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 //! [`Option`]s 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(...)`][`Some`]) or
  66 //! not ([`None`]).
  67 //!
  68 //! ```
  69 //! let optional = None;
  70 //! check_optional(optional);
  71 //!
  72 //! let optional = 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!("has value {}", p),
  78 //!         None => println!("has 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 //! if let Some(ref m) = msg {
  97 //!     println!("{}", *m);
  98 //! }
  99 //!
 100 //! // Remove the contained string, destroying the Option
 101 //! let unwrapped_msg = msg.unwrap_or("default message");
 102 //! ```
 103 //!
 104 //! Initialize a result to [`None`] before a loop:
 105 //!
 106 //! ```
 107 //! enum Kingdom { Plant(u32, &'static str), Animal(u32, &'static str) }
 108 //!
 109 //! // A list of data to search through.
 110 //! let all_the_big_things = [
 111 //!     Kingdom::Plant(250, "redwood"),
 112 //!     Kingdom::Plant(230, "noble fir"),
 113 //!     Kingdom::Plant(229, "sugar pine"),
 114 //!     Kingdom::Animal(25, "blue whale"),
 115 //!     Kingdom::Animal(19, "fin whale"),
 116 //!     Kingdom::Animal(15, "north pacific right whale"),
 117 //! ];
 118 //!
 119 //! // We're going to search for the name of the biggest animal,
 120 //! // but to start with we've just got `None`.
 121 //! let mut name_of_biggest_animal = None;
 122 //! let mut size_of_biggest_animal = 0;
 123 //! for big_thing in &all_the_big_things {
 124 //!     match *big_thing {
 125 //!         Kingdom::Animal(size, name) if size > size_of_biggest_animal => {
 126 //!             // Now we've found the name of some big animal
 127 //!             size_of_biggest_animal = size;
 128 //!             name_of_biggest_animal = Some(name);
 129 //!         }
 130 //!         Kingdom::Animal(..) | Kingdom::Plant(..) => ()
 131 //!     }
 132 //! }
 133 //!
 134 //! match name_of_biggest_animal {
 135 //!     Some(name) => println!("the biggest animal is {}", name),
 136 //!     None => println!("there are no animals :("),
 137 //! }
 138 //! ```
 139 //!
 140 //! [`Option`]: enum.Option.html
 141 //! [`Some`]: enum.Option.html#variant.Some
 142 //! [`None`]: enum.Option.html#variant.None
 143 //! [`Box<T>`]: ../../std/boxed/struct.Box.html
 144 //! [`i32`]: ../../std/primitive.i32.html
 145
 146 #![stable(feature = "rust1", since = "1.0.0")]
 147
 148 use iter::{FromIterator, FusedIterator, TrustedLen};
 149 use {mem, ops};
 150
 151 // Note that this is not a lang item per se, but it has a hidden dependency on
 152 // `Iterator`, which is one. The compiler assumes that the `next` method of
 153 // `Iterator` is an enumeration with one type parameter and two variants,
 154 // which basically means it must be `Option`.
 155
 156 /// The `Option` type. See [the module level documentation](index.html) for more.
 157 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 158 #[stable(feature = "rust1", since = "1.0.0")]
 159 pub enum Option<T> {
 160     /// No value
 161     #[stable(feature = "rust1", since = "1.0.0")]
 162     None,
 163     /// Some value `T`
 164     #[stable(feature = "rust1", since = "1.0.0")]
 165     Some(#[stable(feature = "rust1", since = "1.0.0")] T),
 166 }
 167
 168 /////////////////////////////////////////////////////////////////////////////
 169 // Type implementation
 170 /////////////////////////////////////////////////////////////////////////////
 171
 172 impl<T> Option<T> {
 173     /////////////////////////////////////////////////////////////////////////
 174     // Querying the contained values
 175     /////////////////////////////////////////////////////////////////////////
 176
 177     /// Returns `true` if the option is a [`Some`] value.
 178     ///
 179     /// # Examples
 180     ///
 181     /// ```
 182     /// let x: Option<u32> = Some(2);
 183     /// assert_eq!(x.is_some(), true);
 184     ///
 185     /// let x: Option<u32> = None;
 186     /// assert_eq!(x.is_some(), false);
 187     /// ```
 188     ///
 189     /// [`Some`]: #variant.Some
 190     #[inline]
 191     #[stable(feature = "rust1", since = "1.0.0")]
 192     pub fn is_some(&self) -> bool {
 193         match *self {
 194             Some(_) => true,
 195             None => false,
 196         }
 197     }
 198
 199     /// Returns `true` if the option is a [`None`] value.
 200     ///
 201     /// # Examples
 202     ///
 203     /// ```
 204     /// let x: Option<u32> = Some(2);
 205     /// assert_eq!(x.is_none(), false);
 206     ///
 207     /// let x: Option<u32> = None;
 208     /// assert_eq!(x.is_none(), true);
 209     /// ```
 210     ///
 211     /// [`None`]: #variant.None
 212     #[inline]
 213     #[stable(feature = "rust1", since = "1.0.0")]
 214     pub fn is_none(&self) -> bool {
 215         !self.is_some()
 216     }
 217
 218     /////////////////////////////////////////////////////////////////////////
 219     // Adapter for working with references
 220     /////////////////////////////////////////////////////////////////////////
 221
 222     /// Converts from `Option<T>` to `Option<&T>`.
 223     ///
 224     /// # Examples
 225     ///
 226     /// Convert an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, preserving the original.
 227     /// The [`map`] method takes the `self` argument by value, consuming the original,
 228     /// so this technique uses `as_ref` to first take an `Option` to a reference
 229     /// to the value inside the original.
 230     ///
 231     /// [`map`]: enum.Option.html#method.map
 232     /// [`String`]: ../../std/string/struct.String.html
 233     /// [`usize`]: ../../std/primitive.usize.html
 234     ///
 235     /// ```
 236     /// let text: Option<String> = Some("Hello, world!".to_string());
 237     /// // First, cast `Option<String>` to `Option<&String>` with `as_ref`,
 238     /// // then consume *that* with `map`, leaving `text` on the stack.
 239     /// let text_length: Option<usize> = text.as_ref().map(|s| s.len());
 240     /// println!("still can print text: {:?}", text);
 241     /// ```
 242     #[inline]
 243     #[stable(feature = "rust1", since = "1.0.0")]
 244     pub fn as_ref(&self) -> Option<&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(&mut self) -> Option<&mut T> {
 266         match *self {
 267             Some(ref mut x) => Some(x),
 268             None => None,
 269         }
 270     }
 271
 272     /////////////////////////////////////////////////////////////////////////
 273     // Getting to contained values
 274     /////////////////////////////////////////////////////////////////////////
 275
 276     /// Unwraps an option, yielding the content of a [`Some`].
 277     ///
 278     /// # Panics
 279     ///
 280     /// Panics if the value is a [`None`] with a custom panic message provided by
 281     /// `msg`.
 282     ///
 283     /// [`Some`]: #variant.Some
 284     /// [`None`]: #variant.None
 285     ///
 286     /// # Examples
 287     ///
 288     /// ```
 289     /// let x = Some("value");
 290     /// assert_eq!(x.expect("the world is ending"), "value");
 291     /// ```
 292     ///
 293     /// ```{.should_panic}
 294     /// let x: Option<&str> = None;
 295     /// x.expect("the world is ending"); // panics with `the world is ending`
 296     /// ```
 297     #[inline]
 298     #[stable(feature = "rust1", since = "1.0.0")]
 299     pub fn expect(self, msg: &str) -> T {
 300         match self {
 301             Some(val) => val,
 302             None => expect_failed(msg),
 303         }
 304     }
 305
 306     /// Moves the value `v` out of the `Option<T>` if it is [`Some(v)`].
 307     ///
 308     /// In general, because this function may panic, its use is discouraged.
 309     /// Instead, prefer to use pattern matching and handle the [`None`]
 310     /// case explicitly.
 311     ///
 312     /// # Panics
 313     ///
 314     /// Panics if the self value equals [`None`].
 315     ///
 316     /// [`Some(v)`]: #variant.Some
 317     /// [`None`]: #variant.None
 318     ///
 319     /// # Examples
 320     ///
 321     /// ```
 322     /// let x = Some("air");
 323     /// assert_eq!(x.unwrap(), "air");
 324     /// ```
 325     ///
 326     /// ```{.should_panic}
 327     /// let x: Option<&str> = None;
 328     /// assert_eq!(x.unwrap(), "air"); // fails
 329     /// ```
 330     #[inline]
 331     #[stable(feature = "rust1", since = "1.0.0")]
 332     pub fn unwrap(self) -> T {
 333         match self {
 334             Some(val) => val,
 335             None => panic!("called `Option::unwrap()` on a `None` value"),
 336         }
 337     }
 338
 339     /// Returns the contained value or a default.
 340     ///
 341     /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
 342     /// the result of a function call, it is recommended to use [`unwrap_or_else`],
 343     /// which is lazily evaluated.
 344     ///
 345     /// [`unwrap_or_else`]: #method.unwrap_or_else
 346     ///
 347     /// # Examples
 348     ///
 349     /// ```
 350     /// assert_eq!(Some("car").unwrap_or("bike"), "car");
 351     /// assert_eq!(None.unwrap_or("bike"), "bike");
 352     /// ```
 353     #[inline]
 354     #[stable(feature = "rust1", since = "1.0.0")]
 355     pub fn unwrap_or(self, def: T) -> T {
 356         match self {
 357             Some(x) => x,
 358             None => def,
 359         }
 360     }
 361
 362     /// Returns the contained value or computes it from a closure.
 363     ///
 364     /// # Examples
 365     ///
 366     /// ```
 367     /// let k = 10;
 368     /// assert_eq!(Some(4).unwrap_or_else(|| 2 * k), 4);
 369     /// assert_eq!(None.unwrap_or_else(|| 2 * k), 20);
 370     /// ```
 371     #[inline]
 372     #[stable(feature = "rust1", since = "1.0.0")]
 373     pub fn unwrap_or_else<F: FnOnce() -> T>(self, f: F) -> T {
 374         match self {
 375             Some(x) => x,
 376             None => f(),
 377         }
 378     }
 379
 380     /////////////////////////////////////////////////////////////////////////
 381     // Transforming contained values
 382     /////////////////////////////////////////////////////////////////////////
 383
 384     /// Maps an `Option<T>` to `Option<U>` by applying a function to a contained value.
 385     ///
 386     /// # Examples
 387     ///
 388     /// Convert an `Option<`[`String`]`>` into an `Option<`[`usize`]`>`, consuming the original:
 389     ///
 390     /// [`String`]: ../../std/string/struct.String.html
 391     /// [`usize`]: ../../std/primitive.usize.html
 392     ///
 393     /// ```
 394     /// let maybe_some_string = Some(String::from("Hello, World!"));
 395     /// // `Option::map` takes self *by value*, consuming `maybe_some_string`
 396     /// let maybe_some_len = maybe_some_string.map(|s| s.len());
 397     ///
 398     /// assert_eq!(maybe_some_len, Some(13));
 399     /// ```
 400     #[inline]
 401     #[stable(feature = "rust1", since = "1.0.0")]
 402     pub fn map<U, F: FnOnce(T) -> U>(self, f: F) -> Option<U> {
 403         match self {
 404             Some(x) => Some(f(x)),
 405             None => None,
 406         }
 407     }
 408
 409     /// Applies a function to the contained value (if any),
 410     /// or returns the provided default (if not).
 411     ///
 412     /// # Examples
 413     ///
 414     /// ```
 415     /// let x = Some("foo");
 416     /// assert_eq!(x.map_or(42, |v| v.len()), 3);
 417     ///
 418     /// let x: Option<&str> = None;
 419     /// assert_eq!(x.map_or(42, |v| v.len()), 42);
 420     /// ```
 421     #[inline]
 422     #[stable(feature = "rust1", since = "1.0.0")]
 423     pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
 424         match self {
 425             Some(t) => f(t),
 426             None => default,
 427         }
 428     }
 429
 430     /// Applies a function to the contained value (if any),
 431     /// or computes a default (if not).
 432     ///
 433     /// # Examples
 434     ///
 435     /// ```
 436     /// let k = 21;
 437     ///
 438     /// let x = Some("foo");
 439     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 3);
 440     ///
 441     /// let x: Option<&str> = None;
 442     /// assert_eq!(x.map_or_else(|| 2 * k, |v| v.len()), 42);
 443     /// ```
 444     #[inline]
 445     #[stable(feature = "rust1", since = "1.0.0")]
 446     pub fn map_or_else<U, D: FnOnce() -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
 447         match self {
 448             Some(t) => f(t),
 449             None => default(),
 450         }
 451     }
 452
 453     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
 454     /// [`Ok(v)`] and [`None`] to [`Err(err)`].
 455     ///
 456     /// Arguments passed to `ok_or` are eagerly evaluated; if you are passing the
 457     /// result of a function call, it is recommended to use [`ok_or_else`], which is
 458     /// lazily evaluated.
 459     ///
 460     /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 461     /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
 462     /// [`Err(err)`]: ../../std/result/enum.Result.html#variant.Err
 463     /// [`None`]: #variant.None
 464     /// [`Some(v)`]: #variant.Some
 465     /// [`ok_or_else`]: #method.ok_or_else
 466     ///
 467     /// # Examples
 468     ///
 469     /// ```
 470     /// let x = Some("foo");
 471     /// assert_eq!(x.ok_or(0), Ok("foo"));
 472     ///
 473     /// let x: Option<&str> = None;
 474     /// assert_eq!(x.ok_or(0), Err(0));
 475     /// ```
 476     #[inline]
 477     #[stable(feature = "rust1", since = "1.0.0")]
 478     pub fn ok_or<E>(self, err: E) -> Result<T, E> {
 479         match self {
 480             Some(v) => Ok(v),
 481             None => Err(err),
 482         }
 483     }
 484
 485     /// Transforms the `Option<T>` into a [`Result<T, E>`], mapping [`Some(v)`] to
 486     /// [`Ok(v)`] and [`None`] to [`Err(err())`].
 487     ///
 488     /// [`Result<T, E>`]: ../../std/result/enum.Result.html
 489     /// [`Ok(v)`]: ../../std/result/enum.Result.html#variant.Ok
 490     /// [`Err(err())`]: ../../std/result/enum.Result.html#variant.Err
 491     /// [`None`]: #variant.None
 492     /// [`Some(v)`]: #variant.Some
 493     ///
 494     /// # Examples
 495     ///
 496     /// ```
 497     /// let x = Some("foo");
 498     /// assert_eq!(x.ok_or_else(|| 0), Ok("foo"));
 499     ///
 500     /// let x: Option<&str> = None;
 501     /// assert_eq!(x.ok_or_else(|| 0), Err(0));
 502     /// ```
 503     #[inline]
 504     #[stable(feature = "rust1", since = "1.0.0")]
 505     pub fn ok_or_else<E, F: FnOnce() -> E>(self, err: F) -> Result<T, E> {
 506         match self {
 507             Some(v) => Ok(v),
 508             None => Err(err()),
 509         }
 510     }
 511
 512     /////////////////////////////////////////////////////////////////////////
 513     // Iterator constructors
 514     /////////////////////////////////////////////////////////////////////////
 515
 516     /// Returns an iterator over the possibly contained value.
 517     ///
 518     /// # Examples
 519     ///
 520     /// ```
 521     /// let x = Some(4);
 522     /// assert_eq!(x.iter().next(), Some(&4));
 523     ///
 524     /// let x: Option<u32> = None;
 525     /// assert_eq!(x.iter().next(), None);
 526     /// ```
 527     #[inline]
 528     #[stable(feature = "rust1", since = "1.0.0")]
 529     pub fn iter(&self) -> Iter<T> {
 530         Iter { inner: Item { opt: self.as_ref() } }
 531     }
 532
 533     /// Returns a mutable iterator over the possibly contained value.
 534     ///
 535     /// # Examples
 536     ///
 537     /// ```
 538     /// let mut x = Some(4);
 539     /// match x.iter_mut().next() {
 540     ///     Some(v) => *v = 42,
 541     ///     None => {},
 542     /// }
 543     /// assert_eq!(x, Some(42));
 544     ///
 545     /// let mut x: Option<u32> = None;
 546     /// assert_eq!(x.iter_mut().next(), None);
 547     /// ```
 548     #[inline]
 549     #[stable(feature = "rust1", since = "1.0.0")]
 550     pub fn iter_mut(&mut self) -> IterMut<T> {
 551         IterMut { inner: Item { opt: self.as_mut() } }
 552     }
 553
 554     /////////////////////////////////////////////////////////////////////////
 555     // Boolean operations on the values, eager and lazy
 556     /////////////////////////////////////////////////////////////////////////
 557
 558     /// Returns [`None`] if the option is [`None`], otherwise returns `optb`.
 559     ///
 560     /// [`None`]: #variant.None
 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     /// [`None`]: #variant.None
 596     ///
 597     /// # Examples
 598     ///
 599     /// ```
 600     /// fn sq(x: u32) -> Option<u32> { Some(x * x) }
 601     /// fn nope(_: u32) -> Option<u32> { None }
 602     ///
 603     /// assert_eq!(Some(2).and_then(sq).and_then(sq), Some(16));
 604     /// assert_eq!(Some(2).and_then(sq).and_then(nope), None);
 605     /// assert_eq!(Some(2).and_then(nope).and_then(sq), None);
 606     /// assert_eq!(None.and_then(sq).and_then(sq), None);
 607     /// ```
 608     #[inline]
 609     #[stable(feature = "rust1", since = "1.0.0")]
 610     pub fn and_then<U, F: FnOnce(T) -> Option<U>>(self, f: F) -> Option<U> {
 611         match self {
 612             Some(x) => f(x),
 613             None => None,
 614         }
 615     }
 616
 617     /// Returns `None` if the option is `None`, otherwise calls `predicate`
 618     /// with the wrapped value and returns:
 619     ///
 620     /// - `Some(t)` if `predicate` returns `true` (where `t` is the wrapped
 621     ///   value), and
 622     /// - `None` if `predicate` returns `false`.
 623     ///
 624     /// This function works similar to `Iterator::filter()`. You can imagine
 625     /// the `Option<T>` being an iterator over one or zero elements. `filter()`
 626     /// lets you decide which elements to keep.
 627     ///
 628     /// # Examples
 629     ///
 630     /// ```rust
 631     /// #![feature(option_filter)]
 632     ///
 633     /// fn is_even(n: &i32) -> bool {
 634     ///     n % 2 == 0
 635     /// }
 636     ///
 637     /// assert_eq!(None.filter(is_even), None);
 638     /// assert_eq!(Some(3).filter(is_even), None);
 639     /// assert_eq!(Some(4).filter(is_even), Some(4));
 640     /// ```
 641     #[inline]
 642     #[unstable(feature = "option_filter", issue = "45860")]
 643     pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
 644         if let Some(x) = self {
 645             if predicate(&x) {
 646                 return Some(x)
 647             }
 648         }
 649         None
 650     }
 651
 652     /// Returns the option if it contains a value, otherwise returns `optb`.
 653     ///
 654     /// Arguments passed to `or` are eagerly evaluated; if you are passing the
 655     /// result of a function call, it is recommended to use [`or_else`], which is
 656     /// lazily evaluated.
 657     ///
 658     /// [`or_else`]: #method.or_else
 659     ///
 660     /// # Examples
 661     ///
 662     /// ```
 663     /// let x = Some(2);
 664     /// let y = None;
 665     /// assert_eq!(x.or(y), Some(2));
 666     ///
 667     /// let x = None;
 668     /// let y = Some(100);
 669     /// assert_eq!(x.or(y), Some(100));
 670     ///
 671     /// let x = Some(2);
 672     /// let y = Some(100);
 673     /// assert_eq!(x.or(y), Some(2));
 674     ///
 675     /// let x: Option<u32> = None;
 676     /// let y = None;
 677     /// assert_eq!(x.or(y), None);
 678     /// ```
 679     #[inline]
 680     #[stable(feature = "rust1", since = "1.0.0")]
 681     pub fn or(self, optb: Option<T>) -> Option<T> {
 682         match self {
 683             Some(_) => self,
 684             None => optb,
 685         }
 686     }
 687
 688     /// Returns the option if it contains a value, otherwise calls `f` and
 689     /// returns the result.
 690     ///
 691     /// # Examples
 692     ///
 693     /// ```
 694     /// fn nobody() -> Option<&'static str> { None }
 695     /// fn vikings() -> Option<&'static str> { Some("vikings") }
 696     ///
 697     /// assert_eq!(Some("barbarians").or_else(vikings), Some("barbarians"));
 698     /// assert_eq!(None.or_else(vikings), Some("vikings"));
 699     /// assert_eq!(None.or_else(nobody), None);
 700     /// ```
 701     #[inline]
 702     #[stable(feature = "rust1", since = "1.0.0")]
 703     pub fn or_else<F: FnOnce() -> Option<T>>(self, f: F) -> Option<T> {
 704         match self {
 705             Some(_) => self,
 706             None => f(),
 707         }
 708     }
 709
 710     /////////////////////////////////////////////////////////////////////////
 711     // Entry-like operations to insert if None and return a reference
 712     /////////////////////////////////////////////////////////////////////////
 713
 714     /// Inserts `v` into the option if it is [`None`], then
 715     /// returns a mutable reference to the contained value.
 716     ///
 717     /// [`None`]: #variant.None
 718     ///
 719     /// # Examples
 720     ///
 721     /// ```
 722     /// let mut x = None;
 723     ///
 724     /// {
 725     ///     let y: &mut u32 = x.get_or_insert(5);
 726     ///     assert_eq!(y, &5);
 727     ///
 728     ///     *y = 7;
 729     /// }
 730     ///
 731     /// assert_eq!(x, Some(7));
 732     /// ```
 733     #[inline]
 734     #[stable(feature = "option_entry", since = "1.20.0")]
 735     pub fn get_or_insert(&mut self, v: T) -> &mut T {
 736         match *self {
 737             None => *self = Some(v),
 738             _ => (),
 739         }
 740
 741         match *self {
 742             Some(ref mut v) => v,
 743             _ => unreachable!(),
 744         }
 745     }
 746
 747     /// Inserts a value computed from `f` into the option if it is [`None`], then
 748     /// returns a mutable reference to the contained value.
 749     ///
 750     /// [`None`]: #variant.None
 751     ///
 752     /// # Examples
 753     ///
 754     /// ```
 755     /// let mut x = None;
 756     ///
 757     /// {
 758     ///     let y: &mut u32 = x.get_or_insert_with(|| 5);
 759     ///     assert_eq!(y, &5);
 760     ///
 761     ///     *y = 7;
 762     /// }
 763     ///
 764     /// assert_eq!(x, Some(7));
 765     /// ```
 766     #[inline]
 767     #[stable(feature = "option_entry", since = "1.20.0")]
 768     pub fn get_or_insert_with<F: FnOnce() -> T>(&mut self, f: F) -> &mut T {
 769         match *self {
 770             None => *self = Some(f()),
 771             _ => (),
 772         }
 773
 774         match *self {
 775             Some(ref mut v) => v,
 776             _ => unreachable!(),
 777         }
 778     }
 779
 780     /////////////////////////////////////////////////////////////////////////
 781     // Misc
 782     /////////////////////////////////////////////////////////////////////////
 783
 784     /// Takes the value out of the option, leaving a [`None`] in its place.
 785     ///
 786     /// [`None`]: #variant.None
 787     ///
 788     /// # Examples
 789     ///
 790     /// ```
 791     /// let mut x = Some(2);
 792     /// x.take();
 793     /// assert_eq!(x, None);
 794     ///
 795     /// let mut x: Option<u32> = None;
 796     /// x.take();
 797     /// assert_eq!(x, None);
 798     /// ```
 799     #[inline]
 800     #[stable(feature = "rust1", since = "1.0.0")]
 801     pub fn take(&mut self) -> Option<T> {
 802         mem::replace(self, None)
 803     }
 804 }
 805
 806 impl<'a, T: Clone> Option<&'a T> {
 807     /// Maps an `Option<&T>` to an `Option<T>` by cloning the contents of the
 808     /// option.
 809     ///
 810     /// # Examples
 811     ///
 812     /// ```
 813     /// let x = 12;
 814     /// let opt_x = Some(&x);
 815     /// assert_eq!(opt_x, Some(&12));
 816     /// let cloned = opt_x.cloned();
 817     /// assert_eq!(cloned, Some(12));
 818     /// ```
 819     #[stable(feature = "rust1", since = "1.0.0")]
 820     pub fn cloned(self) -> Option<T> {
 821         self.map(|t| t.clone())
 822     }
 823 }
 824
 825 impl<'a, T: Clone> Option<&'a mut T> {
 826     /// Maps an `Option<&mut T>` to an `Option<T>` by cloning the contents of the
 827     /// option.
 828     ///
 829     /// # Examples
 830     ///
 831     /// ```
 832     /// let mut x = 12;
 833     /// let opt_x = Some(&mut x);
 834     /// assert_eq!(opt_x, Some(&mut 12));
 835     /// let cloned = opt_x.cloned();
 836     /// assert_eq!(cloned, Some(12));
 837     /// ```
 838     #[stable(since = "1.26.0", feature = "option_ref_mut_cloned")]
 839     pub fn cloned(self) -> Option<T> {
 840         self.map(|t| t.clone())
 841     }
 842 }
 843
 844 impl<T: Default> Option<T> {
 845     /// Returns the contained value or a default
 846     ///
 847     /// Consumes the `self` argument then, if [`Some`], returns the contained
 848     /// value, otherwise if [`None`], returns the [default value] for that
 849     /// type.
 850     ///
 851     /// # Examples
 852     ///
 853     /// Convert a string to an integer, turning poorly-formed strings
 854     /// into 0 (the default value for integers). [`parse`] converts
 855     /// a string to any other type that implements [`FromStr`], returning
 856     /// [`None`] on error.
 857     ///
 858     /// ```
 859     /// let good_year_from_input = "1909";
 860     /// let bad_year_from_input = "190blarg";
 861     /// let good_year = good_year_from_input.parse().ok().unwrap_or_default();
 862     /// let bad_year = bad_year_from_input.parse().ok().unwrap_or_default();
 863     ///
 864     /// assert_eq!(1909, good_year);
 865     /// assert_eq!(0, bad_year);
 866     /// ```
 867     ///
 868     /// [`Some`]: #variant.Some
 869     /// [`None`]: #variant.None
 870     /// [default value]: ../default/trait.Default.html#tymethod.default
 871     /// [`parse`]: ../../std/primitive.str.html#method.parse
 872     /// [`FromStr`]: ../../std/str/trait.FromStr.html
 873     #[inline]
 874     #[stable(feature = "rust1", since = "1.0.0")]
 875     pub fn unwrap_or_default(self) -> T {
 876         match self {
 877             Some(x) => x,
 878             None => Default::default(),
 879         }
 880     }
 881 }
 882
 883 impl<T, E> Option<Result<T, E>> {
 884     /// Transposes an `Option` of a `Result` into a `Result` of an `Option`.
 885     ///
 886     /// `None` will be mapped to `Ok(None)`.
 887     /// `Some(Ok(_))` and `Some(Err(_))` will be mapped to `Ok(Some(_))` and `Err(_)`.
 888     ///
 889     /// # Examples
 890     ///
 891     /// ```
 892     /// #![feature(transpose_result)]
 893     ///
 894     /// #[derive(Debug, Eq, PartialEq)]
 895     /// struct SomeErr;
 896     ///
 897     /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
 898     /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
 899     /// assert_eq!(x, y.transpose());
 900     /// ```
 901     #[inline]
 902     #[unstable(feature = "transpose_result", issue = "47338")]
 903     pub fn transpose(self) -> Result<Option<T>, E> {
 904         match self {
 905             Some(Ok(x)) => Ok(Some(x)),
 906             Some(Err(e)) => Err(e),
 907             None => Ok(None),
 908         }
 909     }
 910 }
 911
 912 // This is a separate function to reduce the code size of .expect() itself.
 913 #[inline(never)]
 914 #[cold]
 915 fn expect_failed(msg: &str) -> ! {
 916     panic!("{}", msg)
 917 }
 918
 919
 920 /////////////////////////////////////////////////////////////////////////////
 921 // Trait implementations
 922 /////////////////////////////////////////////////////////////////////////////
 923
 924 #[stable(feature = "rust1", since = "1.0.0")]
 925 impl<T> Default for Option<T> {
 926     /// Returns [`None`].
 927     ///
 928     /// [`None`]: #variant.None
 929     #[inline]
 930     fn default() -> Option<T> { None }
 931 }
 932
 933 #[stable(feature = "rust1", since = "1.0.0")]
 934 impl<T> IntoIterator for Option<T> {
 935     type Item = T;
 936     type IntoIter = IntoIter<T>;
 937
 938     /// Returns a consuming iterator over the possibly contained value.
 939     ///
 940     /// # Examples
 941     ///
 942     /// ```
 943     /// let x = Some("string");
 944     /// let v: Vec<&str> = x.into_iter().collect();
 945     /// assert_eq!(v, ["string"]);
 946     ///
 947     /// let x = None;
 948     /// let v: Vec<&str> = x.into_iter().collect();
 949     /// assert!(v.is_empty());
 950     /// ```
 951     #[inline]
 952     fn into_iter(self) -> IntoIter<T> {
 953         IntoIter { inner: Item { opt: self } }
 954     }
 955 }
 956
 957 #[stable(since = "1.4.0", feature = "option_iter")]
 958 impl<'a, T> IntoIterator for &'a Option<T> {
 959     type Item = &'a T;
 960     type IntoIter = Iter<'a, T>;
 961
 962     fn into_iter(self) -> Iter<'a, T> {
 963         self.iter()
 964     }
 965 }
 966
 967 #[stable(since = "1.4.0", feature = "option_iter")]
 968 impl<'a, T> IntoIterator for &'a mut Option<T> {
 969     type Item = &'a mut T;
 970     type IntoIter = IterMut<'a, T>;
 971
 972     fn into_iter(self) -> IterMut<'a, T> {
 973         self.iter_mut()
 974     }
 975 }
 976
 977 #[stable(since = "1.12.0", feature = "option_from")]
 978 impl<T> From<T> for Option<T> {
 979     fn from(val: T) -> Option<T> {
 980         Some(val)
 981     }
 982 }
 983
 984 /////////////////////////////////////////////////////////////////////////////
 985 // The Option Iterators
 986 /////////////////////////////////////////////////////////////////////////////
 987
 988 #[derive(Clone, Debug)]
 989 struct Item<A> {
 990     opt: Option<A>
 991 }
 992
 993 impl<A> Iterator for Item<A> {
 994     type Item = A;
 995
 996     #[inline]
 997     fn next(&mut self) -> Option<A> {
 998         self.opt.take()
 999     }
1000
1001     #[inline]
1002     fn size_hint(&self) -> (usize, Option<usize>) {
1003         match self.opt {
1004             Some(_) => (1, Some(1)),
1005             None => (0, Some(0)),
1006         }
1007     }
1008 }
1009
1010 impl<A> DoubleEndedIterator for Item<A> {
1011     #[inline]
1012     fn next_back(&mut self) -> Option<A> {
1013         self.opt.take()
1014     }
1015 }
1016
1017 impl<A> ExactSizeIterator for Item<A> {}
1018 impl<A> FusedIterator for Item<A> {}
1019 unsafe impl<A> TrustedLen for Item<A> {}
1020
1021 /// An iterator over a reference to the [`Some`] variant of an [`Option`].
1022 ///
1023 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1024 ///
1025 /// This `struct` is created by the [`Option::iter`] function.
1026 ///
1027 /// [`Option`]: enum.Option.html
1028 /// [`Some`]: enum.Option.html#variant.Some
1029 /// [`Option::iter`]: enum.Option.html#method.iter
1030 #[stable(feature = "rust1", since = "1.0.0")]
1031 #[derive(Debug)]
1032 pub struct Iter<'a, A: 'a> { inner: Item<&'a A> }
1033
1034 #[stable(feature = "rust1", since = "1.0.0")]
1035 impl<'a, A> Iterator for Iter<'a, A> {
1036     type Item = &'a A;
1037
1038     #[inline]
1039     fn next(&mut self) -> Option<&'a A> { self.inner.next() }
1040     #[inline]
1041     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
1042 }
1043
1044 #[stable(feature = "rust1", since = "1.0.0")]
1045 impl<'a, A> DoubleEndedIterator for Iter<'a, A> {
1046     #[inline]
1047     fn next_back(&mut self) -> Option<&'a A> { self.inner.next_back() }
1048 }
1049
1050 #[stable(feature = "rust1", since = "1.0.0")]
1051 impl<'a, A> ExactSizeIterator for Iter<'a, A> {}
1052
1053 #[stable(feature = "fused", since = "1.26.0")]
1054 impl<'a, A> FusedIterator for Iter<'a, A> {}
1055
1056 #[unstable(feature = "trusted_len", issue = "37572")]
1057 unsafe impl<'a, A> TrustedLen for Iter<'a, A> {}
1058
1059 #[stable(feature = "rust1", since = "1.0.0")]
1060 impl<'a, A> Clone for Iter<'a, A> {
1061     fn clone(&self) -> Iter<'a, A> {
1062         Iter { inner: self.inner.clone() }
1063     }
1064 }
1065
1066 /// An iterator over a mutable reference to the [`Some`] variant of an [`Option`].
1067 ///
1068 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1069 ///
1070 /// This `struct` is created by the [`Option::iter_mut`] function.
1071 ///
1072 /// [`Option`]: enum.Option.html
1073 /// [`Some`]: enum.Option.html#variant.Some
1074 /// [`Option::iter_mut`]: enum.Option.html#method.iter_mut
1075 #[stable(feature = "rust1", since = "1.0.0")]
1076 #[derive(Debug)]
1077 pub struct IterMut<'a, A: 'a> { inner: Item<&'a mut A> }
1078
1079 #[stable(feature = "rust1", since = "1.0.0")]
1080 impl<'a, A> Iterator for IterMut<'a, A> {
1081     type Item = &'a mut A;
1082
1083     #[inline]
1084     fn next(&mut self) -> Option<&'a mut A> { self.inner.next() }
1085     #[inline]
1086     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
1087 }
1088
1089 #[stable(feature = "rust1", since = "1.0.0")]
1090 impl<'a, A> DoubleEndedIterator for IterMut<'a, A> {
1091     #[inline]
1092     fn next_back(&mut self) -> Option<&'a mut A> { self.inner.next_back() }
1093 }
1094
1095 #[stable(feature = "rust1", since = "1.0.0")]
1096 impl<'a, A> ExactSizeIterator for IterMut<'a, A> {}
1097
1098 #[stable(feature = "fused", since = "1.26.0")]
1099 impl<'a, A> FusedIterator for IterMut<'a, A> {}
1100 #[unstable(feature = "trusted_len", issue = "37572")]
1101 unsafe impl<'a, A> TrustedLen for IterMut<'a, A> {}
1102
1103 /// An iterator over the value in [`Some`] variant of an [`Option`].
1104 ///
1105 /// The iterator yields one value if the [`Option`] is a [`Some`], otherwise none.
1106 ///
1107 /// This `struct` is created by the [`Option::into_iter`] function.
1108 ///
1109 /// [`Option`]: enum.Option.html
1110 /// [`Some`]: enum.Option.html#variant.Some
1111 /// [`Option::into_iter`]: enum.Option.html#method.into_iter
1112 #[derive(Clone, Debug)]
1113 #[stable(feature = "rust1", since = "1.0.0")]
1114 pub struct IntoIter<A> { inner: Item<A> }
1115
1116 #[stable(feature = "rust1", since = "1.0.0")]
1117 impl<A> Iterator for IntoIter<A> {
1118     type Item = A;
1119
1120     #[inline]
1121     fn next(&mut self) -> Option<A> { self.inner.next() }
1122     #[inline]
1123     fn size_hint(&self) -> (usize, Option<usize>) { self.inner.size_hint() }
1124 }
1125
1126 #[stable(feature = "rust1", since = "1.0.0")]
1127 impl<A> DoubleEndedIterator for IntoIter<A> {
1128     #[inline]
1129     fn next_back(&mut self) -> Option<A> { self.inner.next_back() }
1130 }
1131
1132 #[stable(feature = "rust1", since = "1.0.0")]
1133 impl<A> ExactSizeIterator for IntoIter<A> {}
1134
1135 #[stable(feature = "fused", since = "1.26.0")]
1136 impl<A> FusedIterator for IntoIter<A> {}
1137
1138 #[unstable(feature = "trusted_len", issue = "37572")]
1139 unsafe impl<A> TrustedLen for IntoIter<A> {}
1140
1141 /////////////////////////////////////////////////////////////////////////////
1142 // FromIterator
1143 /////////////////////////////////////////////////////////////////////////////
1144
1145 #[stable(feature = "rust1", since = "1.0.0")]
1146 impl<A, V: FromIterator<A>> FromIterator<Option<A>> for Option<V> {
1147     /// Takes each element in the [`Iterator`]: if it is [`None`], no further
1148     /// elements are taken, and the [`None`] is returned. Should no [`None`] occur, a
1149     /// container with the values of each `Option` is returned.
1150     ///
1151     /// Here is an example which increments every integer in a vector,
1152     /// checking for overflow:
1153     ///
1154     /// ```
1155     /// use std::u16;
1156     ///
1157     /// let v = vec![1, 2];
1158     /// let res: Option<Vec<u16>> = v.iter().map(|&x: &u16|
1159     ///     if x == u16::MAX { None }
1160     ///     else { Some(x + 1) }
1161     /// ).collect();
1162     /// assert!(res == Some(vec![2, 3]));
1163     /// ```
1164     ///
1165     /// [`Iterator`]: ../iter/trait.Iterator.html
1166     /// [`None`]: enum.Option.html#variant.None
1167     #[inline]
1168     fn from_iter<I: IntoIterator<Item=Option<A>>>(iter: I) -> Option<V> {
1169         // FIXME(#11084): This could be replaced with Iterator::scan when this
1170         // performance bug is closed.
1171
1172         struct Adapter<Iter> {
1173             iter: Iter,
1174             found_none: bool,
1175         }
1176
1177         impl<T, Iter: Iterator<Item=Option<T>>> Iterator for Adapter<Iter> {
1178             type Item = T;
1179
1180             #[inline]
1181             fn next(&mut self) -> Option<T> {
1182                 match self.iter.next() {
1183                     Some(Some(value)) => Some(value),
1184                     Some(None) => {
1185                         self.found_none = true;
1186                         None
1187                     }
1188                     None => None,
1189                 }
1190             }
1191
1192             #[inline]
1193             fn size_hint(&self) -> (usize, Option<usize>) {
1194                 if self.found_none {
1195                     (0, Some(0))
1196                 } else {
1197                     let (_, upper) = self.iter.size_hint();
1198                     (0, upper)
1199                 }
1200             }
1201         }
1202
1203         let mut adapter = Adapter { iter: iter.into_iter(), found_none: false };
1204         let v: V = FromIterator::from_iter(adapter.by_ref());
1205
1206         if adapter.found_none {
1207             None
1208         } else {
1209             Some(v)
1210         }
1211     }
1212 }
1213
1214 /// The error type that results from applying the try operator (`?`) to a `None` value. If you wish
1215 /// to allow `x?` (where `x` is an `Option<T>`) to be converted into your error type, you can
1216 /// implement `impl From<NoneError>` for `YourErrorType`. In that case, `x?` within a function that
1217 /// returns `Result<_, YourErrorType>` will translate a `None` value into an `Err` result.
1218 #[unstable(feature = "try_trait", issue = "42327")]
1219 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
1220 pub struct NoneError;
1221
1222 #[unstable(feature = "try_trait", issue = "42327")]
1223 impl<T> ops::Try for Option<T> {
1224     type Ok = T;
1225     type Error = NoneError;
1226
1227     fn into_result(self) -> Result<T, NoneError> {
1228         self.ok_or(NoneError)
1229     }
1230
1231     fn from_ok(v: T) -> Self {
1232         Some(v)
1233     }
1234
1235     fn from_error(_: NoneError) -> Self {
1236         None
1237     }
1238 }