src/libcore/result.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 //! Error handling with the `Result` type.
  12 //!
  13 //! `Result<T, E>` is the type used for returning and propagating
  14 //! errors. It is an enum with the variants, `Ok(T)`, representing
  15 //! success and containing a value, and `Err(E)`, representing error
  16 //! and containing an error value.
  17 //!
  18 //! ```
  19 //! # #[allow(dead_code)]
  20 //! enum Result<T, E> {
  21 //!    Ok(T),
  22 //!    Err(E)
  23 //! }
  24 //! ```
  25 //!
  26 //! Functions return `Result` whenever errors are expected and
  27 //! recoverable. In the `std` crate `Result` is most prominently used
  28 //! for [I/O](../../std/io/index.html).
  29 //!
  30 //! A simple function returning `Result` might be
  31 //! defined and used like so:
  32 //!
  33 //! ```
  34 //! #[derive(Debug)]
  35 //! enum Version { Version1, Version2 }
  36 //!
  37 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
  38 //!     match header.get(0) {
  39 //!         None => Err("invalid header length"),
  40 //!         Some(&1) => Ok(Version::Version1),
  41 //!         Some(&2) => Ok(Version::Version2),
  42 //!         Some(_) => Err("invalid version")
  43 //!     }
  44 //! }
  45 //!
  46 //! let version = parse_version(&[1, 2, 3, 4]);
  47 //! match version {
  48 //!     Ok(v) => println!("working with version: {:?}", v),
  49 //!     Err(e) => println!("error parsing header: {:?}", e),
  50 //! }
  51 //! ```
  52 //!
  53 //! Pattern matching on `Result`s is clear and straightforward for
  54 //! simple cases, but `Result` comes with some convenience methods
  55 //! that make working with it more succinct.
  56 //!
  57 //! ```
  58 //! let good_result: Result<i32, i32> = Ok(10);
  59 //! let bad_result: Result<i32, i32> = Err(10);
  60 //!
  61 //! // The `is_ok` and `is_err` methods do what they say.
  62 //! assert!(good_result.is_ok() && !good_result.is_err());
  63 //! assert!(bad_result.is_err() && !bad_result.is_ok());
  64 //!
  65 //! // `map` consumes the `Result` and produces another.
  66 //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
  67 //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
  68 //!
  69 //! // Use `and_then` to continue the computation.
  70 //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
  71 //!
  72 //! // Use `or_else` to handle the error.
  73 //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
  74 //!
  75 //! // Consume the result and return the contents with `unwrap`.
  76 //! let final_awesome_result = good_result.unwrap();
  77 //! ```
  78 //!
  79 //! # Results must be used
  80 //!
  81 //! A common problem with using return values to indicate errors is
  82 //! that it is easy to ignore the return value, thus failing to handle
  83 //! the error. Result is annotated with the #[must_use] attribute,
  84 //! which will cause the compiler to issue a warning when a Result
  85 //! value is ignored. This makes `Result` especially useful with
  86 //! functions that may encounter errors but don't otherwise return a
  87 //! useful value.
  88 //!
  89 //! Consider the `write_all` method defined for I/O types
  90 //! by the [`Write`](../../std/io/trait.Write.html) trait:
  91 //!
  92 //! ```
  93 //! use std::io;
  94 //!
  95 //! trait Write {
  96 //!     fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
  97 //! }
  98 //! ```
  99 //!
 100 //! *Note: The actual definition of `Write` uses `io::Result`, which
 101 //! is just a synonym for `Result<T, io::Error>`.*
 102 //!
 103 //! This method doesn't produce a value, but the write may
 104 //! fail. It's crucial to handle the error case, and *not* write
 105 //! something like this:
 106 //!
 107 //! ```no_run
 108 //! # #![allow(unused_must_use)] // \o/
 109 //! use std::fs::File;
 110 //! use std::io::prelude::*;
 111 //!
 112 //! let mut file = File::create("valuable_data.txt").unwrap();
 113 //! // If `write_all` errors, then we'll never know, because the return
 114 //! // value is ignored.
 115 //! file.write_all(b"important message");
 116 //! ```
 117 //!
 118 //! If you *do* write that in Rust, the compiler will give you a
 119 //! warning (by default, controlled by the `unused_must_use` lint).
 120 //!
 121 //! You might instead, if you don't want to handle the error, simply
 122 //! assert success with `expect`. This will panic if the
 123 //! write fails, providing a marginally useful message indicating why:
 124 //!
 125 //! ```{.no_run}
 126 //! use std::fs::File;
 127 //! use std::io::prelude::*;
 128 //!
 129 //! let mut file = File::create("valuable_data.txt").unwrap();
 130 //! file.write_all(b"important message").expect("failed to write message");
 131 //! ```
 132 //!
 133 //! You might also simply assert success:
 134 //!
 135 //! ```{.no_run}
 136 //! # use std::fs::File;
 137 //! # use std::io::prelude::*;
 138 //! # let mut file = File::create("valuable_data.txt").unwrap();
 139 //! assert!(file.write_all(b"important message").is_ok());
 140 //! ```
 141 //!
 142 //! Or propagate the error up the call stack with `try!`:
 143 //!
 144 //! ```
 145 //! # use std::fs::File;
 146 //! # use std::io::prelude::*;
 147 //! # use std::io;
 148 //! # #[allow(dead_code)]
 149 //! fn write_message() -> io::Result<()> {
 150 //!     let mut file = try!(File::create("valuable_data.txt"));
 151 //!     try!(file.write_all(b"important message"));
 152 //!     Ok(())
 153 //! }
 154 //! ```
 155 //!
 156 //! # The `try!` macro
 157 //!
 158 //! When writing code that calls many functions that return the
 159 //! `Result` type, the error handling can be tedious.  The `try!`
 160 //! macro hides some of the boilerplate of propagating errors up the
 161 //! call stack.
 162 //!
 163 //! It replaces this:
 164 //!
 165 //! ```
 166 //! # #![allow(dead_code)]
 167 //! use std::fs::File;
 168 //! use std::io::prelude::*;
 169 //! use std::io;
 170 //!
 171 //! struct Info {
 172 //!     name: String,
 173 //!     age: i32,
 174 //!     rating: i32,
 175 //! }
 176 //!
 177 //! fn write_info(info: &Info) -> io::Result<()> {
 178 //!     let mut file = try!(File::create("my_best_friends.txt"));
 179 //!     // Early return on error
 180 //!     if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
 181 //!         return Err(e)
 182 //!     }
 183 //!     if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
 184 //!         return Err(e)
 185 //!     }
 186 //!     if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
 187 //!         return Err(e)
 188 //!     }
 189 //!     Ok(())
 190 //! }
 191 //! ```
 192 //!
 193 //! With this:
 194 //!
 195 //! ```
 196 //! # #![allow(dead_code)]
 197 //! use std::fs::File;
 198 //! use std::io::prelude::*;
 199 //! use std::io;
 200 //!
 201 //! struct Info {
 202 //!     name: String,
 203 //!     age: i32,
 204 //!     rating: i32,
 205 //! }
 206 //!
 207 //! fn write_info(info: &Info) -> io::Result<()> {
 208 //!     let mut file = try!(File::create("my_best_friends.txt"));
 209 //!     // Early return on error
 210 //!     try!(file.write_all(format!("name: {}\n", info.name).as_bytes()));
 211 //!     try!(file.write_all(format!("age: {}\n", info.age).as_bytes()));
 212 //!     try!(file.write_all(format!("rating: {}\n", info.rating).as_bytes()));
 213 //!     Ok(())
 214 //! }
 215 //! ```
 216 //!
 217 //! *It's much nicer!*
 218 //!
 219 //! Wrapping an expression in `try!` will result in the unwrapped
 220 //! success (`Ok`) value, unless the result is `Err`, in which case
 221 //! `Err` is returned early from the enclosing function. Its simple definition
 222 //! makes it clear:
 223 //!
 224 //! ```
 225 //! macro_rules! try {
 226 //!     ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
 227 //! }
 228 //! ```
 229 //!
 230 //! `try!` is imported by the prelude and is available everywhere, but it can only
 231 //! be used in functions that return `Result` because of the early return of
 232 //! `Err` that it provides.
 233
 234 #![stable(feature = "rust1", since = "1.0.0")]
 235
 236 use self::Result::{Ok, Err};
 237
 238 use clone::Clone;
 239 use fmt;
 240 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSizeIterator, IntoIterator};
 241 use ops::FnOnce;
 242 use option::Option::{self, None, Some};
 243
 244 /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
 245 ///
 246 /// See the [`std::result`](index.html) module documentation for details.
 247 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 248 #[must_use]
 249 #[stable(feature = "rust1", since = "1.0.0")]
 250 pub enum Result<T, E> {
 251     /// Contains the success value
 252     #[stable(feature = "rust1", since = "1.0.0")]
 253     Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
 254
 255     /// Contains the error value
 256     #[stable(feature = "rust1", since = "1.0.0")]
 257     Err(#[stable(feature = "rust1", since = "1.0.0")] E)
 258 }
 259
 260 /////////////////////////////////////////////////////////////////////////////
 261 // Type implementation
 262 /////////////////////////////////////////////////////////////////////////////
 263
 264 impl<T, E> Result<T, E> {
 265     /////////////////////////////////////////////////////////////////////////
 266     // Querying the contained values
 267     /////////////////////////////////////////////////////////////////////////
 268
 269     /// Returns true if the result is `Ok`
 270     ///
 271     /// # Examples
 272     ///
 273     /// ```
 274     /// let x: Result<i32, &str> = Ok(-3);
 275     /// assert_eq!(x.is_ok(), true);
 276     ///
 277     /// let x: Result<i32, &str> = Err("Some error message");
 278     /// assert_eq!(x.is_ok(), false);
 279     /// ```
 280     #[inline]
 281     #[stable(feature = "rust1", since = "1.0.0")]
 282     pub fn is_ok(&self) -> bool {
 283         match *self {
 284             Ok(_) => true,
 285             Err(_) => false
 286         }
 287     }
 288
 289     /// Returns true if the result is `Err`
 290     ///
 291     /// # Examples
 292     ///
 293     /// ```
 294     /// let x: Result<i32, &str> = Ok(-3);
 295     /// assert_eq!(x.is_err(), false);
 296     ///
 297     /// let x: Result<i32, &str> = Err("Some error message");
 298     /// assert_eq!(x.is_err(), true);
 299     /// ```
 300     #[inline]
 301     #[stable(feature = "rust1", since = "1.0.0")]
 302     pub fn is_err(&self) -> bool {
 303         !self.is_ok()
 304     }
 305
 306     /////////////////////////////////////////////////////////////////////////
 307     // Adapter for each variant
 308     /////////////////////////////////////////////////////////////////////////
 309
 310     /// Converts from `Result<T, E>` to `Option<T>`
 311     ///
 312     /// Converts `self` into an `Option<T>`, consuming `self`,
 313     /// and discarding the error, if any.
 314     ///
 315     /// # Examples
 316     ///
 317     /// ```
 318     /// let x: Result<u32, &str> = Ok(2);
 319     /// assert_eq!(x.ok(), Some(2));
 320     ///
 321     /// let x: Result<u32, &str> = Err("Nothing here");
 322     /// assert_eq!(x.ok(), None);
 323     /// ```
 324     #[inline]
 325     #[stable(feature = "rust1", since = "1.0.0")]
 326     pub fn ok(self) -> Option<T> {
 327         match self {
 328             Ok(x)  => Some(x),
 329             Err(_) => None,
 330         }
 331     }
 332
 333     /// Converts from `Result<T, E>` to `Option<E>`
 334     ///
 335     /// Converts `self` into an `Option<E>`, consuming `self`,
 336     /// and discarding the success value, if any.
 337     ///
 338     /// # Examples
 339     ///
 340     /// ```
 341     /// let x: Result<u32, &str> = Ok(2);
 342     /// assert_eq!(x.err(), None);
 343     ///
 344     /// let x: Result<u32, &str> = Err("Nothing here");
 345     /// assert_eq!(x.err(), Some("Nothing here"));
 346     /// ```
 347     #[inline]
 348     #[stable(feature = "rust1", since = "1.0.0")]
 349     pub fn err(self) -> Option<E> {
 350         match self {
 351             Ok(_)  => None,
 352             Err(x) => Some(x),
 353         }
 354     }
 355
 356     /////////////////////////////////////////////////////////////////////////
 357     // Adapter for working with references
 358     /////////////////////////////////////////////////////////////////////////
 359
 360     /// Converts from `Result<T, E>` to `Result<&T, &E>`
 361     ///
 362     /// Produces a new `Result`, containing a reference
 363     /// into the original, leaving the original in place.
 364     ///
 365     /// ```
 366     /// let x: Result<u32, &str> = Ok(2);
 367     /// assert_eq!(x.as_ref(), Ok(&2));
 368     ///
 369     /// let x: Result<u32, &str> = Err("Error");
 370     /// assert_eq!(x.as_ref(), Err(&"Error"));
 371     /// ```
 372     #[inline]
 373     #[stable(feature = "rust1", since = "1.0.0")]
 374     pub fn as_ref(&self) -> Result<&T, &E> {
 375         match *self {
 376             Ok(ref x) => Ok(x),
 377             Err(ref x) => Err(x),
 378         }
 379     }
 380
 381     /// Converts from `Result<T, E>` to `Result<&mut T, &mut E>`
 382     ///
 383     /// ```
 384     /// fn mutate(r: &mut Result<i32, i32>) {
 385     ///     match r.as_mut() {
 386     ///         Ok(&mut ref mut v) => *v = 42,
 387     ///         Err(&mut ref mut e) => *e = 0,
 388     ///     }
 389     /// }
 390     ///
 391     /// let mut x: Result<i32, i32> = Ok(2);
 392     /// mutate(&mut x);
 393     /// assert_eq!(x.unwrap(), 42);
 394     ///
 395     /// let mut x: Result<i32, i32> = Err(13);
 396     /// mutate(&mut x);
 397     /// assert_eq!(x.unwrap_err(), 0);
 398     /// ```
 399     #[inline]
 400     #[stable(feature = "rust1", since = "1.0.0")]
 401     pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
 402         match *self {
 403             Ok(ref mut x) => Ok(x),
 404             Err(ref mut x) => Err(x),
 405         }
 406     }
 407
 408     /////////////////////////////////////////////////////////////////////////
 409     // Transforming contained values
 410     /////////////////////////////////////////////////////////////////////////
 411
 412     /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
 413     /// contained `Ok` value, leaving an `Err` value untouched.
 414     ///
 415     /// This function can be used to compose the results of two functions.
 416     ///
 417     /// # Examples
 418     ///
 419     /// Print the numbers on each line of a string multiplied by two.
 420     ///
 421     /// ```
 422     /// let line = "1\n2\n3\n4\n";
 423     ///
 424     /// for num in line.lines() {
 425     ///     match num.parse::<i32>().map(|i| i * 2) {
 426     ///         Ok(n) => println!("{}", n),
 427     ///         Err(..) => {}
 428     ///     }
 429     /// }
 430     /// ```
 431     #[inline]
 432     #[stable(feature = "rust1", since = "1.0.0")]
 433     pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
 434         match self {
 435             Ok(t) => Ok(op(t)),
 436             Err(e) => Err(e)
 437         }
 438     }
 439
 440     /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
 441     /// contained `Err` value, leaving an `Ok` value untouched.
 442     ///
 443     /// This function can be used to pass through a successful result while handling
 444     /// an error.
 445     ///
 446     /// # Examples
 447     ///
 448     /// ```
 449     /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
 450     ///
 451     /// let x: Result<u32, u32> = Ok(2);
 452     /// assert_eq!(x.map_err(stringify), Ok(2));
 453     ///
 454     /// let x: Result<u32, u32> = Err(13);
 455     /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
 456     /// ```
 457     #[inline]
 458     #[stable(feature = "rust1", since = "1.0.0")]
 459     pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
 460         match self {
 461             Ok(t) => Ok(t),
 462             Err(e) => Err(op(e))
 463         }
 464     }
 465
 466     /////////////////////////////////////////////////////////////////////////
 467     // Iterator constructors
 468     /////////////////////////////////////////////////////////////////////////
 469
 470     /// Returns an iterator over the possibly contained value.
 471     ///
 472     /// # Examples
 473     ///
 474     /// ```
 475     /// let x: Result<u32, &str> = Ok(7);
 476     /// assert_eq!(x.iter().next(), Some(&7));
 477     ///
 478     /// let x: Result<u32, &str> = Err("nothing!");
 479     /// assert_eq!(x.iter().next(), None);
 480     /// ```
 481     #[inline]
 482     #[stable(feature = "rust1", since = "1.0.0")]
 483     pub fn iter(&self) -> Iter<T> {
 484         Iter { inner: self.as_ref().ok() }
 485     }
 486
 487     /// Returns a mutable iterator over the possibly contained value.
 488     ///
 489     /// # Examples
 490     ///
 491     /// ```
 492     /// let mut x: Result<u32, &str> = Ok(7);
 493     /// match x.iter_mut().next() {
 494     ///     Some(v) => *v = 40,
 495     ///     None => {},
 496     /// }
 497     /// assert_eq!(x, Ok(40));
 498     ///
 499     /// let mut x: Result<u32, &str> = Err("nothing!");
 500     /// assert_eq!(x.iter_mut().next(), None);
 501     /// ```
 502     #[inline]
 503     #[stable(feature = "rust1", since = "1.0.0")]
 504     pub fn iter_mut(&mut self) -> IterMut<T> {
 505         IterMut { inner: self.as_mut().ok() }
 506     }
 507
 508     ////////////////////////////////////////////////////////////////////////
 509     // Boolean operations on the values, eager and lazy
 510     /////////////////////////////////////////////////////////////////////////
 511
 512     /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
 513     ///
 514     /// # Examples
 515     ///
 516     /// ```
 517     /// let x: Result<u32, &str> = Ok(2);
 518     /// let y: Result<&str, &str> = Err("late error");
 519     /// assert_eq!(x.and(y), Err("late error"));
 520     ///
 521     /// let x: Result<u32, &str> = Err("early error");
 522     /// let y: Result<&str, &str> = Ok("foo");
 523     /// assert_eq!(x.and(y), Err("early error"));
 524     ///
 525     /// let x: Result<u32, &str> = Err("not a 2");
 526     /// let y: Result<&str, &str> = Err("late error");
 527     /// assert_eq!(x.and(y), Err("not a 2"));
 528     ///
 529     /// let x: Result<u32, &str> = Ok(2);
 530     /// let y: Result<&str, &str> = Ok("different result type");
 531     /// assert_eq!(x.and(y), Ok("different result type"));
 532     /// ```
 533     #[inline]
 534     #[stable(feature = "rust1", since = "1.0.0")]
 535     pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
 536         match self {
 537             Ok(_) => res,
 538             Err(e) => Err(e),
 539         }
 540     }
 541
 542     /// Calls `op` if the result is `Ok`, otherwise returns the `Err` value of `self`.
 543     ///
 544     /// This function can be used for control flow based on result values.
 545     ///
 546     /// # Examples
 547     ///
 548     /// ```
 549     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 550     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 551     ///
 552     /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
 553     /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
 554     /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
 555     /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
 556     /// ```
 557     #[inline]
 558     #[stable(feature = "rust1", since = "1.0.0")]
 559     pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
 560         match self {
 561             Ok(t) => op(t),
 562             Err(e) => Err(e),
 563         }
 564     }
 565
 566     /// Returns `res` if the result is `Err`, otherwise returns the `Ok` value of `self`.
 567     ///
 568     /// # Examples
 569     ///
 570     /// ```
 571     /// let x: Result<u32, &str> = Ok(2);
 572     /// let y: Result<u32, &str> = Err("late error");
 573     /// assert_eq!(x.or(y), Ok(2));
 574     ///
 575     /// let x: Result<u32, &str> = Err("early error");
 576     /// let y: Result<u32, &str> = Ok(2);
 577     /// assert_eq!(x.or(y), Ok(2));
 578     ///
 579     /// let x: Result<u32, &str> = Err("not a 2");
 580     /// let y: Result<u32, &str> = Err("late error");
 581     /// assert_eq!(x.or(y), Err("late error"));
 582     ///
 583     /// let x: Result<u32, &str> = Ok(2);
 584     /// let y: Result<u32, &str> = Ok(100);
 585     /// assert_eq!(x.or(y), Ok(2));
 586     /// ```
 587     #[inline]
 588     #[stable(feature = "rust1", since = "1.0.0")]
 589     pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
 590         match self {
 591             Ok(v) => Ok(v),
 592             Err(_) => res,
 593         }
 594     }
 595
 596     /// Calls `op` if the result is `Err`, otherwise returns the `Ok` value of `self`.
 597     ///
 598     /// This function can be used for control flow based on result values.
 599     ///
 600     /// # Examples
 601     ///
 602     /// ```
 603     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 604     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 605     ///
 606     /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
 607     /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
 608     /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
 609     /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
 610     /// ```
 611     #[inline]
 612     #[stable(feature = "rust1", since = "1.0.0")]
 613     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
 614         match self {
 615             Ok(t) => Ok(t),
 616             Err(e) => op(e),
 617         }
 618     }
 619
 620     /// Unwraps a result, yielding the content of an `Ok`.
 621     /// Else it returns `optb`.
 622     ///
 623     /// # Examples
 624     ///
 625     /// ```
 626     /// let optb = 2;
 627     /// let x: Result<u32, &str> = Ok(9);
 628     /// assert_eq!(x.unwrap_or(optb), 9);
 629     ///
 630     /// let x: Result<u32, &str> = Err("error");
 631     /// assert_eq!(x.unwrap_or(optb), optb);
 632     /// ```
 633     #[inline]
 634     #[stable(feature = "rust1", since = "1.0.0")]
 635     pub fn unwrap_or(self, optb: T) -> T {
 636         match self {
 637             Ok(t) => t,
 638             Err(_) => optb
 639         }
 640     }
 641
 642     /// Unwraps a result, yielding the content of an `Ok`.
 643     /// If the value is an `Err` then it calls `op` with its value.
 644     ///
 645     /// # Examples
 646     ///
 647     /// ```
 648     /// fn count(x: &str) -> usize { x.len() }
 649     ///
 650     /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
 651     /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
 652     /// ```
 653     #[inline]
 654     #[stable(feature = "rust1", since = "1.0.0")]
 655     pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
 656         match self {
 657             Ok(t) => t,
 658             Err(e) => op(e)
 659         }
 660     }
 661 }
 662
 663 impl<T, E: fmt::Debug> Result<T, E> {
 664     /// Unwraps a result, yielding the content of an `Ok`.
 665     ///
 666     /// # Panics
 667     ///
 668     /// Panics if the value is an `Err`, with a panic message provided by the
 669     /// `Err`'s value.
 670     ///
 671     /// # Examples
 672     ///
 673     /// ```
 674     /// let x: Result<u32, &str> = Ok(2);
 675     /// assert_eq!(x.unwrap(), 2);
 676     /// ```
 677     ///
 678     /// ```{.should_panic}
 679     /// let x: Result<u32, &str> = Err("emergency failure");
 680     /// x.unwrap(); // panics with `emergency failure`
 681     /// ```
 682     #[inline]
 683     #[stable(feature = "rust1", since = "1.0.0")]
 684     pub fn unwrap(self) -> T {
 685         match self {
 686             Ok(t) => t,
 687             Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", e),
 688         }
 689     }
 690
 691     /// Unwraps a result, yielding the content of an `Ok`.
 692     ///
 693     /// # Panics
 694     ///
 695     /// Panics if the value is an `Err`, with a panic message including the
 696     /// passed message, and the content of the `Err`.
 697     ///
 698     /// # Examples
 699     /// ```{.should_panic}
 700     /// let x: Result<u32, &str> = Err("emergency failure");
 701     /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
 702     /// ```
 703     #[inline]
 704     #[stable(feature = "result_expect", since = "1.4.0")]
 705     pub fn expect(self, msg: &str) -> T {
 706         match self {
 707             Ok(t) => t,
 708             Err(e) => unwrap_failed(msg, e),
 709         }
 710     }
 711 }
 712
 713 impl<T: fmt::Debug, E> Result<T, E> {
 714     /// Unwraps a result, yielding the content of an `Err`.
 715     ///
 716     /// # Panics
 717     ///
 718     /// Panics if the value is an `Ok`, with a custom panic message provided
 719     /// by the `Ok`'s value.
 720     ///
 721     /// # Examples
 722     ///
 723     /// ```{.should_panic}
 724     /// let x: Result<u32, &str> = Ok(2);
 725     /// x.unwrap_err(); // panics with `2`
 726     /// ```
 727     ///
 728     /// ```
 729     /// let x: Result<u32, &str> = Err("emergency failure");
 730     /// assert_eq!(x.unwrap_err(), "emergency failure");
 731     /// ```
 732     #[inline]
 733     #[stable(feature = "rust1", since = "1.0.0")]
 734     pub fn unwrap_err(self) -> E {
 735         match self {
 736             Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", t),
 737             Err(e) => e,
 738         }
 739     }
 740 }
 741
 742 // This is a separate function to reduce the code size of the methods
 743 #[inline(never)]
 744 #[cold]
 745 fn unwrap_failed<E: fmt::Debug>(msg: &str, error: E) -> ! {
 746     panic!("{}: {:?}", msg, error)
 747 }
 748
 749 /////////////////////////////////////////////////////////////////////////////
 750 // Trait implementations
 751 /////////////////////////////////////////////////////////////////////////////
 752
 753 #[stable(feature = "rust1", since = "1.0.0")]
 754 impl<T, E> IntoIterator for Result<T, E> {
 755     type Item = T;
 756     type IntoIter = IntoIter<T>;
 757
 758     /// Returns a consuming iterator over the possibly contained value.
 759     ///
 760     /// # Examples
 761     ///
 762     /// ```
 763     /// let x: Result<u32, &str> = Ok(5);
 764     /// let v: Vec<u32> = x.into_iter().collect();
 765     /// assert_eq!(v, [5]);
 766     ///
 767     /// let x: Result<u32, &str> = Err("nothing!");
 768     /// let v: Vec<u32> = x.into_iter().collect();
 769     /// assert_eq!(v, []);
 770     /// ```
 771     #[inline]
 772     fn into_iter(self) -> IntoIter<T> {
 773         IntoIter { inner: self.ok() }
 774     }
 775 }
 776
 777 #[stable(since = "1.4.0", feature = "result_iter")]
 778 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
 779     type Item = &'a T;
 780     type IntoIter = Iter<'a, T>;
 781
 782     fn into_iter(self) -> Iter<'a, T> {
 783         self.iter()
 784     }
 785 }
 786
 787 #[stable(since = "1.4.0", feature = "result_iter")]
 788 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
 789     type Item = &'a mut T;
 790     type IntoIter = IterMut<'a, T>;
 791
 792     fn into_iter(mut self) -> IterMut<'a, T> {
 793         self.iter_mut()
 794     }
 795 }
 796
 797 /////////////////////////////////////////////////////////////////////////////
 798 // The Result Iterators
 799 /////////////////////////////////////////////////////////////////////////////
 800
 801 /// An iterator over a reference to the `Ok` variant of a `Result`.
 802 #[derive(Debug)]
 803 #[stable(feature = "rust1", since = "1.0.0")]
 804 pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }
 805
 806 #[stable(feature = "rust1", since = "1.0.0")]
 807 impl<'a, T> Iterator for Iter<'a, T> {
 808     type Item = &'a T;
 809
 810     #[inline]
 811     fn next(&mut self) -> Option<&'a T> { self.inner.take() }
 812     #[inline]
 813     fn size_hint(&self) -> (usize, Option<usize>) {
 814         let n = if self.inner.is_some() {1} else {0};
 815         (n, Some(n))
 816     }
 817 }
 818
 819 #[stable(feature = "rust1", since = "1.0.0")]
 820 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
 821     #[inline]
 822     fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
 823 }
 824
 825 #[stable(feature = "rust1", since = "1.0.0")]
 826 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
 827
 828 #[stable(feature = "rust1", since = "1.0.0")]
 829 impl<'a, T> Clone for Iter<'a, T> {
 830     fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
 831 }
 832
 833 /// An iterator over a mutable reference to the `Ok` variant of a `Result`.
 834 #[derive(Debug)]
 835 #[stable(feature = "rust1", since = "1.0.0")]
 836 pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }
 837
 838 #[stable(feature = "rust1", since = "1.0.0")]
 839 impl<'a, T> Iterator for IterMut<'a, T> {
 840     type Item = &'a mut T;
 841
 842     #[inline]
 843     fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
 844     #[inline]
 845     fn size_hint(&self) -> (usize, Option<usize>) {
 846         let n = if self.inner.is_some() {1} else {0};
 847         (n, Some(n))
 848     }
 849 }
 850
 851 #[stable(feature = "rust1", since = "1.0.0")]
 852 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
 853     #[inline]
 854     fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
 855 }
 856
 857 #[stable(feature = "rust1", since = "1.0.0")]
 858 impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
 859
 860 /// An iterator over the value in a `Ok` variant of a `Result`.
 861 #[derive(Debug)]
 862 #[stable(feature = "rust1", since = "1.0.0")]
 863 pub struct IntoIter<T> { inner: Option<T> }
 864
 865 #[stable(feature = "rust1", since = "1.0.0")]
 866 impl<T> Iterator for IntoIter<T> {
 867     type Item = T;
 868
 869     #[inline]
 870     fn next(&mut self) -> Option<T> { self.inner.take() }
 871     #[inline]
 872     fn size_hint(&self) -> (usize, Option<usize>) {
 873         let n = if self.inner.is_some() {1} else {0};
 874         (n, Some(n))
 875     }
 876 }
 877
 878 #[stable(feature = "rust1", since = "1.0.0")]
 879 impl<T> DoubleEndedIterator for IntoIter<T> {
 880     #[inline]
 881     fn next_back(&mut self) -> Option<T> { self.inner.take() }
 882 }
 883
 884 #[stable(feature = "rust1", since = "1.0.0")]
 885 impl<T> ExactSizeIterator for IntoIter<T> {}
 886
 887 /////////////////////////////////////////////////////////////////////////////
 888 // FromIterator
 889 /////////////////////////////////////////////////////////////////////////////
 890
 891 #[stable(feature = "rust1", since = "1.0.0")]
 892 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
 893     /// Takes each element in the `Iterator`: if it is an `Err`, no further
 894     /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
 895     /// container with the values of each `Result` is returned.
 896     ///
 897     /// Here is an example which increments every integer in a vector,
 898     /// checking for overflow:
 899     ///
 900     /// ```
 901     /// use std::u32;
 902     ///
 903     /// let v = vec!(1, 2);
 904     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32|
 905     ///     if x == u32::MAX { Err("Overflow!") }
 906     ///     else { Ok(x + 1) }
 907     /// ).collect();
 908     /// assert!(res == Ok(vec!(2, 3)));
 909     /// ```
 910     #[inline]
 911     fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
 912         // FIXME(#11084): This could be replaced with Iterator::scan when this
 913         // performance bug is closed.
 914
 915         struct Adapter<Iter, E> {
 916             iter: Iter,
 917             err: Option<E>,
 918         }
 919
 920         impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
 921             type Item = T;
 922
 923             #[inline]
 924             fn next(&mut self) -> Option<T> {
 925                 match self.iter.next() {
 926                     Some(Ok(value)) => Some(value),
 927                     Some(Err(err)) => {
 928                         self.err = Some(err);
 929                         None
 930                     }
 931                     None => None,
 932                 }
 933             }
 934         }
 935
 936         let mut adapter = Adapter { iter: iter.into_iter(), err: None };
 937         let v: V = FromIterator::from_iter(adapter.by_ref());
 938
 939         match adapter.err {
 940             Some(err) => Err(err),
 941             None => Ok(v),
 942         }
 943     }
 944 }