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 //! `int`. Notice that in order to use the inner `int` 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<int>> = None;
70 //! check_optional(&optional);
72 //! let optional: Option<Box<int>> = Some(box 9000);
73 //! check_optional(&optional);
75 //! fn check_optional(optional: &Option<Box<int>>) {
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(uint, &'static str), Animal(uint, &'static str) }
113 //! // A list of data to search through.
114 //! let all_the_big_things = [
115 //! Plant(250, "redwood"),
116 //! Plant(230, "noble fir"),
117 //! Plant(229, "sugar pine"),
118 //! Animal(25, "blue whale"),
119 //! Animal(19, "fin whale"),
120 //! 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 //! 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 //! Animal(..) | Plant(..) => ()
138 //! match name_of_biggest_animal {
139 //! Some(name) => println!("the biggest animal is {}", name),
140 //! None => println!("there are no animals :(")
144 use cmp::{PartialEq, Eq, Ord};
145 use default::Default;
146 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSize};
151 #[deriving(Clone, PartialEq, PartialOrd, Eq, Ord, 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<String>` 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<String> = Some("10".to_string());
198 /// // First, cast `Option<String>` to `Option<&String>` 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
243 pub fn expect(self, msg: &str) -> T {
250 /// Moves a value out of an option type and returns it, consuming the `Option`.
254 /// Fails if the self value equals `None`.
258 /// In general, because this function may fail, its use is discouraged.
259 /// Instead, prefer to use pattern matching and handle the `None`
262 pub fn unwrap(self) -> T {
265 None => fail!("called `Option::unwrap()` on a `None` value"),
269 /// Returns the contained value or a default.
271 pub fn unwrap_or(self, def: T) -> T {
278 /// Returns the contained value or computes it from a closure.
280 pub fn unwrap_or_else(self, f: || -> T) -> T {
287 /////////////////////////////////////////////////////////////////////////
288 // Transforming contained values
289 /////////////////////////////////////////////////////////////////////////
291 /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
295 /// Convert an `Option<String>` into an `Option<uint>`, consuming the original:
298 /// let num_as_str: Option<String> = Some("10".to_string());
299 /// // `Option::map` takes self *by value*, consuming `num_as_str`
300 /// let num_as_int: Option<uint> = num_as_str.map(|n| n.len());
303 pub fn map<U>(self, f: |T| -> U) -> Option<U> {
304 match self { Some(x) => Some(f(x)), None => None }
307 /// Applies a function to the contained value or returns a default.
309 pub fn map_or<U>(self, def: U, f: |T| -> U) -> U {
310 match self { None => def, Some(t) => f(t) }
313 /// Applies a function to the contained value or does nothing.
314 /// Returns true if the contained value was mutated.
315 pub fn mutate(&mut self, f: |T| -> T) -> bool {
317 *self = Some(f(self.take_unwrap()));
322 /// Applies a function to the contained value or sets it to a default.
323 /// Returns true if the contained value was mutated, or false if set to the default.
324 pub fn mutate_or_set(&mut self, def: T, f: |T| -> T) -> bool {
326 *self = Some(f(self.take_unwrap()));
334 /////////////////////////////////////////////////////////////////////////
335 // Iterator constructors
336 /////////////////////////////////////////////////////////////////////////
338 /// Returns an iterator over the possibly contained value.
340 pub fn iter<'r>(&'r self) -> Item<&'r T> {
341 Item{opt: self.as_ref()}
344 /// Returns a mutable iterator over the possibly contained value.
346 pub fn mut_iter<'r>(&'r mut self) -> Item<&'r mut T> {
347 Item{opt: self.as_mut()}
350 /// Returns a consuming iterator over the possibly contained value.
352 pub fn move_iter(self) -> Item<T> {
356 /////////////////////////////////////////////////////////////////////////
357 // Boolean operations on the values, eager and lazy
358 /////////////////////////////////////////////////////////////////////////
360 /// Returns `None` if the option is `None`, otherwise returns `optb`.
362 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
369 /// Returns `None` if the option is `None`, otherwise calls `f` with the
370 /// wrapped value and returns the result.
372 pub fn and_then<U>(self, f: |T| -> Option<U>) -> Option<U> {
379 /// Returns the option if it contains a value, otherwise returns `optb`.
381 pub fn or(self, optb: Option<T>) -> Option<T> {
388 /// Returns the option if it contains a value, otherwise calls `f` and
389 /// returns the result.
391 pub fn or_else(self, f: || -> Option<T>) -> Option<T> {
398 /////////////////////////////////////////////////////////////////////////
400 /////////////////////////////////////////////////////////////////////////
402 /// Takes the value out of the option, leaving a `None` in its place.
404 pub fn take(&mut self) -> Option<T> {
405 mem::replace(self, None)
408 /// Filters an optional value using a given function.
410 pub fn filtered(self, f: |t: &T| -> bool) -> Option<T> {
412 Some(x) => if f(&x) { Some(x) } else { None },
417 /// Applies a function zero or more times until the result is `None`.
419 pub fn while_some(self, f: |v: T| -> Option<T>) {
423 Some(x) => opt = f(x),
429 /////////////////////////////////////////////////////////////////////////
430 // Common special cases
431 /////////////////////////////////////////////////////////////////////////
433 /// The option dance. Moves a value out of an option type and returns it,
434 /// replacing the original with `None`.
438 /// Fails if the value equals `None`.
440 pub fn take_unwrap(&mut self) -> T {
443 None => fail!("called `Option::take_unwrap()` on a `None` value")
447 /// Gets an immutable reference to the value inside an option.
451 /// Fails if the value equals `None`
455 /// In general, because this function may fail, its use is discouraged
456 /// (calling `get` on `None` is akin to dereferencing a null pointer).
457 /// Instead, prefer to use pattern matching and handle the `None`
460 pub fn get_ref<'a>(&'a self) -> &'a T {
463 None => fail!("called `Option::get_ref()` on a `None` value"),
467 /// Gets a mutable reference to the value inside an option.
471 /// Fails if the value equals `None`
475 /// In general, because this function may fail, its use is discouraged
476 /// (calling `get` on `None` is akin to dereferencing a null pointer).
477 /// Instead, prefer to use pattern matching and handle the `None`
480 pub fn get_mut_ref<'a>(&'a mut self) -> &'a mut T {
482 Some(ref mut x) => x,
483 None => fail!("called `Option::get_mut_ref()` on a `None` value"),
488 impl<T: Default> Option<T> {
489 /// Returns the contained value or a default
491 /// Consumes the `self` argument then, if `Some`, returns the contained
492 /// value, otherwise if `None`, returns the default value for that
497 /// Convert a string to an integer, turning poorly-formed strings
498 /// into 0 (the default value for integers). `from_str` converts
499 /// a string to any other type that implements `FromStr`, returning
503 /// let good_year_from_input = "1909";
504 /// let bad_year_from_input = "190blarg";
505 /// let good_year = from_str(good_year_from_input).unwrap_or_default();
506 /// let bad_year = from_str(bad_year_from_input).unwrap_or_default();
508 /// assert_eq!(1909, good_year);
509 /// assert_eq!(0, bad_year);
512 pub fn unwrap_or_default(self) -> T {
515 None => Default::default()
520 /////////////////////////////////////////////////////////////////////////////
521 // Trait implementations
522 /////////////////////////////////////////////////////////////////////////////
524 impl<T> Default for Option<T> {
526 fn default() -> Option<T> { None }
529 /////////////////////////////////////////////////////////////////////////////
530 // The Option Iterator
531 /////////////////////////////////////////////////////////////////////////////
533 /// An `Option` iterator that yields either one or zero elements
535 /// The `Item` iterator is returned by the `iter`, `mut_iter` and `move_iter`
536 /// methods on `Option`.
542 impl<A> Iterator<A> for Item<A> {
544 fn next(&mut self) -> Option<A> {
549 fn size_hint(&self) -> (uint, Option<uint>) {
551 Some(_) => (1, Some(1)),
552 None => (0, Some(0)),
557 impl<A> DoubleEndedIterator<A> for Item<A> {
559 fn next_back(&mut self) -> Option<A> {
564 impl<A> ExactSize<A> for Item<A> {}
566 /////////////////////////////////////////////////////////////////////////////
568 /////////////////////////////////////////////////////////////////////////////
570 /// Takes each element in the `Iterator`: if it is `None`, no further
571 /// elements are taken, and the `None` is returned. Should no `None` occur, a
572 /// vector containing the values of each `Option` is returned.
574 /// Here is an example which increments every integer in a vector,
575 /// checking for overflow:
581 /// let v = vec!(1u, 2u);
582 /// let res: Option<Vec<uint>> = option::collect(v.iter().map(|x: &uint|
583 /// if *x == uint::MAX { None }
584 /// else { Some(x + 1) }
586 /// assert!(res == Some(vec!(2u, 3u)));
589 pub fn collect<T, Iter: Iterator<Option<T>>, V: FromIterator<T>>(iter: Iter) -> Option<V> {
590 // FIXME(#11084): This should be twice as fast once this bug is closed.
591 let mut iter = iter.scan(false, |state, x| {
601 let v: V = FromIterator::from_iter(iter.by_ref());
610 /////////////////////////////////////////////////////////////////////////////
612 /////////////////////////////////////////////////////////////////////////////
616 use realstd::vec::Vec;
617 use realstd::string::String;
620 use realstd::str::{Str, StrAllocating};
625 use slice::ImmutableVector;
631 let addr_x: *int = ::mem::transmute(&*x);
633 let y = opt.unwrap();
634 let addr_y: *int = ::mem::transmute(&*y);
635 assert_eq!(addr_x, addr_y);
641 let x = "test".to_string();
642 let addr_x = x.as_slice().as_ptr();
644 let y = opt.unwrap();
645 let addr_y = y.as_slice().as_ptr();
646 assert_eq!(addr_x, addr_y);
650 fn test_get_resource() {
659 impl ::ops::Drop for R {
662 let i = *ii.borrow();
663 *ii.borrow_mut() = i + 1;
667 fn r(i: Rc<RefCell<int>>) -> R {
673 fn realclone<T: ::realstd::clone::Clone>(t: &T) -> T {
674 use realstd::clone::Clone;
678 let i = Rc::new(RefCell::new(0));
680 let x = r(realclone(&i));
682 let _y = opt.unwrap();
684 assert_eq!(*i.borrow(), 1);
688 fn test_option_dance() {
693 y2 = y.take_unwrap();
696 assert!(y.is_none());
699 #[test] #[should_fail]
700 fn test_option_too_much_dance() {
701 let mut y = Some(marker::NoCopy);
702 let _y2 = y.take_unwrap();
703 let _y3 = y.take_unwrap();
708 let x: Option<int> = Some(1);
709 assert_eq!(x.and(Some(2)), Some(2));
710 assert_eq!(x.and(None::<int>), None);
712 let x: Option<int> = None;
713 assert_eq!(x.and(Some(2)), None);
714 assert_eq!(x.and(None::<int>), None);
719 let x: Option<int> = Some(1);
720 assert_eq!(x.and_then(|x| Some(x + 1)), Some(2));
721 assert_eq!(x.and_then(|_| None::<int>), None);
723 let x: Option<int> = None;
724 assert_eq!(x.and_then(|x| Some(x + 1)), None);
725 assert_eq!(x.and_then(|_| None::<int>), None);
730 let x: Option<int> = Some(1);
731 assert_eq!(x.or(Some(2)), Some(1));
732 assert_eq!(x.or(None), Some(1));
734 let x: Option<int> = None;
735 assert_eq!(x.or(Some(2)), Some(2));
736 assert_eq!(x.or(None), None);
741 let x: Option<int> = Some(1);
742 assert_eq!(x.or_else(|| Some(2)), Some(1));
743 assert_eq!(x.or_else(|| None), Some(1));
745 let x: Option<int> = None;
746 assert_eq!(x.or_else(|| Some(2)), Some(2));
747 assert_eq!(x.or_else(|| None), None);
751 fn test_option_while_some() {
753 Some(10).while_some(|j| {
766 assert_eq!(Some(1).unwrap(), 1);
767 let s = Some("hello".to_string()).unwrap();
768 assert_eq!(s.as_slice(), "hello");
773 fn test_unwrap_fail1() {
774 let x: Option<int> = None;
780 fn test_unwrap_fail2() {
781 let x: Option<String> = None;
786 fn test_unwrap_or() {
787 let x: Option<int> = Some(1);
788 assert_eq!(x.unwrap_or(2), 1);
790 let x: Option<int> = None;
791 assert_eq!(x.unwrap_or(2), 2);
795 fn test_unwrap_or_else() {
796 let x: Option<int> = Some(1);
797 assert_eq!(x.unwrap_or_else(|| 2), 1);
799 let x: Option<int> = None;
800 assert_eq!(x.unwrap_or_else(|| 2), 2);
805 let some_stuff = Some(42);
806 let modified_stuff = some_stuff.filtered(|&x| {x < 10});
807 assert_eq!(some_stuff.unwrap(), 42);
808 assert!(modified_stuff.is_none());
816 let mut it = x.iter();
818 assert_eq!(it.size_hint(), (1, Some(1)));
819 assert_eq!(it.next(), Some(&val));
820 assert_eq!(it.size_hint(), (0, Some(0)));
821 assert!(it.next().is_none());
829 let mut x = Some(val);
831 let mut it = x.mut_iter();
833 assert_eq!(it.size_hint(), (1, Some(1)));
837 assert_eq!(*interior, val);
840 None => assert!(false),
843 assert_eq!(it.size_hint(), (0, Some(0)));
844 assert!(it.next().is_none());
846 assert_eq!(x, Some(new_val));
851 let small = Some(1.0);
853 let nan = Some(0.0/0.0);
854 assert!(!(nan < big));
855 assert!(!(nan > big));
856 assert!(small < big);
863 let mut x = Some(3i);
864 assert!(x.mutate(|i| i+1));
865 assert_eq!(x, Some(4i));
866 assert!(x.mutate_or_set(0, |i| i+1));
867 assert_eq!(x, Some(5i));
869 assert!(!x.mutate(|i| i+1));
871 assert!(!x.mutate_or_set(0i, |i| i+1));
872 assert_eq!(x, Some(0i));
877 let v: Option<Vec<int>> = collect(range(0, 0)
879 assert!(v == Some(vec![]));
881 let v: Option<Vec<int>> = collect(range(0, 3)
883 assert!(v == Some(vec![0, 1, 2]));
885 let v: Option<Vec<int>> = collect(range(0, 3)
886 .map(|x| if x > 1 { None } else { Some(x) }));
889 // test that it does not take more elements than it needs
890 let mut functions = [|| Some(()), || None, || fail!()];
892 let v: Option<Vec<()>> = collect(functions.mut_iter().map(|f| (*f)()));