k), 20);

fn map<U, F>(self, f: F) -> Option<U> where F: FnOnce(T), <F as FnOnce(T)>::Output == U

Maps an Option<T> to Option<U> by applying a function to a contained value

Examples

Convert an Option<String> into an Option<usize>, consuming the original:

let num_as_str: Option<String> = Some("10".to_string());
// `Option::map` takes self *by value*, consuming `num_as_str`
let num_as_int: Option<usize> = num_as_str.map(|n| n.len());

fn map_or<U, F>(self, def: U, f: F) -> U where F: FnOnce(T), <F as FnOnce(T)>::Output == U

Applies a function to the contained value or returns a default.

Examples

let x = Some("foo");
assert_eq!(x.map_or(42, |v| v.len()), 3);

let x: Option<&str> = None;
assert_eq!(x.map_or(42, |v| v.len()), 42);

fn map_or_else<U, D, F>(self, def: D, f: F) -> U where D: FnOnce(), <D as FnOnce()>::Output == U, F: FnOnce(T), <F as FnOnce(T)>::Output == U

Applies a function to the contained value or computes a default.

Examples

let k = 21;

let x = Some("foo");
assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);

let x: Option<&str> = None;
assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);

fn ok_or<E>(self, err: E) -> Result<T, E>

Transforms the Option<T> into a Result<T, E>, mapping Some(v) to Ok(v) and None to Err(err).

Examples

let x = Some("foo");
assert_eq!(x.ok_or(0), Ok("foo"));

let x: Option<&str> = None;
assert_eq!(x.ok_or(0), Err(0));

fn ok_or_else<E, F>(self, err: F) -> Result<T, E> where F: FnOnce(), <F as FnOnce()>::Output == E

Transforms the Option<T> into a Result<T, E>, mapping Some(v) to Ok(v) and None to Err(err()).

Examples

let x = Some("foo");
assert_eq!(x.ok_or_else(|| 0), Ok("foo"));

let x: Option<&str> = None;
assert_eq!(x.ok_or_else(|| 0), Err(0));

fn iter(&self) -> Iter<T>

Returns an iterator over the possibly contained value.

Examples

let x = Some(4);
assert_eq!(x.iter().next(), Some(&4));

let x: Option<u32> = None;
assert_eq!(x.iter().next(), None);

fn iter_mut(&mut self) -> IterMut<T>

Returns a mutable iterator over the possibly contained value.

Examples

let mut x = Some(4);
match x.iter_mut().next() {
    Some(&mut ref mut v) => *v = 42,
    None => {},
}
assert_eq!(x, Some(42));

let mut x: Option<u32> = None;
assert_eq!(x.iter_mut().next(), None);

fn into_iter(self) -> IntoIter<T>

Returns a consuming iterator over the possibly contained value.

Examples

let x = Some("string");
let v: Vec<&str> = x.into_iter().collect();
assert_eq!(v, ["string"]);

let x = None;
let v: Vec<&str> = x.into_iter().collect();
assert!(v.is_empty());

fn and<U>(self, optb: Option<U>) -> Option<U>

Returns None if the option is None, otherwise returns optb.

Examples

let x = Some(2);
let y: Option<&str> = None;
assert_eq!(x.and(y), None);

let x: Option<u32> = None;
let y = Some("foo");
assert_eq!(x.and(y), None);

let x = Some(2);
let y = Some("foo");
assert_eq!(x.and(y), Some("foo"));

let x: Option<u32> = None;
let y: Option<&str> = None;
assert_eq!(x.and(y), None);

fn and_then<U, F>(self, f: F) -> Option<U> where F: FnOnce(T), <F as FnOnce(T)>::Output == Option<U>

Returns None if the option is None, otherwise calls f with the wrapped value and returns the result.

Some languages call this operation flatmap.

Examples

fn sq(x: u32) -> Option<u32> { Some(x * x) }
fn nope(_: u32) -> Option<u32> { None }

assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
assert_eq!(Some(2).and_then(sq).and_then(nope), None);
assert_eq!(Some(2).and_then(nope).and_then(sq), None);
assert_eq!(None.and_then(sq).and_then(sq), None);

fn or(self, optb: Option<T>) -> Option<T>

Returns the option if it contains a value, otherwise returns optb.

Examples

let x = Some(2);
let y = None;
assert_eq!(x.or(y), Some(2));

let x = None;
let y = Some(100);
assert_eq!(x.or(y), Some(100));

let x = Some(2);
let y = Some(100);
assert_eq!(x.or(y), Some(2));

let x: Option<u32> = None;
let y = None;
assert_eq!(x.or(y), None);

fn or_else<F>(self, f: F) -> Option<T> where F: FnOnce(), <F as FnOnce()>::Output == Option<T>

Returns the option if it contains a value, otherwise calls f and returns the result.

Examples

fn nobody() -> Option<&'static str> { None }
fn vikings() -> Option<&'static str> { Some("vikings") }

assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
assert_eq!(None.or_else(vikings), Some("vikings"));
assert_eq!(None.or_else(nobody), None);

fn take(&mut self) -> Option<T>

Takes the value out of the option, leaving a None in its place.

Examples

let mut x = Some(2);
x.take();
assert_eq!(x, None);

let mut x: Option<u32> = None;
x.take();
assert_eq!(x, None);

fn as_slice(&'a self) -> &'a [T]

Convert from Option<T> to &[T] (without copying)

impl<'a, T> Option<&'a T> where T: Clone

fn cloned(self) -> Option<T>

Maps an Option<&T> to an Option by cloning the contents of the Option.

impl<T> Option<T> where T: Default

fn unwrap_or_default(self) -> T

Returns the contained value or a default

Consumes the self argument then, if Some, returns the contained value, otherwise if None, returns the default value for that type.

Examples

Convert 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 None on error.

let good_year_from_input = "1909";
let bad_year_from_input = "190blarg";
let good_year = good_year_from_input.parse().ok().unwrap_or_default();
let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();

assert_eq!(1909, good_year);
assert_eq!(0, bad_year);

Trait Implementations

impl<T> Default for Option<T>

fn default() -> Option<T>

impl<A, V> FromIterator<Option<A>> for Option<V> where V: FromIterator<A>

fn from_iter<I>(iter: I) -> Option<V> where I: IntoIterator, <I as IntoIterator>::Item == Option<A>

Takes each element in the Iterator: if it is None, no further elements are taken, and the None is returned. Should no None occur, a container with the values of each Option is returned.

Here is an example which increments every integer in a vector, checking for overflow:

use std::u16;

let v = vec!(1, 2);
let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
    if x == u16::MAX { None }
    else { Some(x + 1) }
).collect();
assert!(res == Some(vec!(2, 3)));

Derived Implementations

impl<T> Hash for Option<T> where T: Hash, T: Hash

fn hash<__H>(&self, __arg_0: &mut __H) where __H: Hasher

impl<T> Debug for Option<T> where T: Debug, T: Debug

fn fmt(&self, __arg_0: &mut Formatter) -> Result<(), Error>

impl<T> Ord for Option<T> where T: Ord, T: Ord

fn cmp(&self, __arg_0: &Option<T>) -> Ordering

impl<T> Eq for Option<T> where T: Eq, T: Eq

impl<T> PartialOrd<Option<T>> for Option<T> where T: PartialOrd<T>, T: PartialOrd<T>

fn partial_cmp(&self, __arg_0: &Option<T>) -> Option<Ordering>

fn lt(&self, __arg_0: &Option<T>) -> bool

fn le(&self, __arg_0: &Option<T>) -> bool

fn gt(&self, __arg_0: &Option<T>) -> bool

fn ge(&self, __arg_0: &Option<T>) -> bool

impl<T> PartialEq<Option<T>> for Option<T> where T: PartialEq<T>, T: PartialEq<T>

fn eq(&self, __arg_0: &Option<T>) -> bool

fn ne(&self, __arg_0: &Option<T>) -> bool

impl<T> Copy for Option<T> where T: Copy, T: Copy

impl<T> Clone for Option<T> where T: Clone, T: Clone

fn clone(&self) -> Option<T>