src/libcore/option.rs

   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.
   4 //
   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.
  10
  11 //! Optional values
  12 //!
  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:
  17 //!
  18 //! * Initial values
  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
  22 //!   returned on error
  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
  28 //!
  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.
  31 //!
  32 //! ```
  33 //! fn divide(numerator: f64, denominator: f64) -> Option<f64> {
  34 //!     if denominator == 0.0 {
  35 //!         None
  36 //!     } else {
  37 //!         Some(numerator / denominator)
  38 //!     }
  39 //! }
  40 //!
  41 //! // The return value of the function is an option
  42 //! let result = divide(2.0, 3.0);
  43 //!
  44 //! // Pattern match to retrieve the value
  45 //! match result {
  46 //!     // The division was valid
  47 //!     Some(x) => println!("Result: {}", x),
  48 //!     // The division was invalid
  49 //!     None    => println!("Cannot divide by 0"),
  50 //! }
  51 //! ```
  52 //!
  53 //
  54 // FIXME: Show how `Option` is used in practice, with lots of methods
  55 //
  56 //! # Options and pointers ("nullable" pointers)
  57 //!
  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>>`.
  61 //!
  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
  66 //! not (`None`).
  67 //!
  68 //! ```
  69 //! let optional: Option<Box<i32>> = None;
  70 //! check_optional(&optional);
  71 //!
  72 //! let optional: Option<Box<i32>> = Some(Box::new(9000));
  73 //! check_optional(&optional);
  74 //!
  75 //! fn check_optional(optional: &Option<Box<i32>>) {
  76 //!     match *optional {
  77 //!         Some(ref p) => println!("have value {}", p),
  78 //!         None => println!("have no value"),
  79 //!     }
  80 //! }
  81 //! ```
  82 //!
  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.
  87 //!
  88 //! # Examples
  89 //!
  90 //! Basic pattern matching on `Option`:
  91 //!
  92 //! ```
  93 //! let msg = Some("howdy");
  94 //!
  95 //! // Take a reference to the contained string
  96 //! match msg {
  97 //!     Some(ref m) => println!("{}", *m),
  98 //!     None => (),
  99 //! }
 100 //!
 101 //! // Remove the contained string, destroying the Option
 102 //! let unwrapped_msg = match msg {
 103 //!     Some(m) => m,
 104 //!     None => "default message",
 105 //! };
 106 //! ```
 107 //!
 108 //! Initialize a result to `None` before a loop:
 109 //!
 110 //! ```
 111 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
 112 //!
 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"),
 121 //! ];
 122 //!
 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 {
 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);
 133 //!         }
 134 //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
 135 //!     }
 136 //! }
 137 //!
 138 //! match name_of_biggest_animal {
 139 //!     Some(name) => println!("the biggest animal is {}", name),
 140 //!     None => println!("there are no animals :("),
 141 //! }
 142 //! ```
 143
 144 #![stable(feature = "rust1", since = "1.0.0")]
 145
 146 use self::Option::*;
 147
 148 use clone::Clone;
 149 use cmp::{Eq, Ord};
 150 use default::Default;
 151 use iter::ExactSizeIterator;
 152 use iter::{Iterator, DoubleEndedIterator, FromIterator, IntoIterator};
 153 use mem;
 154 use ops::FnOnce;
 155 use result::Result::{Ok, Err};
 156 use result::Result;
 157 use slice;
 158
 159 // Note that this is not a lang item per se, but it has a hidden dependency on
 160 // `Iterator`, which is one. The compiler assumes that the `next` method of
 161 // `Iterator` is an enumeration with one type parameter and two variants,
 162 // which basically means it must be `Option`.
 163
 164 /// The `Option` type. See [the module level documentation](index.html) for more.
 165 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 166 #[stable(feature = "rust1", since = "1.0.0")]
 167 pub enum Option<T> {
 168     /// No value
 169     #[stable(feature = "rust1", since = "1.0.0")]
 170     None,
 171     /// Some value `T`
 172     #[stable(feature = "rust1", since = "1.0.0")]
 173     Some(T)
 174 }
 175
 176 /////////////////////////////////////////////////////////////////////////////
 177 // Type implementation
 178 /////////////////////////////////////////////////////////////////////////////
 179
 180 impl<T> Option<T> {
 181     /////////////////////////////////////////////////////////////////////////
 182     // Querying the contained values
 183     /////////////////////////////////////////////////////////////////////////
 184
 185     /// Returns `true` if the option is a `Some` value
 186     ///
 187     /// # Examples
 188     ///
 189     /// ```
 190     /// let x: Option<u32> = Some(2);
 191     /// assert_eq!(x.is_some(), true);
 192     ///
 193     /// let x: Option<u32> = None;
 194     /// assert_eq!(x.is_some(), false);
 195     /// ```
 196     #[inline]
 197     #[stable(feature = "rust1", since = "1.0.0")]
 198     pub fn is_some(&self) -> bool {
 199         match *self {
 200             Some(_) => true,
 201             None => false,
 202         }
 203     }
 204
 205     /// Returns `true` if the option is a `None` value
 206     ///
 207     /// # Examples
 208     ///
 209     /// ```
 210     /// let x: Option<u32> = Some(2);
 211     /// assert_eq!(x.is_none(), false);
 212     ///
 213     /// let x: Option<u32> = None;
 214     /// assert_eq!(x.is_none(), true);
 215     /// ```
 216     #[inline]
 217     #[stable(feature = "rust1", since = "1.0.0")]
 218     pub fn is_none(&self) -> bool {
 219         !self.is_some()
 220     }
 221
 222     /////////////////////////////////////////////////////////////////////////
 223     // Adapter for working with references
 224     /////////////////////////////////////////////////////////////////////////
 225
 226     /// Converts from `Option<T>` to `Option<&T>`
 227     ///
 228     /// # Examples
 229     ///
 230     /// Convert an `Option<String>` into an `Option<usize>`, preserving the original.
 231     /// The `map` method takes the `self` argument by value, consuming the original,
 232     /// so this technique uses `as_ref` to first take an `Option` to a reference
 233     /// to the value inside the original.
 234     ///
 235     /// ```
 236     /// let num_as_str: Option<String> = Some("10".to_string());
 237     /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
 238     /// // then consume *that* with `map`, leaving `num_as_str` on the stack.
 239     /// let num_as_int: Option<usize> = num_as_str.as_ref().map(|n| n.len());
 240     /// println!("still can print num_as_str: {:?}", num_as_str);
 241     /// ```
 242     #[inline]
 243     #[stable(feature = "rust1", since = "1.0.0")]
 244     pub fn as_ref<'r>(&'r self) -> Option<&'r T> {
 245         match *self {
 246             Some(ref x) => Some(x),
 247             None => None,
 248         }
 249     }
 250
 251     /// Converts from `Option<T>` to `Option<&mut T>`
 252     ///
 253     /// # Examples
 254     ///
 255     /// ```
 256     /// let mut x = Some(2);
 257     /// match x.as_mut() {
 258     ///     Some(v) => *v = 42,
 259     ///     None => {},
 260     /// }
 261     /// assert_eq!(x, Some(42));
 262     /// ```
 263     #[inline]
 264     #[stable(feature = "rust1", since = "1.0.0")]
 265     pub fn as_mut<'r>(&'r mut self) -> Option<&'r mut T> {
 266         match *self {
 267             Some(ref mut x) => Some(x),
 268             None => None,
 269         }
 270     }
 271
 272     /// Converts from `Option<T>` to `&mut [T]` (without copying)
 273     ///
 274     /// # Examples
 275     ///
 276     /// ```
 277     /// #![feature(as_slice)]
 278     ///
 279     /// let mut x = Some("Diamonds");
 280     /// {
 281     ///     let v = x.as_mut_slice();
 282     ///     assert!(v == ["Diamonds"]);
 283     ///     v[0] = "Dirt";
 284     ///     assert!(v == ["Dirt"]);
 285     /// }
 286     /// assert_eq!(x, Some("Dirt"));
 287     /// ```
 288     #[inline]
 289     #[unstable(feature = "as_slice",
 290                reason = "waiting for mut conventions")]
 291     pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
 292         match *self {
 293             Some(ref mut x) => {
 294                 let result: &mut [T] = slice::mut_ref_slice(x);
 295                 result
 296             }
 297             None => {
 298                 let result: &mut [T] = &mut [];
 299                 result
 300             }
 301         }
 302     }
 303
 304     /////////////////////////////////////////////////////////////////////////
 305     // Getting to contained values
 306     /////////////////////////////////////////////////////////////////////////
 307
 308     /// Unwraps an option, yielding the content of a `Some`
 309     ///
 310     /// # Panics
 311     ///
 312     /// Panics if the value is a `None` with a custom panic message provided by
 313     /// `msg`.
 314     ///
 315     /// # Examples
 316     ///
 317     /// ```
 318     /// let x = Some("value");
 319     /// assert_eq!(x.expect("the world is ending"), "value");
 320     /// ```
 321     ///
 322     /// ```{.should_panic}
 323     /// let x: Option<&str> = None;
 324     /// x.expect("the world is ending"); // panics with `the world is ending`
 325     /// ```
 326     #[inline]
 327     #[stable(feature = "rust1", since = "1.0.0")]
 328     pub fn expect(self, msg: &str) -> T {
 329         match self {
 330             Some(val) => val,
 331             None => panic!("{}", msg),
 332         }
 333     }
 334
 335     /// Moves the value `v` out of the `Option<T>` if it is `Some(v)`.
 336     ///
 337     /// # Panics
 338     ///
 339     /// Panics if the self value equals `None`.
 340     ///
 341     /// # Safety note
 342     ///
 343     /// In general, because this function may panic, its use is discouraged.
 344     /// Instead, prefer to use pattern matching and handle the `None`
 345     /// case explicitly.
 346     ///
 347     /// # Examples
 348     ///
 349     /// ```
 350     /// let x = Some("air");
 351     /// assert_eq!(x.unwrap(), "air");
 352     /// ```
 353     ///
 354     /// ```{.should_panic}
 355     /// let x: Option<&str> = None;
 356     /// assert_eq!(x.unwrap(), "air"); // fails
 357     /// ```
 358     #[inline]
 359     #[stable(feature = "rust1", since = "1.0.0")]
 360     pub fn unwrap(self) -> T {
 361         match self {
 362             Some(val) => val,
 363             None => panic!("called `Option::unwrap()` on a `None` value"),
 364         }
 365     }
 366
 367     /// Returns the contained value or a default.
 368     ///
 369     /// # Examples
 370     ///
 371     /// ```
 372     /// assert_eq!(Some("car").unwrap_or("bike"), "car");
 373     /// assert_eq!(None.unwrap_or("bike"), "bike");
 374     /// ```
 375     #[inline]
 376     #[stable(feature = "rust1", since = "1.0.0")]
 377     pub fn unwrap_or(self, def: T) -> T {
 378         match self {
 379             Some(x) => x,
 380             None => def,
 381         }
 382     }
 383
 384     /// Returns the contained value or computes it from a closure.
 385     ///
 386     /// # Examples
 387     ///
 388     /// ```
 389     /// let k = 10;
 390     /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
 391     /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
 392     /// ```
 393     #[inline]
 394     #[stable(feature = "rust1", since = "1.0.0")]
 395     pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
 396         match self {
 397             Some(x) => x,
 398             None => f(),
 399         }
 400     }
 401
 402     /////////////////////////////////////////////////////////////////////////
 403     // Transforming contained values
 404     /////////////////////////////////////////////////////////////////////////
 405
 406     /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value
 407     ///
 408     /// # Examples
 409     ///
 410     /// Convert an `Option<String>` into an `Option<usize>`, consuming the original:
 411     ///
 412     /// ```
 413     /// let maybe_some_string = Some(String::from("Hello, World!"));
 414     /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
 415     /// let maybe_some_len = maybe_some_string.map(|s| s.len());
 416     ///
 417     /// assert_eq!(maybe_some_len, Some(13));
 418     /// ```
 419     #[inline]
 420     #[stable(feature = "rust1", since = "1.0.0")]
 421     pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
 422         match self {
 423             Some(x) => Some(f(x)),
 424             None => None,
 425         }
 426     }
 427
 428     /// Applies a function to the contained value (if any),
 429     /// or returns a `default` (if not).
 430     ///
 431     /// # Examples
 432     ///
 433     /// ```
 434     /// let x = Some("foo");
 435     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 436     ///
 437     /// let x: Option<&str> = None;
 438     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 439     /// ```
 440     #[inline]
 441     #[stable(feature = "rust1", since = "1.0.0")]
 442     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
 443         match self {
 444             Some(t) => f(t),
 445             None => default,
 446         }
 447     }
 448
 449     /// Applies a function to the contained value (if any),
 450     /// or computes a `default` (if not).
 451     ///
 452     /// # Examples
 453     ///
 454     /// ```
 455     /// let k = 21;
 456     ///
 457     /// let x = Some("foo");
 458     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
 459     ///
 460     /// let x: Option<&str> = None;
 461     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
 462     /// ```
 463     #[inline]
 464     #[stable(feature = "rust1", since = "1.0.0")]
 465     pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
 466         match self {
 467             Some(t) => f(t),
 468             None => default(),
 469         }
 470     }
 471
 472     /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
 473     /// `Ok(v)` and `None` to `Err(err)`.
 474     ///
 475     /// # Examples
 476     ///
 477     /// ```
 478     /// let x = Some("foo");
 479     /// assert_eq!(x.ok_or(0), Ok("foo"));
 480     ///
 481     /// let x: Option<&str> = None;
 482     /// assert_eq!(x.ok_or(0), Err(0));
 483     /// ```
 484     #[inline]
 485     #[stable(feature = "rust1", since = "1.0.0")]
 486     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
 487         match self {
 488             Some(v) => Ok(v),
 489             None => Err(err),
 490         }
 491     }
 492
 493     /// Transforms the `Option<T>` into a `Result<T, E>`, mapping `Some(v)` to
 494     /// `Ok(v)` and `None` to `Err(err())`.
 495     ///
 496     /// # Examples
 497     ///
 498     /// ```
 499     /// let x = Some("foo");
 500     /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
 501     ///
 502     /// let x: Option<&str> = None;
 503     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
 504     /// ```
 505     #[inline]
 506     #[stable(feature = "rust1", since = "1.0.0")]
 507     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
 508         match self {
 509             Some(v) => Ok(v),
 510             None => Err(err()),
 511         }
 512     }
 513
 514     /////////////////////////////////////////////////////////////////////////
 515     // Iterator constructors
 516     /////////////////////////////////////////////////////////////////////////
 517
 518     /// Returns an iterator over the possibly contained value.
 519     ///
 520     /// # Examples
 521     ///
 522     /// ```
 523     /// let x = Some(4);
 524     /// assert_eq!(x.iter().next(), Some(&4));
 525     ///
 526     /// let x: Option<u32> = None;
 527     /// assert_eq!(x.iter().next(), None);
 528     /// ```
 529     #[inline]
 530     #[stable(feature = "rust1", since = "1.0.0")]
 531     pub fn iter(&self) -> Iter<T> {
 532         Iter { inner: Item { opt: self.as_ref() } }
 533     }
 534
 535     /// Returns a mutable iterator over the possibly contained value.
 536     ///
 537     /// # Examples
 538     ///
 539     /// ```
 540     /// let mut x = Some(4);
 541     /// match x.iter_mut().next() {
 542     ///     Some(&mut ref mut v) => *v = 42,
 543     ///     None => {},
 544     /// }
 545     /// assert_eq!(x, Some(42));
 546     ///
 547     /// let mut x: Option<u32> = None;
 548     /// assert_eq!(x.iter_mut().next(), None);
 549     /// ```
 550     #[inline]
 551     #[stable(feature = "rust1", since = "1.0.0")]
 552     pub fn iter_mut(&mut self) -> IterMut<T> {
 553         IterMut { inner: Item { opt: self.as_mut() } }
 554     }
 555
 556     /////////////////////////////////////////////////////////////////////////
 557     // Boolean operations on the values, eager and lazy
 558     /////////////////////////////////////////////////////////////////////////
 559
 560     /// Returns `None` if the option is `None`, otherwise returns `optb`.
 561     ///
 562     /// # Examples
 563     ///
 564     /// ```
 565     /// let x = Some(2);
 566     /// let y: Option<&str> = None;
 567     /// assert_eq!(x.and(y), None);
 568     ///
 569     /// let x: Option<u32> = None;
 570     /// let y = Some("foo");
 571     /// assert_eq!(x.and(y), None);
 572     ///
 573     /// let x = Some(2);
 574     /// let y = Some("foo");
 575     /// assert_eq!(x.and(y), Some("foo"));
 576     ///
 577     /// let x: Option<u32> = None;
 578     /// let y: Option<&str> = None;
 579     /// assert_eq!(x.and(y), None);
 580     /// ```
 581     #[inline]
 582     #[stable(feature = "rust1", since = "1.0.0")]
 583     pub fn and<U>(self, optb: Option<U>) -> Option<U> {
 584         match self {
 585             Some(_) => optb,
 586             None => None,
 587         }
 588     }
 589
 590     /// Returns `None` if the option is `None`, otherwise calls `f` with the
 591     /// wrapped value and returns the result.
 592     ///
 593     /// Some languages call this operation flatmap.
 594     ///
 595     /// # Examples
 596     ///
 597     /// ```
 598     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 599     /// fn nope(_: u32) -> Option<u32> { None }
 600     ///
 601     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 602     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 603     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 604     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 605     /// ```
 606     #[inline]
 607     #[stable(feature = "rust1", since = "1.0.0")]
 608     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 609         match self {
 610             Some(x) => f(x),
 611             None => None,
 612         }
 613     }
 614
 615     /// Returns the option if it contains a value, otherwise returns `optb`.
 616     ///
 617     /// # Examples
 618     ///
 619     /// ```
 620     /// let x = Some(2);
 621     /// let y = None;
 622     /// assert_eq!(x.or(y), Some(2));
 623     ///
 624     /// let x = None;
 625     /// let y = Some(100);
 626     /// assert_eq!(x.or(y), Some(100));
 627     ///
 628     /// let x = Some(2);
 629     /// let y = Some(100);
 630     /// assert_eq!(x.or(y), Some(2));
 631     ///
 632     /// let x: Option<u32> = None;
 633     /// let y = None;
 634     /// assert_eq!(x.or(y), None);
 635     /// ```
 636     #[inline]
 637     #[stable(feature = "rust1", since = "1.0.0")]
 638     pub fn or(self, optb: Option<T>) -> Option<T> {
 639         match self {
 640             Some(_) => self,
 641             None => optb,
 642         }
 643     }
 644
 645     /// Returns the option if it contains a value, otherwise calls `f` and
 646     /// returns the result.
 647     ///
 648     /// # Examples
 649     ///
 650     /// ```
 651     /// fn nobody() -> Option<&'static str> { None }
 652     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 653     ///
 654     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 655     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 656     /// assert_eq!(None.or_else(nobody), None);
 657     /// ```
 658     #[inline]
 659     #[stable(feature = "rust1", since = "1.0.0")]
 660     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 661         match self {
 662             Some(_) => self,
 663             None => f(),
 664         }
 665     }
 666
 667     /////////////////////////////////////////////////////////////////////////
 668     // Misc
 669     /////////////////////////////////////////////////////////////////////////
 670
 671     /// Takes the value out of the option, leaving a `None` in its place.
 672     ///
 673     /// # Examples
 674     ///
 675     /// ```
 676     /// let mut x = Some(2);
 677     /// x.take();
 678     /// assert_eq!(x, None);
 679     ///
 680     /// let mut x: Option<u32> = None;
 681     /// x.take();
 682     /// assert_eq!(x, None);
 683     /// ```
 684     #[inline]
 685     #[stable(feature = "rust1", since = "1.0.0")]
 686     pub fn take(&mut self) -> Option<T> {
 687         mem::replace(self, None)
 688     }
 689
 690     /// Converts from `Option<T>` to `&[T]` (without copying)
 691     #[inline]
 692     #[unstable(feature = "as_slice", since = "unsure of the utility here")]
 693     pub fn as_slice<'a>(&'a self) -> &'a [T] {
 694         match *self {
 695             Some(ref x) => slice::ref_slice(x),
 696             None => {
 697                 let result: &[_] = &[];
 698                 result
 699             }
 700         }
 701     }
 702 }
 703
 704 impl<'a, T: Clone> Option<&'a T> {
 705     /// Maps an Option<&T> to an Option<T> by cloning the contents of the Option.
 706     #[stable(feature = "rust1", since = "1.0.0")]
 707     pub fn cloned(self) -> Option<T> {
 708         self.map(|t| t.clone())
 709     }
 710 }
 711
 712 impl<T: Default> Option<T> {
 713     /// Returns the contained value or a default
 714     ///
 715     /// Consumes the `self` argument then, if `Some`, returns the contained
 716     /// value, otherwise if `None`, returns the default value for that
 717     /// type.
 718     ///
 719     /// # Examples
 720     ///
 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
 724     /// `None` on error.
 725     ///
 726     /// ```
 727     /// let good_year_from_input = "1909";
 728     /// let bad_year_from_input = "190blarg";
 729     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
 730     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
 731     ///
 732     /// assert_eq!(1909, good_year);
 733     /// assert_eq!(0, bad_year);
 734     /// ```
 735     #[inline]
 736     #[stable(feature = "rust1", since = "1.0.0")]
 737     pub fn unwrap_or_default(self) -> T {
 738         match self {
 739             Some(x) => x,
 740             None => Default::default(),
 741         }
 742     }
 743 }
 744
 745 /////////////////////////////////////////////////////////////////////////////
 746 // Trait implementations
 747 /////////////////////////////////////////////////////////////////////////////
 748
 749 #[stable(feature = "rust1", since = "1.0.0")]
 750 impl<T> Default for Option<T> {
 751     #[inline]
 752     #[stable(feature = "rust1", since = "1.0.0")]
 753     fn default() -> Option<T> { None }
 754 }
 755
 756 #[stable(feature = "rust1", since = "1.0.0")]
 757 impl<T> IntoIterator for Option<T> {
 758     type Item = T;
 759     type IntoIter = IntoIter<T>;
 760
 761     /// Returns a consuming iterator over the possibly contained value.
 762     ///
 763     /// # Examples
 764     ///
 765     /// ```
 766     /// let x = Some("string");
 767     /// let v: Vec<&str> = x.into_iter().collect();
 768     /// assert_eq!(v, ["string"]);
 769     ///
 770     /// let x = None;
 771     /// let v: Vec<&str> = x.into_iter().collect();
 772     /// assert!(v.is_empty());
 773     /// ```
 774     #[inline]
 775     fn into_iter(self) -> IntoIter<T> {
 776         IntoIter { inner: Item { opt: self } }
 777     }
 778 }
 779
 780 /////////////////////////////////////////////////////////////////////////////
 781 // The Option Iterators
 782 /////////////////////////////////////////////////////////////////////////////
 783
 784 #[derive(Clone)]
 785 struct Item<A> {
 786     opt: Option<A>
 787 }
 788
 789 impl<A> Iterator for Item<A> {
 790     type Item = A;
 791
 792     #[inline]
 793     fn next(&mut self) -> Option<A> {
 794         self.opt.take()
 795     }
 796
 797     #[inline]
 798     fn size_hint(&self) -> (usize, Option<usize>) {
 799         match self.opt {
 800             Some(_) => (1, Some(1)),
 801             None => (0, Some(0)),
 802         }
 803     }
 804 }
 805
 806 impl<A> DoubleEndedIterator for Item<A> {
 807     #[inline]
 808     fn next_back(&mut self) -> Option<A> {
 809         self.opt.take()
 810     }
 811 }
 812
 813 impl<A> ExactSizeIterator for Item<A> {}
 814
 815 /// An iterator over a reference of the contained item in an Option.
 816 #[stable(feature = "rust1", since = "1.0.0")]
 817 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
 818
 819 #[stable(feature = "rust1", since = "1.0.0")]
 820 impl<'a, A> Iterator for Iter<'a, A> {
 821     type Item = &'a A;
 822
 823     #[inline]
 824     fn next(&mut self) -> Option<&'a A> { self.inner.next() }
 825     #[inline]
 826     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 827 }
 828
 829 #[stable(feature = "rust1", since = "1.0.0")]
 830 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
 831     #[inline]
 832     fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
 833 }
 834
 835 #[stable(feature = "rust1", since = "1.0.0")]
 836 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
 837
 838 #[stable(feature = "rust1", since = "1.0.0")]
 839 impl<'a, A> Clone for Iter<'a, A> {
 840     fn clone(&self) -> Iter<'a, A> {
 841         Iter { inner: self.inner.clone() }
 842     }
 843 }
 844
 845 /// An iterator over a mutable reference of the contained item in an Option.
 846 #[stable(feature = "rust1", since = "1.0.0")]
 847 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
 848
 849 #[stable(feature = "rust1", since = "1.0.0")]
 850 impl<'a, A> Iterator for IterMut<'a, A> {
 851     type Item = &'a mut A;
 852
 853     #[inline]
 854     fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
 855     #[inline]
 856     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 857 }
 858
 859 #[stable(feature = "rust1", since = "1.0.0")]
 860 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
 861     #[inline]
 862     fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
 863 }
 864
 865 #[stable(feature = "rust1", since = "1.0.0")]
 866 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
 867
 868 /// An iterator over the item contained inside an Option.
 869 #[derive(Clone)]
 870 #[stable(feature = "rust1", since = "1.0.0")]
 871 pub struct IntoIter<A> { inner: Item<A> }
 872
 873 #[stable(feature = "rust1", since = "1.0.0")]
 874 impl<A> Iterator for IntoIter<A> {
 875     type Item = A;
 876
 877     #[inline]
 878     fn next(&mut self) -> Option<A> { self.inner.next() }
 879     #[inline]
 880     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
 881 }
 882
 883 #[stable(feature = "rust1", since = "1.0.0")]
 884 impl<A> DoubleEndedIterator for IntoIter<A> {
 885     #[inline]
 886     fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
 887 }
 888
 889 #[stable(feature = "rust1", since = "1.0.0")]
 890 impl<A> ExactSizeIterator for IntoIter<A> {}
 891
 892 /////////////////////////////////////////////////////////////////////////////
 893 // FromIterator
 894 /////////////////////////////////////////////////////////////////////////////
 895
 896 #[stable(feature = "rust1", since = "1.0.0")]
 897 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
 898     /// Takes each element in the `Iterator`: if it is `None`, no further
 899     /// elements are taken, and the `None` is returned. Should no `None` occur, a
 900     /// container with the values of each `Option` is returned.
 901     ///
 902     /// Here is an example which increments every integer in a vector,
 903     /// checking for overflow:
 904     ///
 905     /// ```
 906     /// use std::u16;
 907     ///
 908     /// let v = vec!(1, 2);
 909     /// let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
 910     ///     if x == u16::MAX { None }
 911     ///     else { Some(x + 1) }
 912     /// ).collect();
 913     /// assert!(res == Some(vec!(2, 3)));
 914     /// ```
 915     #[inline]
 916     #[stable(feature = "rust1", since = "1.0.0")]
 917     fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
 918         // FIXME(#11084): This could be replaced with Iterator::scan when this
 919         // performance bug is closed.
 920
 921         struct Adapter<Iter> {
 922             iter: Iter,
 923             found_none: bool,
 924         }
 925
 926         impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
 927             type Item = T;
 928
 929             #[inline]
 930             fn next(&mut self) -> Option<T> {
 931                 match self.iter.next() {
 932                     Some(Some(value)) => Some(value),
 933                     Some(None) => {
 934                         self.found_none = true;
 935                         None
 936                     }
 937                     None => None,
 938                 }
 939             }
 940         }
 941
 942         let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
 943         let v: V = FromIterator::from_iter(adapter.by_ref());
 944
 945         if adapter.found_none {
 946             None
 947         } else {
 948             Some(v)
 949         }
 950     }
 951 }