1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
13 //! Type `Option` represents an optional value: every `Option`
14 //! is either `Some` and contains a value, or `None`, and
15 //! does not. `Option` types are very common in Rust code, as
16 //! they have a number of uses:
19 //! * Return values for functions that are not defined
20 //! over their entire input range (partial functions)
21 //! * Return value for otherwise reporting simple errors, where `None` is
23 //! * Optional struct fields
24 //! * Struct fields that can be loaned or "taken"
25 //! * Optional function arguments
26 //! * Nullable pointers
27 //! * Swapping things out of difficult situations
29 //! Options are commonly paired with pattern matching to query the presence
30 //! of a value and take action, always accounting for the `None` case.
33 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
34 //! if denominator == 0.0 {
37 //! Some(numerator / denominator)
41 //! // The return value of the function is an option
42 //! let result = divide(2.0, 3.0);
44 //! // Pattern match to retrieve the value
46 //! // The division was valid
47 //! Some(x) => println!("Result: {}", x),
48 //! // The division was invalid
49 //! None => println!("Cannot divide by 0")
54 // FIXME: Show how `Option` is used in practice, with lots of methods
56 //! # Options and pointers ("nullable" pointers)
58 //! Rust's pointer types must always point to a valid location; there are
59 //! no "null" pointers. Instead, Rust has *optional* pointers, like
60 //! the optional owned box, `Option<Box<T>>`.
62 //! The following example uses `Option` to create an optional box of
63 //! `i32`. Notice that in order to use the inner `i32` value first the
64 //! `check_optional` function needs to use pattern matching to
65 //! determine whether the box has a value (i.e. it is `Some(...)`) or
69 //! let optional: Option<Box<i32>> = None;
70 //! check_optional(&optional);
72 //! let optional: Option<Box<i32>> = Some(Box::new(9000));
73 //! check_optional(&optional);
75 //! fn check_optional(optional: &Option<Box<i32>>) {
77 //! Some(ref p) => println!("have value {}", p),
78 //! None => println!("have no value")
83 //! This usage of `Option` to create safe nullable pointers is so
84 //! common that Rust does special optimizations to make the
85 //! representation of `Option<Box<T>>` a single pointer. Optional pointers
86 //! in Rust are stored as efficiently as any other pointer type.
90 //! Basic pattern matching on `Option`:
93 //! let msg = Some("howdy");
95 //! // Take a reference to the contained string
97 //! Some(ref m) => println!("{}", *m),
101 //! // Remove the contained string, destroying the Option
102 //! let unwrapped_msg = match msg {
104 //! None => "default message"
108 //! Initialize a result to `None` before a loop:
111 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
113 //! // A list of data to search through.
114 //! let all_the_big_things = [
115 //! Kingdom::Plant(250, "redwood"),
116 //! Kingdom::Plant(230, "noble fir"),
117 //! Kingdom::Plant(229, "sugar pine"),
118 //! Kingdom::Animal(25, "blue whale"),
119 //! Kingdom::Animal(19, "fin whale"),
120 //! Kingdom::Animal(15, "north pacific right whale"),
123 //! // We're going to search for the name of the biggest animal,
124 //! // but to start with we've just got `None`.
125 //! let mut name_of_biggest_animal = None;
126 //! let mut size_of_biggest_animal = 0;
127 //! for big_thing in all_the_big_things.iter() {
128 //! match *big_thing {
129 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
130 //! // Now we've found the name of some big animal
131 //! size_of_biggest_animal = size;
132 //! name_of_biggest_animal = Some(name);
134 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
138 //! match name_of_biggest_animal {
139 //! Some(name) => println!("the biggest animal is {}", name),
140 //! None => println!("there are no animals :(")
144 #![stable(feature = "rust1", since = "1.0.0")]
150 use default::Default;
151 use iter::{ExactSizeIterator};
152 use iter::{Iterator, IteratorExt, DoubleEndedIterator, FromIterator, IntoIterator};
154 use ops::{Deref, FnOnce};
155 use result::Result::{Ok, Err};
160 // Note that this is not a lang item per se, but it has a hidden dependency on
161 // `Iterator`, which is one. The compiler assumes that the `next` method of
162 // `Iterator` is an enumeration with one type parameter and two variants,
163 // which basically means it must be `Option`.
165 /// The `Option` type. See [the module level documentation](../index.html) for more.
166 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
167 #[stable(feature = "rust1", since = "1.0.0")]
170 #[stable(feature = "rust1", since = "1.0.0")]
173 #[stable(feature = "rust1", since = "1.0.0")]
177 /////////////////////////////////////////////////////////////////////////////
178 // Type implementation
179 /////////////////////////////////////////////////////////////////////////////
182 /////////////////////////////////////////////////////////////////////////
183 // Querying the contained values
184 /////////////////////////////////////////////////////////////////////////
186 /// Returns `true` if the option is a `Some` value
191 /// let x: Option<u32> = Some(2);
192 /// assert_eq!(x.is_some(), true);
194 /// let x: Option<u32> = None;
195 /// assert_eq!(x.is_some(), false);
198 #[stable(feature = "rust1", since = "1.0.0")]
199 pub fn is_some(&self) -> bool {
206 /// Returns `true` if the option is a `None` value
211 /// let x: Option<u32> = Some(2);
212 /// assert_eq!(x.is_none(), false);
214 /// let x: Option<u32> = None;
215 /// assert_eq!(x.is_none(), true);
218 #[stable(feature = "rust1", since = "1.0.0")]
219 pub fn is_none(&self) -> bool {
223 /////////////////////////////////////////////////////////////////////////
224 // Adapter for working with references
225 /////////////////////////////////////////////////////////////////////////
227 /// Convert from `Option<T>` to `Option<&T>`
231 /// Convert an `Option<String>` into an `Option<usize>`, preserving the original.
232 /// The `map` method takes the `self` argument by value, consuming the original,
233 /// so this technique uses `as_ref` to first take an `Option` to a reference
234 /// to the value inside the original.
237 /// let num_as_str: Option<String> = Some("10".to_string());
238 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
239 /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
240 /// let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len());
241 /// println!("still can print num_as_str: {:?}", num_as_str);
244 #[stable(feature = "rust1", since = "1.0.0")]
245 pub fn as_ref<'r>(&'r self) -> Option<&'r T> {
247 Some(ref x) => Some(x),
252 /// Convert from `Option<T>` to `Option<&mut T>`
257 /// let mut x = Some(2);
258 /// match x.as_mut() {
259 /// Some(v) => *v = 42,
262 /// assert_eq!(x, Some(42));
265 #[stable(feature = "rust1", since = "1.0.0")]
266 pub fn as_mut<'r>(&'r mut self) -> Option<&'r mut T> {
268 Some(ref mut x) => Some(x),
273 /// Convert from `Option<T>` to `&mut [T]` (without copying)
278 /// let mut x = Some("Diamonds");
280 /// let v = x.as_mut_slice();
281 /// assert!(v == ["Diamonds"]);
283 /// assert!(v == ["Dirt"]);
285 /// assert_eq!(x, Some("Dirt"));
288 #[unstable(feature = "core",
289 reason = "waiting for mut conventions")]
290 pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
293 let result: &mut [T] = slice::mut_ref_slice(x);
297 let result: &mut [T] = &mut [];
303 /////////////////////////////////////////////////////////////////////////
304 // Getting to contained values
305 /////////////////////////////////////////////////////////////////////////
307 /// Unwraps an option, yielding the content of a `Some`
311 /// Panics if the value is a `None` with a custom panic message provided by
317 /// let x = Some("value");
318 /// assert_eq!(x.expect("the world is ending"), "value");
321 /// ```{.should_fail}
322 /// let x: Option<&str> = None;
323 /// x.expect("the world is ending"); // panics with `world is ending`
326 #[stable(feature = "rust1", since = "1.0.0")]
327 pub fn expect(self, msg: &str) -> T {
330 None => panic!("{}", msg),
334 /// Returns the inner `T` of a `Some(T)`.
338 /// Panics if the self value equals `None`.
342 /// In general, because this function may panic, its use is discouraged.
343 /// Instead, prefer to use pattern matching and handle the `None`
349 /// let x = Some("air");
350 /// assert_eq!(x.unwrap(), "air");
353 /// ```{.should_fail}
354 /// let x: Option<&str> = None;
355 /// assert_eq!(x.unwrap(), "air"); // fails
358 #[stable(feature = "rust1", since = "1.0.0")]
359 pub fn unwrap(self) -> T {
362 None => panic!("called `Option::unwrap()` on a `None` value"),
366 /// Returns the contained value or a default.
371 /// assert_eq!(Some("car").unwrap_or("bike"), "car");
372 /// assert_eq!(None.unwrap_or("bike"), "bike");
375 #[stable(feature = "rust1", since = "1.0.0")]
376 pub fn unwrap_or(self, def: T) -> T {
383 /// Returns the contained value or computes it from a closure.
389 /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
390 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
393 #[stable(feature = "rust1", since = "1.0.0")]
394 pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
401 /////////////////////////////////////////////////////////////////////////
402 // Transforming contained values
403 /////////////////////////////////////////////////////////////////////////
405 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
409 /// Convert an `Option<String>` into an `Option<usize>`, consuming the original:
412 /// let num_as_str: Option<String> = Some("10".to_string());
413 /// // `Option::map` takes self *by value*, consuming `num_as_str`
414 /// let num_as_int: Option<usize> = num_as_str.map(|n| n.len());
417 #[stable(feature = "rust1", since = "1.0.0")]
418 pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
420 Some(x) => Some(f(x)),
425 /// Applies a function to the contained value or returns a default.
430 /// let x = Some("foo");
431 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
433 /// let x: Option<&str> = None;
434 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
437 #[stable(feature = "rust1", since = "1.0.0")]
438 pub fn map_or<U, F: FnOnce(T) -> U>(self, def: U, f: F) -> U {
445 /// Applies a function to the contained value or computes a default.
452 /// let x = Some("foo");
453 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
455 /// let x: Option<&str> = None;
456 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
459 #[stable(feature = "rust1", since = "1.0.0")]
460 pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, def: D, f: F) -> U {
467 /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
468 /// `Ok(v)` and `None` to `Err(err)`.
473 /// let x = Some("foo");
474 /// assert_eq!(x.ok_or(0), Ok("foo"));
476 /// let x: Option<&str> = None;
477 /// assert_eq!(x.ok_or(0), Err(0));
480 #[unstable(feature = "core")]
481 pub fn ok_or<E>(self, err: E) -> Result<T, E> {
488 /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
489 /// `Ok(v)` and `None` to `Err(err())`.
494 /// let x = Some("foo");
495 /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
497 /// let x: Option<&str> = None;
498 /// assert_eq!(x.ok_or_else(|| 0), Err(0));
501 #[unstable(feature = "core")]
502 pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
509 /////////////////////////////////////////////////////////////////////////
510 // Iterator constructors
511 /////////////////////////////////////////////////////////////////////////
513 /// Returns an iterator over the possibly contained value.
519 /// assert_eq!(x.iter().next(), Some(&4));
521 /// let x: Option<u32> = None;
522 /// assert_eq!(x.iter().next(), None);
525 #[stable(feature = "rust1", since = "1.0.0")]
526 pub fn iter(&self) -> Iter<T> {
527 Iter { inner: Item { opt: self.as_ref() } }
530 /// Returns a mutable iterator over the possibly contained value.
535 /// let mut x = Some(4);
536 /// match x.iter_mut().next() {
537 /// Some(&mut ref mut v) => *v = 42,
540 /// assert_eq!(x, Some(42));
542 /// let mut x: Option<u32> = None;
543 /// assert_eq!(x.iter_mut().next(), None);
546 #[unstable(feature = "core",
547 reason = "waiting for iterator conventions")]
548 pub fn iter_mut(&mut self) -> IterMut<T> {
549 IterMut { inner: Item { opt: self.as_mut() } }
552 /// Returns a consuming iterator over the possibly contained value.
557 /// let x = Some("string");
558 /// let v: Vec<&str> = x.into_iter().collect();
559 /// assert_eq!(v, vec!["string"]);
562 /// let v: Vec<&str> = x.into_iter().collect();
563 /// assert!(v.is_empty());
566 #[stable(feature = "rust1", since = "1.0.0")]
567 pub fn into_iter(self) -> IntoIter<T> {
568 IntoIter { inner: Item { opt: self } }
571 /////////////////////////////////////////////////////////////////////////
572 // Boolean operations on the values, eager and lazy
573 /////////////////////////////////////////////////////////////////////////
575 /// Returns `None` if the option is `None`, otherwise returns `optb`.
581 /// let y: Option<&str> = None;
582 /// assert_eq!(x.and(y), None);
584 /// let x: Option<u32> = None;
585 /// let y = Some("foo");
586 /// assert_eq!(x.and(y), None);
589 /// let y = Some("foo");
590 /// assert_eq!(x.and(y), Some("foo"));
592 /// let x: Option<u32> = None;
593 /// let y: Option<&str> = None;
594 /// assert_eq!(x.and(y), None);
597 #[stable(feature = "rust1", since = "1.0.0")]
598 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
605 /// Returns `None` if the option is `None`, otherwise calls `f` with the
606 /// wrapped value and returns the result.
608 /// Some languages call this operation flatmap.
613 /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
614 /// fn nope(_: u32) -> Option<u32> { None }
616 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
617 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
618 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
619 /// assert_eq!(None.and_then(sq).and_then(sq), None);
622 #[stable(feature = "rust1", since = "1.0.0")]
623 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
630 /// Returns the option if it contains a value, otherwise returns `optb`.
637 /// assert_eq!(x.or(y), Some(2));
640 /// let y = Some(100);
641 /// assert_eq!(x.or(y), Some(100));
644 /// let y = Some(100);
645 /// assert_eq!(x.or(y), Some(2));
647 /// let x: Option<u32> = None;
649 /// assert_eq!(x.or(y), None);
652 #[stable(feature = "rust1", since = "1.0.0")]
653 pub fn or(self, optb: Option<T>) -> Option<T> {
660 /// Returns the option if it contains a value, otherwise calls `f` and
661 /// returns the result.
666 /// fn nobody() -> Option<&'static str> { None }
667 /// fn vikings() -> Option<&'static str> { Some("vikings") }
669 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
670 /// assert_eq!(None.or_else(vikings), Some("vikings"));
671 /// assert_eq!(None.or_else(nobody), None);
674 #[stable(feature = "rust1", since = "1.0.0")]
675 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
682 /////////////////////////////////////////////////////////////////////////
684 /////////////////////////////////////////////////////////////////////////
686 /// Takes the value out of the option, leaving a `None` in its place.
691 /// let mut x = Some(2);
693 /// assert_eq!(x, None);
695 /// let mut x: Option<u32> = None;
697 /// assert_eq!(x, None);
700 #[stable(feature = "rust1", since = "1.0.0")]
701 pub fn take(&mut self) -> Option<T> {
702 mem::replace(self, None)
706 impl<'a, T: Clone, D: Deref<Target=T>> Option<D> {
707 /// Maps an Option<D> to an Option<T> by dereffing and cloning the contents of the Option.
708 /// Useful for converting an Option<&T> to an Option<T>.
709 #[unstable(feature = "core",
710 reason = "recently added as part of collections reform")]
711 pub fn cloned(self) -> Option<T> {
712 self.map(|t| t.deref().clone())
716 impl<T: Default> Option<T> {
717 /// Returns the contained value or a default
719 /// Consumes the `self` argument then, if `Some`, returns the contained
720 /// value, otherwise if `None`, returns the default value for that
725 /// Convert a string to an integer, turning poorly-formed strings
726 /// into 0 (the default value for integers). `parse` converts
727 /// a string to any other type that implements `FromStr`, returning
731 /// let good_year_from_input = "1909";
732 /// let bad_year_from_input = "190blarg";
733 /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
734 /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
736 /// assert_eq!(1909, good_year);
737 /// assert_eq!(0, bad_year);
740 #[stable(feature = "rust1", since = "1.0.0")]
741 pub fn unwrap_or_default(self) -> T {
744 None => Default::default()
749 /////////////////////////////////////////////////////////////////////////////
750 // Trait implementations
751 /////////////////////////////////////////////////////////////////////////////
753 #[unstable(feature = "core",
754 reason = "waiting on the stability of the trait itself")]
755 impl<T> AsSlice<T> for Option<T> {
756 /// Convert from `Option<T>` to `&[T]` (without copying)
758 fn as_slice<'a>(&'a self) -> &'a [T] {
760 Some(ref x) => slice::ref_slice(x),
762 let result: &[_] = &[];
769 #[stable(feature = "rust1", since = "1.0.0")]
770 impl<T> Default for Option<T> {
772 #[stable(feature = "rust1", since = "1.0.0")]
773 fn default() -> Option<T> { None }
776 /////////////////////////////////////////////////////////////////////////////
777 // The Option Iterators
778 /////////////////////////////////////////////////////////////////////////////
785 impl<A> Iterator for Item<A> {
789 fn next(&mut self) -> Option<A> {
794 fn size_hint(&self) -> (usize, Option<usize>) {
796 Some(_) => (1, Some(1)),
797 None => (0, Some(0)),
802 impl<A> DoubleEndedIterator for Item<A> {
804 fn next_back(&mut self) -> Option<A> {
809 impl<A> ExactSizeIterator for Item<A> {}
811 /// An iterator over a reference of the contained item in an Option.
812 #[stable(feature = "rust1", since = "1.0.0")]
813 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
815 #[stable(feature = "rust1", since = "1.0.0")]
816 impl<'a, A> Iterator for Iter<'a, A> {
820 fn next(&mut self) -> Option<&'a A> { self.inner.next() }
822 fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
825 #[stable(feature = "rust1", since = "1.0.0")]
826 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
828 fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
831 #[stable(feature = "rust1", since = "1.0.0")]
832 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
834 #[stable(feature = "rust1", since = "1.0.0")]
835 impl<'a, A> Clone for Iter<'a, A> {
836 fn clone(&self) -> Iter<'a, A> {
837 Iter { inner: self.inner.clone() }
841 /// An iterator over a mutable reference of the contained item in an Option.
842 #[stable(feature = "rust1", since = "1.0.0")]
843 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
845 #[stable(feature = "rust1", since = "1.0.0")]
846 impl<'a, A> Iterator for IterMut<'a, A> {
847 type Item = &'a mut A;
850 fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
852 fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
855 #[stable(feature = "rust1", since = "1.0.0")]
856 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
858 fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
861 #[stable(feature = "rust1", since = "1.0.0")]
862 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
864 /// An iterator over the item contained inside an Option.
865 #[stable(feature = "rust1", since = "1.0.0")]
866 pub struct IntoIter<A> { inner: Item<A> }
868 #[stable(feature = "rust1", since = "1.0.0")]
869 impl<A> Iterator for IntoIter<A> {
873 fn next(&mut self) -> Option<A> { self.inner.next() }
875 fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
878 #[stable(feature = "rust1", since = "1.0.0")]
879 impl<A> DoubleEndedIterator for IntoIter<A> {
881 fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
884 #[stable(feature = "rust1", since = "1.0.0")]
885 impl<A> ExactSizeIterator for IntoIter<A> {}
887 /////////////////////////////////////////////////////////////////////////////
889 /////////////////////////////////////////////////////////////////////////////
891 #[stable(feature = "rust1", since = "1.0.0")]
892 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
893 /// Takes each element in the `Iterator`: if it is `None`, no further
894 /// elements are taken, and the `None` is returned. Should no `None` occur, a
895 /// container with the values of each `Option` is returned.
897 /// Here is an example which increments every integer in a vector,
898 /// checking for overflow:
903 /// let v = vec!(1, 2);
904 /// let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
905 /// if x == u16::MAX { None }
906 /// else { Some(x + 1) }
908 /// assert!(res == Some(vec!(2, 3)));
911 #[stable(feature = "rust1", since = "1.0.0")]
912 fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
913 // FIXME(#11084): This could be replaced with Iterator::scan when this
914 // performance bug is closed.
916 struct Adapter<Iter> {
921 impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
925 fn next(&mut self) -> Option<T> {
926 match self.iter.next() {
927 Some(Some(value)) => Some(value),
929 self.found_none = true;
937 let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
938 let v: V = FromIterator::from_iter(adapter.by_ref());
940 if adapter.found_none {