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<Box<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<Box<int>> = None;
67 //! check_optional(&optional);
69 //! let optional: Option<Box<int>> = Some(box 9000);
70 //! check_optional(&optional);
72 //! fn check_optional(optional: &Option<Box<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<Box<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 :(")
141 use cmp::{Eq, TotalEq, TotalOrd};
142 use default::Default;
143 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSize};
148 #[deriving(Clone, Eq, Ord, TotalEq, TotalOrd)]
156 /////////////////////////////////////////////////////////////////////////////
157 // Type implementation
158 /////////////////////////////////////////////////////////////////////////////
161 /////////////////////////////////////////////////////////////////////////
162 // Querying the contained values
163 /////////////////////////////////////////////////////////////////////////
165 /// Returns `true` if the option is a `Some` value
167 pub fn is_some(&self) -> bool {
174 /// Returns `true` if the option is a `None` value
176 pub fn is_none(&self) -> bool {
180 /////////////////////////////////////////////////////////////////////////
181 // Adapter for working with references
182 /////////////////////////////////////////////////////////////////////////
184 /// Convert from `Option<T>` to `Option<&T>`
188 /// Convert an `Option<~str>` into an `Option<int>`, preserving the original.
189 /// The `map` method takes the `self` argument by value, consuming the original,
190 /// so this technique uses `as_ref` to first take an `Option` to a reference
191 /// to the value inside the original.
194 /// let num_as_str: Option<~str> = Some("10".to_owned());
195 /// // First, cast `Option<~str>` to `Option<&~str>` with `as_ref`,
196 /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
197 /// let num_as_int: Option<uint> = num_as_str.as_ref().map(|n| n.len());
198 /// println!("still can print num_as_str: {}", num_as_str);
201 pub fn as_ref<'r>(&'r self) -> Option<&'r T> {
202 match *self { Some(ref x) => Some(x), None => None }
205 /// Convert from `Option<T>` to `Option<&mut T>`
207 pub fn as_mut<'r>(&'r mut self) -> Option<&'r mut T> {
208 match *self { Some(ref mut x) => Some(x), None => None }
211 /// Convert from `Option<T>` to `&[T]` (without copying)
213 pub fn as_slice<'r>(&'r self) -> &'r [T] {
215 Some(ref x) => slice::ref_slice(x),
220 /// Convert from `Option<T>` to `&mut [T]` (without copying)
222 pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
224 Some(ref mut x) => slice::mut_ref_slice(x),
229 /////////////////////////////////////////////////////////////////////////
230 // Getting to contained values
231 /////////////////////////////////////////////////////////////////////////
233 /// Moves a value out of an option type and returns it, consuming the `Option`.
237 /// Fails if the self value equals `None`.
241 /// In general, because this function may fail, its use is discouraged.
242 /// Instead, prefer to use pattern matching and handle the `None`
245 pub fn unwrap(self) -> T {
248 None => fail!("called `Option::unwrap()` on a `None` value"),
252 /// Returns the contained value or a default.
254 pub fn unwrap_or(self, def: T) -> T {
261 /// Returns the contained value or computes it from a closure.
263 pub fn unwrap_or_else(self, f: || -> T) -> T {
270 /////////////////////////////////////////////////////////////////////////
271 // Transforming contained values
272 /////////////////////////////////////////////////////////////////////////
274 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
278 /// Convert an `Option<~str>` into an `Option<uint>`, consuming the original:
281 /// let num_as_str: Option<~str> = Some("10".to_owned());
282 /// // `Option::map` takes self *by value*, consuming `num_as_str`
283 /// let num_as_int: Option<uint> = num_as_str.map(|n| n.len());
286 pub fn map<U>(self, f: |T| -> U) -> Option<U> {
287 match self { Some(x) => Some(f(x)), None => None }
290 /// Applies a function to the contained value or returns a default.
292 pub fn map_or<U>(self, def: U, f: |T| -> U) -> U {
293 match self { None => def, Some(t) => f(t) }
296 /// Applies a function to the contained value or does nothing.
297 /// Returns true if the contained value was mutated.
298 pub fn mutate(&mut self, f: |T| -> T) -> bool {
300 *self = Some(f(self.take_unwrap()));
305 /// Applies a function to the contained value or sets it to a default.
306 /// Returns true if the contained value was mutated, or false if set to the default.
307 pub fn mutate_or_set(&mut self, def: T, f: |T| -> T) -> bool {
309 *self = Some(f(self.take_unwrap()));
317 /////////////////////////////////////////////////////////////////////////
318 // Iterator constructors
319 /////////////////////////////////////////////////////////////////////////
321 /// Returns an iterator over the possibly contained value.
323 pub fn iter<'r>(&'r self) -> Item<&'r T> {
324 Item{opt: self.as_ref()}
327 /// Returns a mutable iterator over the possibly contained value.
329 pub fn mut_iter<'r>(&'r mut self) -> Item<&'r mut T> {
330 Item{opt: self.as_mut()}
333 /// Returns a consuming iterator over the possibly contained value.
335 pub fn move_iter(self) -> Item<T> {
339 /////////////////////////////////////////////////////////////////////////
340 // Boolean operations on the values, eager and lazy
341 /////////////////////////////////////////////////////////////////////////
343 /// Returns `None` if the option is `None`, otherwise returns `optb`.
345 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
352 /// Returns `None` if the option is `None`, otherwise calls `f` with the
353 /// wrapped value and returns the result.
355 pub fn and_then<U>(self, f: |T| -> Option<U>) -> Option<U> {
362 /// Returns the option if it contains a value, otherwise returns `optb`.
364 pub fn or(self, optb: Option<T>) -> Option<T> {
371 /// Returns the option if it contains a value, otherwise calls `f` and
372 /// returns the result.
374 pub fn or_else(self, f: || -> Option<T>) -> Option<T> {
381 /////////////////////////////////////////////////////////////////////////
383 /////////////////////////////////////////////////////////////////////////
385 /// Takes the value out of the option, leaving a `None` in its place.
387 pub fn take(&mut self) -> Option<T> {
388 mem::replace(self, None)
391 /// Filters an optional value using a given function.
393 pub fn filtered(self, f: |t: &T| -> bool) -> Option<T> {
395 Some(x) => if f(&x) { Some(x) } else { None },
400 /// Applies a function zero or more times until the result is `None`.
402 pub fn while_some(self, f: |v: T| -> Option<T>) {
406 Some(x) => opt = f(x),
412 /////////////////////////////////////////////////////////////////////////
413 // Common special cases
414 /////////////////////////////////////////////////////////////////////////
416 /// The option dance. Moves a value out of an option type and returns it,
417 /// replacing the original with `None`.
421 /// Fails if the value equals `None`.
423 pub fn take_unwrap(&mut self) -> T {
426 None => fail!("called `Option::take_unwrap()` on a `None` value")
430 /// Gets an immutable reference to the value inside an option.
434 /// Fails if the value equals `None`
438 /// In general, because this function may fail, its use is discouraged
439 /// (calling `get` on `None` is akin to dereferencing a null pointer).
440 /// Instead, prefer to use pattern matching and handle the `None`
443 pub fn get_ref<'a>(&'a self) -> &'a T {
446 None => fail!("called `Option::get_ref()` on a `None` value"),
450 /// Gets a mutable reference to the value inside an option.
454 /// Fails if the value equals `None`
458 /// In general, because this function may fail, its use is discouraged
459 /// (calling `get` on `None` is akin to dereferencing a null pointer).
460 /// Instead, prefer to use pattern matching and handle the `None`
463 pub fn get_mut_ref<'a>(&'a mut self) -> &'a mut T {
465 Some(ref mut x) => x,
466 None => fail!("called `Option::get_mut_ref()` on a `None` value"),
471 impl<T: Default> Option<T> {
472 /// Returns the contained value or a default
474 /// Consumes the `self` argument then, if `Some`, returns the contained
475 /// value, otherwise if `None`, returns the default value for that
480 /// Convert a string to an integer, turning poorly-formed strings
481 /// into 0 (the default value for integers). `from_str` converts
482 /// a string to any other type that implements `FromStr`, returning
486 /// let good_year_from_input = "1909";
487 /// let bad_year_from_input = "190blarg";
488 /// let good_year = from_str(good_year_from_input).unwrap_or_default();
489 /// let bad_year = from_str(bad_year_from_input).unwrap_or_default();
491 /// assert_eq!(1909, good_year);
492 /// assert_eq!(0, bad_year);
495 pub fn unwrap_or_default(self) -> T {
498 None => Default::default()
503 /////////////////////////////////////////////////////////////////////////////
504 // Trait implementations
505 /////////////////////////////////////////////////////////////////////////////
507 impl<T> Default for Option<T> {
509 fn default() -> Option<T> { None }
512 /////////////////////////////////////////////////////////////////////////////
513 // The Option Iterator
514 /////////////////////////////////////////////////////////////////////////////
516 /// An `Option` iterator that yields either one or zero elements
518 /// The `Item` iterator is returned by the `iter`, `mut_iter` and `move_iter`
519 /// methods on `Option`.
525 impl<A> Iterator<A> for Item<A> {
527 fn next(&mut self) -> Option<A> {
532 fn size_hint(&self) -> (uint, Option<uint>) {
534 Some(_) => (1, Some(1)),
535 None => (0, Some(0)),
540 impl<A> DoubleEndedIterator<A> for Item<A> {
542 fn next_back(&mut self) -> Option<A> {
547 impl<A> ExactSize<A> for Item<A> {}
549 /////////////////////////////////////////////////////////////////////////////
551 /////////////////////////////////////////////////////////////////////////////
553 /// Takes each element in the `Iterator`: if it is `None`, no further
554 /// elements are taken, and the `None` is returned. Should no `None` occur, a
555 /// vector containing the values of each `Option` is returned.
557 /// Here is an example which increments every integer in a vector,
558 /// checking for overflow:
560 /// fn inc_conditionally(x: uint) -> Option<uint> {
561 /// if x == uint::MAX { return None; }
562 /// else { return Some(x+1u); }
564 /// let v = [1u, 2, 3];
565 /// let res = collect(v.iter().map(|&x| inc_conditionally(x)));
566 /// assert!(res == Some(~[2u, 3, 4]));
568 pub fn collect<T, Iter: Iterator<Option<T>>, V: FromIterator<T>>(iter: Iter) -> Option<V> {
569 // FIXME(#11084): This should be twice as fast once this bug is closed.
570 let mut iter = iter.scan(false, |state, x| {
580 let v: V = FromIterator::from_iter(iter.by_ref());
589 /////////////////////////////////////////////////////////////////////////////
591 /////////////////////////////////////////////////////////////////////////////
595 use realstd::option::collect;
596 use realstd::prelude::*;
597 use realstd::iter::range;
601 use slice::ImmutableVector;
607 let addr_x: *int = ::cast::transmute(&*x);
609 let y = opt.unwrap();
610 let addr_y: *int = ::cast::transmute(&*y);
611 assert_eq!(addr_x, addr_y);
617 let x = "test".to_owned();
618 let addr_x = x.as_ptr();
620 let y = opt.unwrap();
621 let addr_y = y.as_ptr();
622 assert_eq!(addr_x, addr_y);
626 fn test_get_resource() {
635 impl ::ops::Drop for R {
638 let i = ii.borrow().clone();
639 *ii.borrow_mut() = i + 1;
643 fn R(i: Rc<RefCell<int>>) -> R {
649 let i = Rc::new(RefCell::new(0));
651 let x = R(i.clone());
653 let _y = opt.unwrap();
655 assert_eq!(*i.borrow(), 1);
659 fn test_option_dance() {
664 y2 = y.take_unwrap();
667 assert!(y.is_none());
670 #[test] #[should_fail]
671 fn test_option_too_much_dance() {
672 let mut y = Some(marker::NoCopy);
673 let _y2 = y.take_unwrap();
674 let _y3 = y.take_unwrap();
679 let x: Option<int> = Some(1);
680 assert_eq!(x.and(Some(2)), Some(2));
681 assert_eq!(x.and(None::<int>), None);
683 let x: Option<int> = None;
684 assert_eq!(x.and(Some(2)), None);
685 assert_eq!(x.and(None::<int>), None);
690 let x: Option<int> = Some(1);
691 assert_eq!(x.and_then(|x| Some(x + 1)), Some(2));
692 assert_eq!(x.and_then(|_| None::<int>), None);
694 let x: Option<int> = None;
695 assert_eq!(x.and_then(|x| Some(x + 1)), None);
696 assert_eq!(x.and_then(|_| None::<int>), None);
701 let x: Option<int> = Some(1);
702 assert_eq!(x.or(Some(2)), Some(1));
703 assert_eq!(x.or(None), Some(1));
705 let x: Option<int> = None;
706 assert_eq!(x.or(Some(2)), Some(2));
707 assert_eq!(x.or(None), None);
712 let x: Option<int> = Some(1);
713 assert_eq!(x.or_else(|| Some(2)), Some(1));
714 assert_eq!(x.or_else(|| None), Some(1));
716 let x: Option<int> = None;
717 assert_eq!(x.or_else(|| Some(2)), Some(2));
718 assert_eq!(x.or_else(|| None), None);
722 fn test_option_while_some() {
724 Some(10).while_some(|j| {
737 assert_eq!(Some(1).unwrap(), 1);
738 assert_eq!(Some("hello".to_owned()).unwrap(), "hello".to_owned());
743 fn test_unwrap_fail1() {
744 let x: Option<int> = None;
750 fn test_unwrap_fail2() {
751 let x: Option<~str> = None;
756 fn test_unwrap_or() {
757 let x: Option<int> = Some(1);
758 assert_eq!(x.unwrap_or(2), 1);
760 let x: Option<int> = None;
761 assert_eq!(x.unwrap_or(2), 2);
765 fn test_unwrap_or_else() {
766 let x: Option<int> = Some(1);
767 assert_eq!(x.unwrap_or_else(|| 2), 1);
769 let x: Option<int> = None;
770 assert_eq!(x.unwrap_or_else(|| 2), 2);
775 let some_stuff = Some(42);
776 let modified_stuff = some_stuff.filtered(|&x| {x < 10});
777 assert_eq!(some_stuff.unwrap(), 42);
778 assert!(modified_stuff.is_none());
786 let mut it = x.iter();
788 assert_eq!(it.size_hint(), (1, Some(1)));
789 assert_eq!(it.next(), Some(&val));
790 assert_eq!(it.size_hint(), (0, Some(0)));
791 assert!(it.next().is_none());
799 let mut x = Some(val);
801 let mut it = x.mut_iter();
803 assert_eq!(it.size_hint(), (1, Some(1)));
807 assert_eq!(*interior, val);
810 None => assert!(false),
813 assert_eq!(it.size_hint(), (0, Some(0)));
814 assert!(it.next().is_none());
816 assert_eq!(x, Some(new_val));
821 let small = Some(1.0);
823 let nan = Some(0.0/0.0);
824 assert!(!(nan < big));
825 assert!(!(nan > big));
826 assert!(small < big);
833 let mut x = Some(3i);
834 assert!(x.mutate(|i| i+1));
835 assert_eq!(x, Some(4i));
836 assert!(x.mutate_or_set(0, |i| i+1));
837 assert_eq!(x, Some(5i));
839 assert!(!x.mutate(|i| i+1));
841 assert!(!x.mutate_or_set(0i, |i| i+1));
842 assert_eq!(x, Some(0i));
847 let v: Option<~[int]> = collect(range(0, 0)
849 assert_eq!(v, Some(box []));
851 let v: Option<~[int]> = collect(range(0, 3)
853 assert_eq!(v, Some(box [0, 1, 2]));
855 let v: Option<~[int]> = collect(range(0, 3)
856 .map(|x| if x > 1 { None } else { Some(x) }));
859 // test that it does not take more elements than it needs
860 let mut functions = [|| Some(()), || None, || fail!()];
862 let v: Option<~[()]> = collect(functions.mut_iter().map(|f| (*f)()));