Enum Option

1.0.0 · Source

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

Expand description

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

Variants§

§1.0.0

None

No value.

§1.0.0

Some(T)

Some value of type T.

Implementations§

Source §

Calculates the length of an Option<String> as an Option<usize> without moving the String. 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 text: Option<String> = Some("Hello, world!".to_string());
// First, cast `Option<String>` to `Option<&String>` with `as_ref`,
// then consume *that* with `map`, leaving `text` on the stack.
let text_length: Option<usize> = text.as_ref().map(|s| s.len());
println!("still can print text: {text:?}");

1.0.0 (const: 1.83.0) · Source

pub const fn as_mut(&mut self) -> Option<&mut T>

Converts from &mut 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));

1.33.0 (const: 1.84.0) · Source

pub const fn as_pin_ref(self: Pin<&Option<T>>) -> Option<Pin<&T>>

Converts from Pin<&Option<T>> to Option<Pin<&T>>.

1.33.0 (const: 1.84.0) · Source

pub const fn as_pin_mut(self: Pin<&mut Option<T>>) -> Option<Pin<&mut T>>

Converts from Pin<&mut Option<T>> to Option<Pin<&mut T>>.

1.75.0 (const: 1.84.0) · Source

pub const fn as_slice(&self) -> &[T]

Returns a slice of the contained value, if any. If this is None, an empty slice is returned. This can be useful to have a single type of iterator over an Option or slice.

Note: Should you have an Option<&T> and wish to get a slice of T, you can unpack it via opt.map_or(&[], std::slice::from_ref).

§Examples

assert_eq!(
    [Some(1234).as_slice(), None.as_slice()],
    [&[1234][..], &[][..]],
);

The inverse of this function is (discounting borrowing) [_]::first:

for i in [Some(1234_u16), None] {
    assert_eq!(i.as_ref(), i.as_slice().first());
}

1.75.0 (const: 1.84.0) · Source

pub const fn as_mut_slice(&mut self) -> &mut [T]

Returns a mutable slice of the contained value, if any. If this is None, an empty slice is returned. This can be useful to have a single type of iterator over an Option or slice.

Note: Should you have an Option<&mut T> instead of a &mut Option<T>, which this method takes, you can obtain a mutable slice via opt.map_or(&mut [], std::slice::from_mut).

§Examples

assert_eq!(
    [Some(1234).as_mut_slice(), None.as_mut_slice()],
    [&mut [1234][..], &mut [][..]],
);

The result is a mutable slice of zero or one items that points into our original Option:

let mut x = Some(1234);
x.as_mut_slice()[0] += 1;
assert_eq!(x, Some(1235));

The inverse of this method (discounting borrowing) is [_]::first_mut:

assert_eq!(Some(123).as_mut_slice().first_mut(), Some(&mut 123))

1.0.0 (const: 1.83.0) · Source

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

Returns the contained Some value, consuming the self value.

§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("fruits are healthy"), "value");

let x: Option<&str> = None;
x.expect("fruits are healthy"); // panics with `fruits are healthy`

§Recommended Message Style

We recommend that expect messages are used to describe the reason you expect the Option should be Some.

let item = slice.get(0)
    .expect("slice should not be empty");

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 (const: 1.83.0) · Source

pub const fn unwrap(self) -> T

Returns the contained Some 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 pattern matching and handle the None case explicitly, or call unwrap_or, unwrap_or_else, or unwrap_or_default. In functions returning Option, you can use the ? (try) operator.

§Panics

Panics if the self value equals None.

§Examples

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

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

1.0.0 (const: unstable) · Source

pub fn unwrap_or(self, default: T) -> T

Returns the contained Some 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

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

1.0.0 (const: unstable) · Source

pub fn unwrap_or_else<F>(self, f: F) -> T
where F: FnOnce() -> T,

Returns the contained Some 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);

1.0.0 (const: unstable) · Source

pub fn unwrap_or_default(self) -> T
where T: Default,

Returns the contained Some 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

let x: Option<u32> = None;
let y: Option<u32> = Some(12);

assert_eq!(x.unwrap_or_default(), 0);
assert_eq!(y.unwrap_or_default(), 12);

1.58.0 (const: 1.83.0) · Source

pub const unsafe fn unwrap_unchecked(self) -> T

Returns the contained Some value, consuming the self value, without checking that the value is not None.

§Safety

Calling this method on None is undefined behavior.

§Examples

let x = Some("air");
assert_eq!(unsafe { x.unwrap_unchecked() }, "air");

let x: Option<&str> = None;
assert_eq!(unsafe { x.unwrap_unchecked() }, "air"); // Undefined behavior!

1.0.0 (const: unstable) · Source

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

Maps an Option<T> to Option by applying a function to a contained value (if Some) or returns None (if None).

§Examples

Calculates the length of an Option<String> as an Option<usize>, consuming the original:

let maybe_some_string = Some(String::from("Hello, World!"));
// `Option::map` takes self *by value*, consuming `maybe_some_string`
let maybe_some_len = maybe_some_string.map(|s| s.len());
assert_eq!(maybe_some_len, Some(13));

let x: Option<&str> = None;
assert_eq!(x.map(|s| s.len()), None);

1.76.0 (const: unstable) · Source

pub fn inspect<F>(self, f: F) -> Option<T>
where F: FnOnce(&T),

Calls a function with a reference to the contained value if Some.

Returns the original option.

§Examples

let list = vec![1, 2, 3];

// prints "got: 2"
let x = list
    .get(1)
    .inspect(|x| println!("got: {x}"))
    .expect("list should be long enough");

// prints nothing
list.get(5).inspect(|x| println!("got: {x}"));

1.0.0 (const: unstable) · Source

pub fn map_or<U, F>(self, default: U, f: F) -> U
where F: FnOnce(T) -> U,

Returns the provided default result (if none), or applies a function to the contained value (if any).

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 = 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);

1.0.0 (const: unstable) · Source

pub fn map_or_else<U, D, F>(self, default: D, f: F) -> U
where D: FnOnce() -> U, F: FnOnce(T) -> U,

Computes a default function result (if none), or applies a different function to the contained value (if any).

§Basic 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);

§Handling a Result-based fallback

A somewhat common occurrence when dealing with optional values in combination with Result<T, E> is the case where one wants to invoke a fallible fallback if the option is not present. This example parses a command line argument (if present), or the contents of a file to an integer. However, unlike accessing the command line argument, reading the file is fallible, so it must be wrapped with Ok.

let v: u64 = std::env::args()
   .nth(1)
   .map_or_else(|| std::fs::read_to_string("/etc/someconfig.conf"), Ok)?
   .parse()?;

Source

pub const fn map_or_default<U, F>(self, f: F) -> U
where U: Default, F: FnOnce(T) -> U,

🔬This is a nightly-only experimental API. (result_option_map_or_default)

Maps an Option<T> to a U by applying function f to the contained value if the option is Some, otherwise if None, returns the default value for the type U.

§Examples

#![feature(result_option_map_or_default)]

let x: Option<&str> = Some("hi");
let y: Option<&str> = None;

assert_eq!(x.map_or_default(|x| x.len()), 2);
assert_eq!(y.map_or_default(|y| y.len()), 0);

1.0.0 (const: unstable) · Source

pub 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).

Arguments passed to ok_or are eagerly evaluated; if you are passing the result of a function call, it is recommended to use ok_or_else, which is lazily evaluated.

§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));

1.0.0 (const: unstable) · Source

pub fn ok_or_else<E, F>(self, err: F) -> Result<T, E>
where F: FnOnce() -> 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));

1.40.0 (const: unstable) · Source

pub fn as_deref(&self) -> Option<&<T as Deref>::Target>
where T: Deref,

Converts from Option<T> (or &Option<T>) to Option<&T::Target>.

Leaves the original Option in-place, creating a new one with a reference to the original one, additionally coercing the contents via Deref.

§Examples

let x: Option<String> = Some("hey".to_owned());
assert_eq!(x.as_deref(), Some("hey"));

let x: Option<String> = None;
assert_eq!(x.as_deref(), None);

1.40.0 (const: unstable) · Source

pub fn as_deref_mut(&mut self) -> Option<&mut <T as Deref>::Target>
where T: DerefMut,

Converts from Option<T> (or &mut Option<T>) to Option<&mut T::Target>.

Leaves the original Option in-place, creating a new one containing a mutable reference to the inner type’s Deref::Target type.

§Examples

let mut x: Option<String> = Some("hey".to_owned());
assert_eq!(x.as_deref_mut().map(|x| {
    x.make_ascii_uppercase();
    x
}), Some("HEY".to_owned().as_mut_str()));

1.0.0 · Source

pub 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);

1.0.0 · Source

pub 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(v) => *v = 42,
    None => {},
}
assert_eq!(x, Some(42));

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

1.0.0 (const: unstable) · Source

pub fn and(self, optb: Option) -> Option

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

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 = 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);

1.0.0 (const: unstable) · Source

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

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_then_to_string(x: u32) -> Option<String> {
    x.checked_mul(x).map(|sq| sq.to_string())
}

assert_eq!(Some(2).and_then(sq_then_to_string), Some(4.to_string()));
assert_eq!(Some(1_000_000).and_then(sq_then_to_string), None); // overflowed!
assert_eq!(None.and_then(sq_then_to_string), None);

Often used to chain fallible operations that may return None.

let arr_2d = [["A0", "A1"], ["B0", "B1"]];

let item_0_1 = arr_2d.get(0).and_then(|row| row.get(1));
assert_eq!(item_0_1, Some(&"A1"));

let item_2_0 = arr_2d.get(2).and_then(|row| row.get(0));
assert_eq!(item_2_0, None);

1.27.0 (const: unstable) · Source

pub fn filter(self, predicate: P) -> Option<T>
where P: FnOnce(&T) -> bool,

Returns None if the option is None, otherwise calls predicate with the wrapped value and returns:

Some(t) if predicate returns true (where t is the wrapped value), and
None if predicate returns false.

This function works similar to Iterator::filter(). You can imagine the Option<T> being an iterator over one or zero elements. filter() lets you decide which elements to keep.

§Examples

fn is_even(n: &i32) -> bool {
    n % 2 == 0
}

assert_eq!(None.filter(is_even), None);
assert_eq!(Some(3).filter(is_even), None);
assert_eq!(Some(4).filter(is_even), Some(4));

1.0.0 (const: unstable) · Source

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

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

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 = 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);

1.0.0 (const: unstable) · Source

pub fn or_else<F>(self, f: F) -> Option<T>
where F: FnOnce() -> 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);

1.37.0 (const: unstable) · Source

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

Returns Some if exactly one of self, optb is Some, otherwise returns None.

§Examples

let x = Some(2);
let y: Option<u32> = None;
assert_eq!(x.xor(y), Some(2));

let x: Option<u32> = None;
let y = Some(2);
assert_eq!(x.xor(y), Some(2));

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

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

1.53.0 (const: unstable) · Source

pub fn insert(&mut self, value: T) -> &mut T

Inserts value into the option, then returns a mutable reference to it.

If the option already contains a value, the old value is dropped.

See also Option::get_or_insert, which doesn’t update the value if the option already contains Some.

§Example

let mut opt = None;
let val = opt.insert(1);
assert_eq!(*val, 1);
assert_eq!(opt.unwrap(), 1);
let val = opt.insert(2);
assert_eq!(*val, 2);
*val = 3;
assert_eq!(opt.unwrap(), 3);

1.20.0 · Source

pub fn get_or_insert(&mut self, value: T) -> &mut T

Inserts value into the option if it is None, then returns a mutable reference to the contained value.

See also Option::insert, which updates the value even if the option already contains Some.

§Examples

let mut x = None;

{
    let y: &mut u32 = x.get_or_insert(5);
    assert_eq!(y, &5);

    *y = 7;
}

assert_eq!(x, Some(7));

1.83.0 (const: unstable) · Source

pub fn get_or_insert_default(&mut self) -> &mut T
where T: Default,

Inserts the default value into the option if it is None, then returns a mutable reference to the contained value.

§Examples

let mut x = None;

{
    let y: &mut u32 = x.get_or_insert_default();
    assert_eq!(y, &0);

    *y = 7;
}

assert_eq!(x, Some(7));

1.20.0 (const: unstable) · Source

pub fn get_or_insert_with<F>(&mut self, f: F) -> &mut T
where F: FnOnce() -> T,

Inserts a value computed from f into the option if it is None, then returns a mutable reference to the contained value.

§Examples

let mut x = None;

{
    let y: &mut u32 = x.get_or_insert_with(|| 5);
    assert_eq!(y, &5);

    *y = 7;
}

assert_eq!(x, Some(7));

1.0.0 (const: 1.83.0) · Source

pub const 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);
let y = x.take();
assert_eq!(x, None);
assert_eq!(y, Some(2));

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

1.80.0 (const: unstable) · Source

pub fn take_if(&mut self, predicate: P) -> Option<T>
where P: FnOnce(&mut T) -> bool,

Takes the value out of the option, but only if the predicate evaluates to true on a mutable reference to the value.

In other words, replaces self with None if the predicate returns true. This method operates similar to Option::take but conditional.

§Examples

let mut x = Some(42);

let prev = x.take_if(|v| if *v == 42 {
    *v += 1;
    false
} else {
    false
});
assert_eq!(x, Some(43));
assert_eq!(prev, None);

let prev = x.take_if(|v| *v == 43);
assert_eq!(x, None);
assert_eq!(prev, Some(43));

1.31.0 (const: 1.83.0) · Source

pub const fn replace(&mut self, value: T) -> Option<T>

Replaces the actual value in the option by the value given in parameter, returning the old value if present, leaving a Some in its place without deinitializing either one.

§Examples

let mut x = Some(2);
let old = x.replace(5);
assert_eq!(x, Some(5));
assert_eq!(old, Some(2));

let mut x = None;
let old = x.replace(3);
assert_eq!(x, Some(3));
assert_eq!(old, None);

1.46.0 (const: unstable) · Source

pub fn zip(self, other: Option) -> Option<(T, U)>

Zips self with another Option.

If self is Some(s) and other is Some(o), this method returns Some((s, o)). Otherwise, None is returned.

§Examples

let x = Some(1);
let y = Some("hi");
let z = None::<u8>;

assert_eq!(x.zip(y), Some((1, "hi")));
assert_eq!(x.zip(z), None);

Source

pub const fn zip_with<U, F, R>(self, other: Option, f: F) -> Option<R>
where F: FnOnce(T, U) -> R,

🔬This is a nightly-only experimental API. (option_zip)

Zips self and another Option with function f.

If self is Some(s) and other is Some(o), this method returns Some(f(s, o)). Otherwise, None is returned.

§Examples

#![feature(option_zip)]

#[derive(Debug, PartialEq)]
struct Point {
    x: f64,
    y: f64,
}

impl Point {
    fn new(x: f64, y: f64) -> Self {
        Self { x, y }
    }
}

let x = Some(17.5);
let y = Some(42.7);

assert_eq!(x.zip_with(y, Point::new), Some(Point { x: 17.5, y: 42.7 }));
assert_eq!(x.zip_with(None, Point::new), None);

Source

pub fn reduce<U, R, F>(self, other: Option, f: F) -> Option<R>
where T: Into<R>, U: Into<R>, F: FnOnce(T, U) -> R,

🔬This is a nightly-only experimental API. (option_reduce)

Reduces two options into one, using the provided function if both are Some.

If self is Some(s) and other is Some(o), this method returns Some(f(s, o)). Otherwise, if only one of self and other is Some, that one is returned. If both self and other are None, None is returned.

§Examples

#![feature(option_reduce)]

let s12 = Some(12);
let s17 = Some(17);
let n = None;
let f = |a, b| a + b;

assert_eq!(s12.reduce(s17, f), Some(29));
assert_eq!(s12.reduce(n, f), Some(12));
assert_eq!(n.reduce(s17, f), Some(17));
assert_eq!(n.reduce(n, f), None);

Source §

impl<T, U> Option<(T, U)>

1.66.0 · Source

pub fn unzip(self) -> (Option<T>, Option)

Unzips an option containing a tuple of two options.

If self is Some((a, b)) this method returns (Some(a), Some(b)). Otherwise, (None, None) is returned.

§Examples

let x = Some((1, "hi"));
let y = None::<(u8, u32)>;

assert_eq!(x.unzip(), (Some(1), Some("hi")));
assert_eq!(y.unzip(), (None, None));

Source §

impl<T> Option<&T>

1.35.0 (const: 1.83.0) · Source

pub const fn copied(self) -> Option<T>
where T: Copy,

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

§Examples

let x = 12;
let opt_x = Some(&x);
assert_eq!(opt_x, Some(&12));
let copied = opt_x.copied();
assert_eq!(copied, Some(12));

1.0.0 · Source

pub fn cloned(self) -> Option<T>
where T: Clone,

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

§Examples

let x = 12;
let opt_x = Some(&x);
assert_eq!(opt_x, Some(&12));
let cloned = opt_x.cloned();
assert_eq!(cloned, Some(12));

Source §

impl<T> Option<&mut T>

1.35.0 (const: 1.83.0) · Source

pub const fn copied(self) -> Option<T>
where T: Copy,

Maps an Option<&mut T> to an Option<T> by copying the contents of the option.

§Examples

let mut x = 12;
let opt_x = Some(&mut x);
assert_eq!(opt_x, Some(&mut 12));
let copied = opt_x.copied();
assert_eq!(copied, Some(12));

1.26.0 · Source

pub fn cloned(self) -> Option<T>
where T: Clone,

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

§Examples

let mut x = 12;
let opt_x = Some(&mut x);
assert_eq!(opt_x, Some(&mut 12));
let cloned = opt_x.cloned();
assert_eq!(cloned, Some(12));

Source §

impl<T, E> Option<Result<T, E>>

1.33.0 (const: 1.83.0) · Source

pub const fn transpose(self) -> Result<Option<T>, E>

Transposes an Option of a Result into a Result of an Option.

Some(Ok(_)) is mapped to Ok(Some(_)), Some(Err(_)) is mapped to Err(_), and None will be mapped to Ok(None).

§Examples

#[derive(Debug, Eq, PartialEq)]
struct SomeErr;

let x: Option<Result<i32, SomeErr>> = Some(Ok(5));
let y: Result<Option<i32>, SomeErr> = Ok(Some(5));
assert_eq!(x.transpose(), y);

Source §

impl<T> Option<Option<T>>

1.40.0 (const: 1.83.0) · Source

pub const fn flatten(self) -> Option<T>

Converts from Option<Option<T>> to Option<T>.

§Examples

Basic usage:

let x: Option<Option<u32>> = Some(Some(6));
assert_eq!(Some(6), x.flatten());

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

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

Flattening only removes one level of nesting at a time:

let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
assert_eq!(Some(Some(6)), x.flatten());
assert_eq!(Some(6), x.flatten().flatten());

Trait Implementations§

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impl<'de, T, Context> BorrowDecode<'de, Context> for Option<T>
where T: BorrowDecode<'de, Context>,

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fn borrow_decode<D>(decoder: &mut D) -> Result<Option<T>, DecodeError>
where D: BorrowDecoder<'de, Context = Context>,

Attempt to decode this type with the given BorrowDecode.

1.0.0 (const: unstable) · Source§

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

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

Returns a duplicate of the value. Read more

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fn clone_from(&mut self, source: &Option<T>)

Performs copy-assignment from source. Read more

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impl<T> Context<T, Infallible> for Option<T>

use anyhow::{Context, Result};

fn maybe_get() -> Option<T> {
    ...
}

fn demo() -> Result<()> {
    let t = maybe_get().context("there is no T")?;
    ...
}

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fn context<C>(self, context: C) -> Result<T, Error>
where C: Display + Send + Sync + 'static,

Wrap the error value with additional context.

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fn default() -> Option<T>

Returns None.

§Examples

let opt: Option<u32> = Option::default();
assert!(opt.is_none());

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impl<'de, T> Deserialize<'de> for Option<T>
where T: Deserialize<'de>,

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fn deserialize<D>( deserializer: D, ) -> Result<Option<T>, <D as Deserializer<'de>>::Error>
where D: Deserializer<'de>,

Deserialize this value from the given Serde deserializer. Read more

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impl<T> Distribution<Option<T>> for Standard
where Standard: Distribution<T>,

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fn sample<R>(&self, rng: &mut R) -> Option<T>
where R: Rng + ?Sized,

Generate a random value of T, using rng as the source of randomness.

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fn sample_iter<R>(self, rng: R) -> DistIter<Self, R, T> ⓘ
where R: Rng, Self: Sized,

Create an iterator that generates random values of T, using rng as the source of randomness. Read more

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fn map<F, S>(self, func: F) -> DistMap<Self, F, T, S>
where F: Fn(T) -> S, Self: Sized,

Create a distribution of values of ‘S’ by mapping the output of Self through the closure F Read more

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

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fn encode<E>(&self, encoder: &mut E) -> Result<(), EncodeError>
where E: Encoder,

Encode a given type.

1.30.0 (const: unstable) · Source§

impl<'a, T> From<&'a Option<T>> for Option<&'a T>

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fn from(o: &'a Option<T>) -> Option<&'a T>

Converts from &Option<T> to Option<&T>.

§Examples

Converts 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 from to first take an Option to a reference to the value inside the original.

let s: Option<String> = Some(String::from("Hello, Rustaceans!"));
let o: Option<usize> = Option::from(&s).map(|ss: &String| ss.len());

println!("Can still print s: {s:?}");

assert_eq!(o, Some(18));

1.30.0 (const: unstable) · Source§

Moves val into a new Some.

§Examples

let o: Option<u8> = Option::from(67);

assert_eq!(Some(67), o);

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Reads a copy of Self from the suffix of the given source. Read more

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Reads a copy of Self from the suffix of the given source. Read more

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Reads a copy of Self from the suffix of the given source. Read more

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impl FromData for Option<Offset16>

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const SIZE: usize = 2usize

Object’s raw data size. Read more

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fn parse(data: &[u8]) -> Option<Option<Offset16>>

Here is an example which increments every integer in a vector. We use the checked variant of add that returns None when the calculation would result in an overflow.

let items = vec![0_u16, 1, 2];

let res: Option<Vec<u16>> = items
    .iter()
    .map(|x| x.checked_add(1))
    .collect();

assert_eq!(res, Some(vec![1, 2, 3]));

As you can see, this will return the expected, valid items.

Here is another example that tries to subtract one from another list of integers, this time checking for underflow:

let items = vec![2_u16, 1, 0];

let res: Option<Vec<u16>> = items
    .iter()
    .map(|x| x.checked_sub(1))
    .collect();

assert_eq!(res, None);

Since the last element is zero, it would underflow. Thus, the resulting value is None.

Here is a variation on the previous example, showing that no further elements are taken from iter after the first None.

let items = vec![3_u16, 2, 1, 10];

let mut shared = 0;

let res: Option<Vec<u16>> = items
    .iter()
    .map(|x| { shared += x; x.checked_sub(2) })
    .collect();

assert_eq!(res, None);
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.

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impl<C, T> FromParallelIterator<Option<T>> for Option<C>
where C: FromParallelIterator<T>, T: Send,

Collect an arbitrary Option-wrapped collection.

If any item is None, then all previous items collected are discarded, and it returns only None.

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fn from_par_iter(par_iter: I) -> Option<C>
where I: IntoParallelIterator<Item = Option<T>>,

Creates an instance of the collection from the parallel iterator par_iter. Read more

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

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

Feeds this value into the given Hasher. Read more

1.3.0 · Source§

fn hash_slice<H>(data: &[Self], state: &mut H)
where H: Hasher, Self: Sized,

Feeds a slice of this type into the given Hasher. Read more

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Gets the bytes of this value. Read more

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fn as_mut_bytes(&mut self) -> &mut [u8] ⓘ
where Self: FromBytes,

Gets the bytes of this value mutably. Read more

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fn write_to(&self, dst: &mut [u8]) -> Result<(), SizeError<&Self, &mut [u8]>>
where Self: Immutable,

Writes a copy of self to dst. Read more

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fn write_to_prefix( &self, dst: &mut [u8], ) -> Result<(), SizeError<&Self, &mut [u8]>>
where Self: Immutable,

Writes a copy of self to the prefix of dst. Read more

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fn write_to_suffix( &self, dst: &mut [u8], ) -> Result<(), SizeError<&Self, &mut [u8]>>
where Self: Immutable,

Writes a copy of self to the suffix of dst. Read more

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Gets the bytes of this value. Read more

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fn as_mut_bytes(&mut self) -> &mut [u8] ⓘ
where Self: FromBytes,

Gets the bytes of this value mutably. Read more

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fn write_to(&self, dst: &mut [u8]) -> Result<(), SizeError<&Self, &mut [u8]>>
where Self: Immutable,

Writes a copy of self to dst. Read more

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fn write_to_prefix( &self, dst: &mut [u8], ) -> Result<(), SizeError<&Self, &mut [u8]>>
where Self: Immutable,

Writes a copy of self to the prefix of dst. Read more

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fn write_to_suffix( &self, dst: &mut [u8], ) -> Result<(), SizeError<&Self, &mut [u8]>>
where Self: Immutable,

Writes a copy of self to the suffix of dst. Read more

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Gets the bytes of this value. Read more

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fn as_mut_bytes(&mut self) -> &mut [u8] ⓘ
where Self: FromBytes,

impl<T> IntoIterator for Option<T>

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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());

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fn into_c(self) -> <Option<T> as IntoReprC>::ReprC

Since the mode is technically only calculated for α > 1, β > 1, those are the only values we allow. We may consider relaxing this constraint in the future.

§Formula

(α - 1) / (α + β - 2)

where α is shapeA and β is shapeB

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impl Mode<Option<f64>> for Cauchy

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fn mode(&self) -> Option<f64>

Returns the mode of the cauchy distribution

§Formula

x_0

where x_0 is the location

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impl Mode<Option<f64>> for Chi

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fn mode(&self) -> Option<f64>

Returns the mode for the chi distribution

§Panics

If freedom < 1.0

§Formula

sqrt(k - 1)

where k is the degrees of freedom

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impl Mode<Option<f64>> for ChiSquared

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fn mode(&self) -> Option<f64>

Returns the mode of the chi-squared distribution

§Formula

k - 2

where k is the degrees of freedom

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impl Mode<Option<f64>> for Dirac

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fn mode(&self) -> Option<f64>

Returns the mode of the dirac distribution

§Formula

where v is the point of the dirac distribution

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impl Mode<Option<f64>> for Erlang

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fn mode(&self) -> Option<f64>

Returns the mode for the erlang distribution

§Remarks

Returns shape if rate ==f64::INFINITY. This behavior is borrowed from the Math.NET implementation

§Formula

(k - 1) / λ

where k is the shape and λ is the rate

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impl Mode<Option<f64>> for Exp

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fn mode(&self) -> Option<f64>

Returns the mode of the exponential distribution

§Formula

(α - 1) / β, where α≥1

where α is the shape and β is the rate

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impl Mode<Option<f64>> for InverseGamma

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fn mode(&self) -> Option<f64>

Returns the mode of the inverse gamma distribution

§Formula

β / (α + 1)

/// where α is the shape and β is the rate

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impl Mode<Option<f64>> for Laplace

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fn mode(&self) -> Option<f64>

Returns the mode of the laplace distribution

§Formula

μ

where μ is the location

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impl Mode<Option<f64>> for LogNormal

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fn mode(&self) -> Option<f64>

Returns the mode of the log-normal distribution

§Formula

e^(μ - σ^2)

where μ is the location and σ is the scale

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impl Mode<Option<f64>> for NegativeBinomial

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fn mode(&self) -> Option<f64>

Returns the mode for the negative binomial distribution.

§Formula

if r > 1 then
    floor((r - 1) * (1-p / p))
else
    0

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impl Mode<Option<f64>> for Normal

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fn mode(&self) -> Option<f64>

Returns the mode of the normal distribution

§Formula

μ

where μ is the mean

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impl Mode<Option<f64>> for Pareto

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fn mode(&self) -> Option<f64>

Returns the mode of the Pareto distribution

§Formula

x_m

where x_m is the scale

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impl Mode<Option<f64>> for StudentsT

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fn mode(&self) -> Option<f64>

Returns the mode of the student’s t-distribution

§Formula

μ

where μ is the location

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impl Mode<Option<f64>> for Triangular

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fn mode(&self) -> Option<f64>

Returns the mode of the triangular distribution

§Formula

mode

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impl Mode<Option<f64>> for Uniform

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fn mode(&self) -> Option<f64>

Returns the mode for the continuous uniform distribution

§Remarks

Since every element has an equal probability, mode simply returns the middle element

§Formula

N/A // (max + min) / 2 for the middle element

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impl Mode<Option<f64>> for Weibull

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fn mode(&self) -> Option<f64>

Returns the median of the weibull distribution

§Formula

if k == 1 {
    0
} else {
    λ((k - 1) / k)^(1 / k)
}

where k is the shape and λ is the scale

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floor((n + 1) * (k + 1) / (N + 2))

where N is population, K is successes, and n is draws

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impl Mode<Option<u64>> for Poisson

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fn mode(&self) -> Option<u64>

Returns the mode of the poisson distribution

§Formula

floor(λ)

where λ is the rate

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impl<T> OptionExt<T> for Option<T>

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fn contains(&self, x: &U) -> bool
where U: PartialEq<T>,

impl<T> StableAbi for Option<T>
where T: StableAbi<IsNonZeroType = True>,

Implementing abi stability for Option<T> is fine if T is a NonZero primitive type.

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const LAYOUT: &'static TypeLayout

The layout of the type provided by implementors.

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type IsNonZeroType = False

Whether this type has a single invalid bit-pattern. Read more

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