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 //! # // FIXME This is not the greatest first example
34 //! // cow_says contains the word "moo"
35 //! let cow_says = Some("moo");
36 //! // dog_says does not contain a value
37 //! let dog_says: Option<&str> = None;
39 //! // Pattern match to retrieve the value
40 //! match (cow_says, dog_says) {
41 //! (Some(cow_words), Some(dog_words)) => {
42 //! println!("Cow says {} and dog says {}!", cow_words, dog_words);
44 //! (Some(cow_words), None) => println!("Cow says {}", cow_words),
45 //! (None, Some(dog_words)) => println!("Dog says {}", dog_words),
46 //! (None, None) => println!("Cow and dog are suspiciously silent")
51 // FIXME: Show how `Option` is used in practice, with lots of methods
53 //! # Options and pointers ("nullable" pointers)
55 //! Rust's pointer types must always point to a valid location; there are
56 //! no "null" pointers. Instead, Rust has *optional* pointers, like
57 //! the optional owned box, `Option<~T>`.
59 //! The following example uses `Option` to create an optional box of
60 //! `int`. Notice that in order to use the inner `int` value first the
61 //! `check_optional` function needs to use pattern matching to
62 //! determine whether the box has a value (i.e. it is `Some(...)`) or
66 //! let optional: Option<~int> = None;
67 //! check_optional(&optional);
69 //! let optional: Option<~int> = Some(~9000);
70 //! check_optional(&optional);
72 //! fn check_optional(optional: &Option<~int>) {
74 //! Some(ref p) => println!("have value {}", p),
75 //! None => println!("have no value")
80 //! This usage of `Option` to create safe nullable pointers is so
81 //! common that Rust does special optimizations to make the
82 //! representation of `Option<~T>` a single pointer. Optional pointers
83 //! in Rust are stored as efficiently as any other pointer type.
87 //! Basic pattern matching on `Option`:
90 //! let msg = Some("howdy");
92 //! // Take a reference to the contained string
94 //! Some(ref m) => println!("{}", *m),
98 //! // Remove the contained string, destroying the Option
99 //! let unwrapped_msg = match msg {
101 //! None => "default message"
105 //! Initialize a result to `None` before a loop:
108 //! enum Kingdom { Plant(uint, &'static str), Animal(uint, &'static str) }
110 //! // A list of data to search through.
111 //! let all_the_big_things = [
112 //! Plant(250, "redwood"),
113 //! Plant(230, "noble fir"),
114 //! Plant(229, "sugar pine"),
115 //! Animal(25, "blue whale"),
116 //! Animal(19, "fin whale"),
117 //! Animal(15, "north pacific right whale"),
120 //! // We're going to search for the name of the biggest animal,
121 //! // but to start with we've just got `None`.
122 //! let mut name_of_biggest_animal = None;
123 //! let mut size_of_biggest_animal = 0;
124 //! for big_thing in all_the_big_things.iter() {
125 //! match *big_thing {
126 //! Animal(size, name) if size > size_of_biggest_animal => {
127 //! // Now we've found the name of some big animal
128 //! size_of_biggest_animal = size;
129 //! name_of_biggest_animal = Some(name);
131 //! Animal(..) | Plant(..) => ()
135 //! match name_of_biggest_animal {
136 //! Some(name) => println!("the biggest animal is {}", name),
137 //! None => println!("there are no animals :(")
143 use cmp::{Eq, TotalEq, TotalOrd};
144 use default::Default;
145 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSize};
151 #[deriving(Clone, Eq, Ord, TotalEq, TotalOrd, Show)]
159 /////////////////////////////////////////////////////////////////////////////
160 // Type implementation
161 /////////////////////////////////////////////////////////////////////////////
164 /////////////////////////////////////////////////////////////////////////
165 // Querying the contained values
166 /////////////////////////////////////////////////////////////////////////
168 /// Returns `true` if the option is a `Some` value
170 pub fn is_some(&self) -> bool {
177 /// Returns `true` if the option is a `None` value
179 pub fn is_none(&self) -> bool {
183 /////////////////////////////////////////////////////////////////////////
184 // Adapter for working with references
185 /////////////////////////////////////////////////////////////////////////
187 /// Convert from `Option<T>` to `Option<&T>`
191 /// Convert an `Option<~str>` into an `Option<int>`, preserving the original.
192 /// The `map` method takes the `self` argument by value, consuming the original,
193 /// so this technique uses `as_ref` to first take an `Option` to a reference
194 /// to the value inside the original.
197 /// let num_as_str: Option<~str> = Some(~"10");
198 /// // First, cast `Option<~str>` to `Option<&~str>` with `as_ref`,
199 /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
200 /// let num_as_int: Option<uint> = num_as_str.as_ref().map(|n| n.len());
201 /// println!("still can print num_as_str: {}", num_as_str);
204 pub fn as_ref<'r>(&'r self) -> Option<&'r T> {
205 match *self { Some(ref x) => Some(x), None => None }
208 /// Convert from `Option<T>` to `Option<&mut T>`
210 pub fn as_mut<'r>(&'r mut self) -> Option<&'r mut T> {
211 match *self { Some(ref mut x) => Some(x), None => None }
214 /// Convert from `Option<T>` to `&[T]` (without copying)
216 pub fn as_slice<'r>(&'r self) -> &'r [T] {
218 Some(ref x) => slice::ref_slice(x),
223 /// Convert from `Option<T>` to `&mut [T]` (without copying)
225 pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
227 Some(ref mut x) => slice::mut_ref_slice(x),
232 /////////////////////////////////////////////////////////////////////////
233 // Getting to contained values
234 /////////////////////////////////////////////////////////////////////////
236 /// Unwraps an option, yielding the content of a `Some`
240 /// Fails if the value is a `None` with a custom failure message provided by `msg`.
242 pub fn expect<M: Any + Send>(self, msg: M) -> T {
249 /// Moves a value out of an option type and returns it, consuming the `Option`.
253 /// Fails if the self value equals `None`.
257 /// In general, because this function may fail, its use is discouraged.
258 /// Instead, prefer to use pattern matching and handle the `None`
261 pub fn unwrap(self) -> T {
264 None => fail!("called `Option::unwrap()` on a `None` value"),
268 /// Returns the contained value or a default.
270 pub fn unwrap_or(self, def: T) -> T {
277 /// Returns the contained value or computes it from a closure.
279 pub fn unwrap_or_else(self, f: || -> T) -> T {
286 /////////////////////////////////////////////////////////////////////////
287 // Transforming contained values
288 /////////////////////////////////////////////////////////////////////////
290 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
294 /// Convert an `Option<~str>` into an `Option<uint>`, consuming the original:
297 /// let num_as_str: Option<~str> = Some(~"10");
298 /// // `Option::map` takes self *by value*, consuming `num_as_str`
299 /// let num_as_int: Option<uint> = num_as_str.map(|n| n.len());
302 pub fn map<U>(self, f: |T| -> U) -> Option<U> {
303 match self { Some(x) => Some(f(x)), None => None }
306 /// Applies a function to the contained value or returns a default.
308 pub fn map_or<U>(self, def: U, f: |T| -> U) -> U {
309 match self { None => def, Some(t) => f(t) }
312 /// Applies a function to the contained value or does nothing.
313 /// Returns true if the contained value was mutated.
314 pub fn mutate(&mut self, f: |T| -> T) -> bool {
316 *self = Some(f(self.take_unwrap()));
321 /// Applies a function to the contained value or sets it to a default.
322 /// Returns true if the contained value was mutated, or false if set to the default.
323 pub fn mutate_or_set(&mut self, def: T, f: |T| -> T) -> bool {
325 *self = Some(f(self.take_unwrap()));
333 /////////////////////////////////////////////////////////////////////////
334 // Iterator constructors
335 /////////////////////////////////////////////////////////////////////////
337 /// Returns an iterator over the possibly contained value.
339 pub fn iter<'r>(&'r self) -> Item<&'r T> {
340 Item{opt: self.as_ref()}
343 /// Returns a mutable iterator over the possibly contained value.
345 pub fn mut_iter<'r>(&'r mut self) -> Item<&'r mut T> {
346 Item{opt: self.as_mut()}
349 /// Returns a consuming iterator over the possibly contained value.
351 pub fn move_iter(self) -> Item<T> {
355 /////////////////////////////////////////////////////////////////////////
356 // Boolean operations on the values, eager and lazy
357 /////////////////////////////////////////////////////////////////////////
359 /// Returns `None` if the option is `None`, otherwise returns `optb`.
361 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
368 /// Returns `None` if the option is `None`, otherwise calls `f` with the
369 /// wrapped value and returns the result.
371 pub fn and_then<U>(self, f: |T| -> Option<U>) -> Option<U> {
378 /// Returns the option if it contains a value, otherwise returns `optb`.
380 pub fn or(self, optb: Option<T>) -> Option<T> {
387 /// Returns the option if it contains a value, otherwise calls `f` and
388 /// returns the result.
390 pub fn or_else(self, f: || -> Option<T>) -> Option<T> {
397 /////////////////////////////////////////////////////////////////////////
399 /////////////////////////////////////////////////////////////////////////
401 /// Takes the value out of the option, leaving a `None` in its place.
403 pub fn take(&mut self) -> Option<T> {
404 mem::replace(self, None)
407 /// Filters an optional value using a given function.
409 pub fn filtered(self, f: |t: &T| -> bool) -> Option<T> {
411 Some(x) => if f(&x) { Some(x) } else { None },
416 /// Applies a function zero or more times until the result is `None`.
418 pub fn while_some(self, f: |v: T| -> Option<T>) {
422 Some(x) => opt = f(x),
428 /////////////////////////////////////////////////////////////////////////
429 // Common special cases
430 /////////////////////////////////////////////////////////////////////////
432 /// The option dance. Moves a value out of an option type and returns it,
433 /// replacing the original with `None`.
437 /// Fails if the value equals `None`.
439 pub fn take_unwrap(&mut self) -> T {
442 None => fail!("called `Option::take_unwrap()` on a `None` value")
446 /// Gets an immutable reference to the value inside an option.
450 /// Fails if the value equals `None`
454 /// In general, because this function may fail, its use is discouraged
455 /// (calling `get` on `None` is akin to dereferencing a null pointer).
456 /// Instead, prefer to use pattern matching and handle the `None`
459 pub fn get_ref<'a>(&'a self) -> &'a T {
462 None => fail!("called `Option::get_ref()` on a `None` value"),
466 /// Gets a mutable reference to the value inside an option.
470 /// Fails if the value equals `None`
474 /// In general, because this function may fail, its use is discouraged
475 /// (calling `get` on `None` is akin to dereferencing a null pointer).
476 /// Instead, prefer to use pattern matching and handle the `None`
479 pub fn get_mut_ref<'a>(&'a mut self) -> &'a mut T {
481 Some(ref mut x) => x,
482 None => fail!("called `Option::get_mut_ref()` on a `None` value"),
487 impl<T: Default> Option<T> {
488 /// Returns the contained value or a default
490 /// Consumes the `self` argument then, if `Some`, returns the contained
491 /// value, otherwise if `None`, returns the default value for that
496 /// Convert a string to an integer, turning poorly-formed strings
497 /// into 0 (the default value for integers). `from_str` converts
498 /// a string to any other type that implements `FromStr`, returning
502 /// let good_year_from_input = "1909";
503 /// let bad_year_from_input = "190blarg";
504 /// let good_year = from_str(good_year_from_input).unwrap_or_default();
505 /// let bad_year = from_str(bad_year_from_input).unwrap_or_default();
507 /// assert_eq!(1909, good_year);
508 /// assert_eq!(0, bad_year);
511 pub fn unwrap_or_default(self) -> T {
514 None => Default::default()
519 /////////////////////////////////////////////////////////////////////////////
520 // Trait implementations
521 /////////////////////////////////////////////////////////////////////////////
523 impl<T> Default for Option<T> {
525 fn default() -> Option<T> { None }
528 /////////////////////////////////////////////////////////////////////////////
529 // The Option Iterator
530 /////////////////////////////////////////////////////////////////////////////
532 /// An `Option` iterator that yields either one or zero elements
534 /// The `Item` iterator is returned by the `iter`, `mut_iter` and `move_iter`
535 /// methods on `Option`.
541 impl<A> Iterator<A> for Item<A> {
543 fn next(&mut self) -> Option<A> {
548 fn size_hint(&self) -> (uint, Option<uint>) {
550 Some(_) => (1, Some(1)),
551 None => (0, Some(0)),
556 impl<A> DoubleEndedIterator<A> for Item<A> {
558 fn next_back(&mut self) -> Option<A> {
563 impl<A> ExactSize<A> for Item<A> {}
565 /////////////////////////////////////////////////////////////////////////////
567 /////////////////////////////////////////////////////////////////////////////
569 /// Takes each element in the `Iterator`: if it is `None`, no further
570 /// elements are taken, and the `None` is returned. Should no `None` occur, a
571 /// vector containing the values of each `Option` is returned.
573 /// Here is an example which increments every integer in a vector,
574 /// checking for overflow:
576 /// fn inc_conditionally(x: uint) -> Option<uint> {
577 /// if x == uint::MAX { return None; }
578 /// else { return Some(x+1u); }
580 /// let v = [1u, 2, 3];
581 /// let res = collect(v.iter().map(|&x| inc_conditionally(x)));
582 /// assert!(res == Some(~[2u, 3, 4]));
584 pub fn collect<T, Iter: Iterator<Option<T>>, V: FromIterator<T>>(iter: Iter) -> Option<V> {
585 // FIXME(#11084): This should be twice as fast once this bug is closed.
586 let mut iter = iter.scan(false, |state, x| {
596 let v: V = FromIterator::from_iterator(&mut iter);
605 /////////////////////////////////////////////////////////////////////////////
607 /////////////////////////////////////////////////////////////////////////////
617 use slice::ImmutableVector;
623 let addr_x: *int = ::cast::transmute(&*x);
625 let y = opt.unwrap();
626 let addr_y: *int = ::cast::transmute(&*y);
627 assert_eq!(addr_x, addr_y);
634 let addr_x = x.as_ptr();
636 let y = opt.unwrap();
637 let addr_y = y.as_ptr();
638 assert_eq!(addr_x, addr_y);
642 fn test_get_resource() {
651 impl ::ops::Drop for R {
653 let ii = self.i.deref();
654 ii.set(ii.get() + 1);
658 fn R(i: Rc<RefCell<int>>) -> R {
664 let i = Rc::new(RefCell::new(0));
666 let x = R(i.clone());
668 let _y = opt.unwrap();
670 assert_eq!(i.deref().get(), 1);
674 fn test_option_dance() {
679 y2 = y.take_unwrap();
682 assert!(y.is_none());
685 #[test] #[should_fail]
686 fn test_option_too_much_dance() {
687 let mut y = Some(marker::NoPod);
688 let _y2 = y.take_unwrap();
689 let _y3 = y.take_unwrap();
694 let x: Option<int> = Some(1);
695 assert_eq!(x.and(Some(2)), Some(2));
696 assert_eq!(x.and(None::<int>), None);
698 let x: Option<int> = None;
699 assert_eq!(x.and(Some(2)), None);
700 assert_eq!(x.and(None::<int>), None);
705 let x: Option<int> = Some(1);
706 assert_eq!(x.and_then(|x| Some(x + 1)), Some(2));
707 assert_eq!(x.and_then(|_| None::<int>), None);
709 let x: Option<int> = None;
710 assert_eq!(x.and_then(|x| Some(x + 1)), None);
711 assert_eq!(x.and_then(|_| None::<int>), None);
716 let x: Option<int> = Some(1);
717 assert_eq!(x.or(Some(2)), Some(1));
718 assert_eq!(x.or(None), Some(1));
720 let x: Option<int> = None;
721 assert_eq!(x.or(Some(2)), Some(2));
722 assert_eq!(x.or(None), None);
727 let x: Option<int> = Some(1);
728 assert_eq!(x.or_else(|| Some(2)), Some(1));
729 assert_eq!(x.or_else(|| None), Some(1));
731 let x: Option<int> = None;
732 assert_eq!(x.or_else(|| Some(2)), Some(2));
733 assert_eq!(x.or_else(|| None), None);
737 fn test_option_while_some() {
739 Some(10).while_some(|j| {
752 assert_eq!(Some(1).unwrap(), 1);
753 assert_eq!(Some(~"hello").unwrap(), ~"hello");
758 fn test_unwrap_fail1() {
759 let x: Option<int> = None;
765 fn test_unwrap_fail2() {
766 let x: Option<~str> = None;
771 fn test_unwrap_or() {
772 let x: Option<int> = Some(1);
773 assert_eq!(x.unwrap_or(2), 1);
775 let x: Option<int> = None;
776 assert_eq!(x.unwrap_or(2), 2);
780 fn test_unwrap_or_else() {
781 let x: Option<int> = Some(1);
782 assert_eq!(x.unwrap_or_else(|| 2), 1);
784 let x: Option<int> = None;
785 assert_eq!(x.unwrap_or_else(|| 2), 2);
790 let some_stuff = Some(42);
791 let modified_stuff = some_stuff.filtered(|&x| {x < 10});
792 assert_eq!(some_stuff.unwrap(), 42);
793 assert!(modified_stuff.is_none());
801 let mut it = x.iter();
803 assert_eq!(it.size_hint(), (1, Some(1)));
804 assert_eq!(it.next(), Some(&val));
805 assert_eq!(it.size_hint(), (0, Some(0)));
806 assert!(it.next().is_none());
814 let mut x = Some(val);
816 let mut it = x.mut_iter();
818 assert_eq!(it.size_hint(), (1, Some(1)));
822 assert_eq!(*interior, val);
825 None => assert!(false),
828 assert_eq!(it.size_hint(), (0, Some(0)));
829 assert!(it.next().is_none());
831 assert_eq!(x, Some(new_val));
836 let small = Some(1.0);
838 let nan = Some(0.0/0.0);
839 assert!(!(nan < big));
840 assert!(!(nan > big));
841 assert!(small < big);
848 let mut x = Some(3i);
849 assert!(x.mutate(|i| i+1));
850 assert_eq!(x, Some(4i));
851 assert!(x.mutate_or_set(0, |i| i+1));
852 assert_eq!(x, Some(5i));
854 assert!(!x.mutate(|i| i+1));
856 assert!(!x.mutate_or_set(0i, |i| i+1));
857 assert_eq!(x, Some(0i));
862 let v: Option<~[int]> = collect(range(0, 0)
864 assert_eq!(v, Some(~[]));
866 let v: Option<~[int]> = collect(range(0, 3)
868 assert_eq!(v, Some(~[0, 1, 2]));
870 let v: Option<~[int]> = collect(range(0, 3)
871 .map(|x| if x > 1 { None } else { Some(x) }));
874 // test that it does not take more elements than it needs
875 let functions = [|| Some(()), || None, || fail!()];
877 let v: Option<~[()]> = collect(functions.iter().map(|f| (*f)()));