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 //! # use std::boxed::Box;
70 //! let optional: Option<Box<int>> = None;
71 //! check_optional(&optional);
73 //! let optional: Option<Box<int>> = Some(Box::new(9000));
74 //! check_optional(&optional);
76 //! fn check_optional(optional: &Option<Box<int>>) {
78 //! Some(ref p) => println!("have value {}", p),
79 //! None => println!("have no value")
84 //! This usage of `Option` to create safe nullable pointers is so
85 //! common that Rust does special optimizations to make the
86 //! representation of `Option<Box<T>>` a single pointer. Optional pointers
87 //! in Rust are stored as efficiently as any other pointer type.
91 //! Basic pattern matching on `Option`:
94 //! let msg = Some("howdy");
96 //! // Take a reference to the contained string
98 //! Some(ref m) => println!("{}", *m),
102 //! // Remove the contained string, destroying the Option
103 //! let unwrapped_msg = match msg {
105 //! None => "default message"
109 //! Initialize a result to `None` before a loop:
112 //! enum Kingdom { Plant(uint, &'static str), Animal(uint, &'static str) }
114 //! // A list of data to search through.
115 //! let all_the_big_things = [
116 //! Kingdom::Plant(250, "redwood"),
117 //! Kingdom::Plant(230, "noble fir"),
118 //! Kingdom::Plant(229, "sugar pine"),
119 //! Kingdom::Animal(25, "blue whale"),
120 //! Kingdom::Animal(19, "fin whale"),
121 //! Kingdom::Animal(15, "north pacific right whale"),
124 //! // We're going to search for the name of the biggest animal,
125 //! // but to start with we've just got `None`.
126 //! let mut name_of_biggest_animal = None;
127 //! let mut size_of_biggest_animal = 0;
128 //! for big_thing in all_the_big_things.iter() {
129 //! match *big_thing {
130 //! Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
131 //! // Now we've found the name of some big animal
132 //! size_of_biggest_animal = size;
133 //! name_of_biggest_animal = Some(name);
135 //! Kingdom::Animal(..) | Kingdom::Plant(..) => ()
139 //! match name_of_biggest_animal {
140 //! Some(name) => println!("the biggest animal is {}", name),
141 //! None => println!("there are no animals :(")
151 use default::Default;
152 use iter::{ExactSizeIterator};
153 use iter::{Iterator, IteratorExt, DoubleEndedIterator, FromIterator};
155 use ops::{Deref, FnOnce};
156 use result::Result::{Ok, Err};
161 // Note that this is not a lang item per se, but it has a hidden dependency on
162 // `Iterator`, which is one. The compiler assumes that the `next` method of
163 // `Iterator` is an enumeration with one type parameter and two variants,
164 // which basically means it must be `Option`.
166 /// The `Option` type.
167 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Show, Hash)]
178 /////////////////////////////////////////////////////////////////////////////
179 // Type implementation
180 /////////////////////////////////////////////////////////////////////////////
183 /////////////////////////////////////////////////////////////////////////
184 // Querying the contained values
185 /////////////////////////////////////////////////////////////////////////
187 /// Returns `true` if the option is a `Some` value
192 /// let x: Option<uint> = Some(2);
193 /// assert_eq!(x.is_some(), true);
195 /// let x: Option<uint> = None;
196 /// assert_eq!(x.is_some(), false);
200 pub fn is_some(&self) -> bool {
207 /// Returns `true` if the option is a `None` value
212 /// let x: Option<uint> = Some(2);
213 /// assert_eq!(x.is_none(), false);
215 /// let x: Option<uint> = None;
216 /// assert_eq!(x.is_none(), true);
220 pub fn is_none(&self) -> bool {
224 /////////////////////////////////////////////////////////////////////////
225 // Adapter for working with references
226 /////////////////////////////////////////////////////////////////////////
228 /// Convert from `Option<T>` to `Option<&T>`
232 /// Convert an `Option<String>` into an `Option<int>`, preserving the original.
233 /// The `map` method takes the `self` argument by value, consuming the original,
234 /// so this technique uses `as_ref` to first take an `Option` to a reference
235 /// to the value inside the original.
238 /// let num_as_str: Option<String> = Some("10".to_string());
239 /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
240 /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
241 /// let num_as_int: Option<uint> = num_as_str.as_ref().map(|n| n.len());
242 /// println!("still can print num_as_str: {:?}", num_as_str);
246 pub fn as_ref<'r>(&'r self) -> Option<&'r T> {
248 Some(ref x) => Some(x),
253 /// Convert from `Option<T>` to `Option<&mut T>`
258 /// let mut x = Some(2u);
259 /// match x.as_mut() {
260 /// Some(v) => *v = 42,
263 /// assert_eq!(x, Some(42u));
267 pub fn as_mut<'r>(&'r mut self) -> Option<&'r mut T> {
269 Some(ref mut x) => Some(x),
274 /// Convert from `Option<T>` to `&mut [T]` (without copying)
279 /// let mut x = Some("Diamonds");
281 /// let v = x.as_mut_slice();
282 /// assert!(v == ["Diamonds"]);
284 /// assert!(v == ["Dirt"]);
286 /// assert_eq!(x, Some("Dirt"));
289 #[unstable = "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`
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
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");
376 pub fn unwrap_or(self, def: T) -> T {
383 /// Returns the contained value or computes it from a closure.
389 /// assert_eq!(Some(4u).unwrap_or_else(|| 2 * k), 4u);
390 /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20u);
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<uint>`, 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<uint> = num_as_str.map(|n| n.len());
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(42u, |v| v.len()), 3u);
433 /// let x: Option<&str> = None;
434 /// assert_eq!(x.map_or(42u, |v| v.len()), 42u);
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()), 3u);
455 /// let x: Option<&str> = None;
456 /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42u);
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(0i), Ok("foo"));
476 /// let x: Option<&str> = None;
477 /// assert_eq!(x.ok_or(0i), Err(0i));
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(|| 0i), Ok("foo"));
497 /// let x: Option<&str> = None;
498 /// assert_eq!(x.ok_or_else(|| 0i), Err(0i));
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.
518 /// let x = Some(4u);
519 /// assert_eq!(x.iter().next(), Some(&4));
521 /// let x: Option<uint> = None;
522 /// assert_eq!(x.iter().next(), None);
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(4u);
536 /// match x.iter_mut().next() {
537 /// Some(&mut ref mut v) => *v = 42u,
540 /// assert_eq!(x, Some(42));
542 /// let mut x: Option<uint> = None;
543 /// assert_eq!(x.iter_mut().next(), None);
546 #[unstable = "waiting for iterator conventions"]
547 pub fn iter_mut(&mut self) -> IterMut<T> {
548 IterMut { inner: Item { opt: self.as_mut() } }
551 /// Returns a consuming iterator over the possibly contained value.
556 /// let x = Some("string");
557 /// let v: Vec<&str> = x.into_iter().collect();
558 /// assert_eq!(v, vec!["string"]);
561 /// let v: Vec<&str> = x.into_iter().collect();
562 /// assert!(v.is_empty());
566 pub fn into_iter(self) -> IntoIter<T> {
567 IntoIter { inner: Item { opt: self } }
570 /////////////////////////////////////////////////////////////////////////
571 // Boolean operations on the values, eager and lazy
572 /////////////////////////////////////////////////////////////////////////
574 /// Returns `None` if the option is `None`, otherwise returns `optb`.
579 /// let x = Some(2u);
580 /// let y: Option<&str> = None;
581 /// assert_eq!(x.and(y), None);
583 /// let x: Option<uint> = None;
584 /// let y = Some("foo");
585 /// assert_eq!(x.and(y), None);
587 /// let x = Some(2u);
588 /// let y = Some("foo");
589 /// assert_eq!(x.and(y), Some("foo"));
591 /// let x: Option<uint> = None;
592 /// let y: Option<&str> = None;
593 /// assert_eq!(x.and(y), None);
597 pub fn and<U>(self, optb: Option<U>) -> Option<U> {
604 /// Returns `None` if the option is `None`, otherwise calls `f` with the
605 /// wrapped value and returns the result.
610 /// fn sq(x: uint) -> Option<uint> { Some(x * x) }
611 /// fn nope(_: uint) -> Option<uint> { None }
613 /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
614 /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
615 /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
616 /// assert_eq!(None.and_then(sq).and_then(sq), None);
620 pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
627 /// Returns the option if it contains a value, otherwise returns `optb`.
632 /// let x = Some(2u);
634 /// assert_eq!(x.or(y), Some(2u));
637 /// let y = Some(100u);
638 /// assert_eq!(x.or(y), Some(100u));
640 /// let x = Some(2u);
641 /// let y = Some(100u);
642 /// assert_eq!(x.or(y), Some(2u));
644 /// let x: Option<uint> = None;
646 /// assert_eq!(x.or(y), None);
650 pub fn or(self, optb: Option<T>) -> Option<T> {
657 /// Returns the option if it contains a value, otherwise calls `f` and
658 /// returns the result.
663 /// fn nobody() -> Option<&'static str> { None }
664 /// fn vikings() -> Option<&'static str> { Some("vikings") }
666 /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
667 /// assert_eq!(None.or_else(vikings), Some("vikings"));
668 /// assert_eq!(None.or_else(nobody), None);
672 pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
679 /////////////////////////////////////////////////////////////////////////
681 /////////////////////////////////////////////////////////////////////////
683 /// Takes the value out of the option, leaving a `None` in its place.
688 /// let mut x = Some(2u);
690 /// assert_eq!(x, None);
692 /// let mut x: Option<uint> = None;
694 /// assert_eq!(x, None);
698 pub fn take(&mut self) -> Option<T> {
699 mem::replace(self, None)
703 impl<'a, T: Clone, D: Deref<Target=T>> Option<D> {
704 /// Maps an Option<D> to an Option<T> by dereffing and cloning the contents of the Option.
705 /// Useful for converting an Option<&T> to an Option<T>.
706 #[unstable = "recently added as part of collections reform"]
707 pub fn cloned(self) -> Option<T> {
708 self.map(|t| t.deref().clone())
712 impl<T: Default> Option<T> {
713 /// Returns the contained value or a default
715 /// Consumes the `self` argument then, if `Some`, returns the contained
716 /// value, otherwise if `None`, returns the default value for that
721 /// Convert a string to an integer, turning poorly-formed strings
722 /// into 0 (the default value for integers). `parse` converts
723 /// a string to any other type that implements `FromStr`, returning
727 /// let good_year_from_input = "1909";
728 /// let bad_year_from_input = "190blarg";
729 /// let good_year = good_year_from_input.parse().unwrap_or_default();
730 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
732 /// assert_eq!(1909i, good_year);
733 /// assert_eq!(0i, bad_year);
737 pub fn unwrap_or_default(self) -> T {
740 None => Default::default()
745 /////////////////////////////////////////////////////////////////////////////
746 // Trait implementations
747 /////////////////////////////////////////////////////////////////////////////
749 #[unstable = "waiting on the stability of the trait itself"]
750 impl<T> AsSlice<T> for Option<T> {
751 /// Convert from `Option<T>` to `&[T]` (without copying)
753 fn as_slice<'a>(&'a self) -> &'a [T] {
755 Some(ref x) => slice::ref_slice(x),
757 let result: &[_] = &[];
765 impl<T> Default for Option<T> {
768 fn default() -> Option<T> { None }
771 /////////////////////////////////////////////////////////////////////////////
772 // The Option Iterators
773 /////////////////////////////////////////////////////////////////////////////
780 impl<A> Iterator for Item<A> {
784 fn next(&mut self) -> Option<A> {
789 fn size_hint(&self) -> (uint, Option<uint>) {
791 Some(_) => (1, Some(1)),
792 None => (0, Some(0)),
797 impl<A> DoubleEndedIterator for Item<A> {
799 fn next_back(&mut self) -> Option<A> {
804 impl<A> ExactSizeIterator for Item<A> {}
806 /// An iterator over a reference of the contained item in an Option.
808 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
811 impl<'a, A> Iterator for Iter<'a, A> {
815 fn next(&mut self) -> Option<&'a A> { self.inner.next() }
817 fn size_hint(&self) -> (uint, Option<uint>) { self.inner.size_hint() }
821 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
823 fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
827 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
830 impl<'a, A> Clone for Iter<'a, A> {
831 fn clone(&self) -> Iter<'a, A> {
832 Iter { inner: self.inner.clone() }
836 /// An iterator over a mutable reference of the contained item in an Option.
838 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
841 impl<'a, A> Iterator for IterMut<'a, A> {
842 type Item = &'a mut A;
845 fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
847 fn size_hint(&self) -> (uint, Option<uint>) { self.inner.size_hint() }
851 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
853 fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
857 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
859 /// An iterator over the item contained inside an Option.
861 pub struct IntoIter<A> { inner: Item<A> }
864 impl<A> Iterator for IntoIter<A> {
868 fn next(&mut self) -> Option<A> { self.inner.next() }
870 fn size_hint(&self) -> (uint, Option<uint>) { self.inner.size_hint() }
874 impl<A> DoubleEndedIterator for IntoIter<A> {
876 fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
880 impl<A> ExactSizeIterator for IntoIter<A> {}
882 /////////////////////////////////////////////////////////////////////////////
884 /////////////////////////////////////////////////////////////////////////////
887 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
888 /// Takes each element in the `Iterator`: if it is `None`, no further
889 /// elements are taken, and the `None` is returned. Should no `None` occur, a
890 /// container with the values of each `Option` is returned.
892 /// Here is an example which increments every integer in a vector,
893 /// checking for overflow:
898 /// let v = vec!(1u, 2u);
899 /// let res: Option<Vec<uint>> = v.iter().map(|&x: &uint|
900 /// if x == uint::MAX { None }
901 /// else { Some(x + 1) }
903 /// assert!(res == Some(vec!(2u, 3u)));
907 fn from_iter<I: Iterator<Item=Option<A>>>(iter: I) -> Option<V> {
908 // FIXME(#11084): This could be replaced with Iterator::scan when this
909 // performance bug is closed.
911 struct Adapter<Iter> {
916 impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
920 fn next(&mut self) -> Option<T> {
921 match self.iter.next() {
922 Some(Some(value)) => Some(value),
924 self.found_none = true;
932 let mut adapter = Adapter { iter: iter, found_none: false };
933 let v: V = FromIterator::from_iter(adapter.by_ref());
935 if adapter.found_none {