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 //! enum Result<T, E> {
  20 //!    Ok(T),
  21 //!    Err(E)
  22 //! }
  23 //! ```
  24 //!
  25 //! Functions return `Result` whenever errors are expected and
  26 //! recoverable. In the `std` crate `Result` is most prominently used
  27 //! for [I/O](../../std/io/index.html).
  28 //!
  29 //! A simple function returning `Result` might be
  30 //! defined and used like so:
  31 //!
  32 //! ```
  33 //! #[derive(Debug)]
  34 //! enum Version { Version1, Version2 }
  35 //!
  36 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
  37 //!     match header.get(0) {
  38 //!         None => Err("invalid header length"),
  39 //!         Some(&1) => Ok(Version::Version1),
  40 //!         Some(&2) => Ok(Version::Version2),
  41 //!         Some(_) => Err("invalid version")
  42 //!     }
  43 //! }
  44 //!
  45 //! let version = parse_version(&[1, 2, 3, 4]);
  46 //! match version {
  47 //!     Ok(v) => println!("working with version: {:?}", v),
  48 //!     Err(e) => println!("error parsing header: {:?}", e),
  49 //! }
  50 //! ```
  51 //!
  52 //! Pattern matching on `Result`s is clear and straightforward for
  53 //! simple cases, but `Result` comes with some convenience methods
  54 //! that make working with it more succinct.
  55 //!
  56 //! ```
  57 //! let good_result: Result<i32, i32> = Ok(10);
  58 //! let bad_result: Result<i32, i32> = Err(10);
  59 //!
  60 //! // The `is_ok` and `is_err` methods do what they say.
  61 //! assert!(good_result.is_ok() && !good_result.is_err());
  62 //! assert!(bad_result.is_err() && !bad_result.is_ok());
  63 //!
  64 //! // `map` consumes the `Result` and produces another.
  65 //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
  66 //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
  67 //!
  68 //! // Use `and_then` to continue the computation.
  69 //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
  70 //!
  71 //! // Use `or_else` to handle the error.
  72 //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
  73 //!
  74 //! // Consume the result and return the contents with `unwrap`.
  75 //! let final_awesome_result = good_result.unwrap();
  76 //! ```
  77 //!
  78 //! # Results must be used
  79 //!
  80 //! A common problem with using return values to indicate errors is
  81 //! that it is easy to ignore the return value, thus failing to handle
  82 //! the error. Result is annotated with the #[must_use] attribute,
  83 //! which will cause the compiler to issue a warning when a Result
  84 //! value is ignored. This makes `Result` especially useful with
  85 //! functions that may encounter errors but don't otherwise return a
  86 //! useful value.
  87 //!
  88 //! Consider the `write_all` method defined for I/O types
  89 //! by the [`Write`](../../std/io/trait.Write.html) trait:
  90 //!
  91 //! ```
  92 //! use std::io;
  93 //!
  94 //! trait Write {
  95 //!     fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
  96 //! }
  97 //! ```
  98 //!
  99 //! *Note: The actual definition of `Write` uses `io::Result`, which
 100 //! is just a synonym for `Result<T, io::Error>`.*
 101 //!
 102 //! This method doesn't produce a value, but the write may
 103 //! fail. It's crucial to handle the error case, and *not* write
 104 //! something like this:
 105 //!
 106 //! ```no_run
 107 //! use std::fs::File;
 108 //! use std::io::prelude::*;
 109 //!
 110 //! let mut file = File::create("valuable_data.txt").unwrap();
 111 //! // If `write_all` errors, then we'll never know, because the return
 112 //! // value is ignored.
 113 //! file.write_all(b"important message");
 114 //! ```
 115 //!
 116 //! If you *do* write that in Rust, the compiler will give you a
 117 //! warning (by default, controlled by the `unused_must_use` lint).
 118 //!
 119 //! You might instead, if you don't want to handle the error, simply
 120 //! panic, by converting to an `Option` with `ok`, then asserting
 121 //! success with `expect`. This will panic if the write fails, proving
 122 //! a marginally useful message indicating why:
 123 //!
 124 //! ```{.no_run}
 125 //! use std::fs::File;
 126 //! use std::io::prelude::*;
 127 //!
 128 //! let mut file = File::create("valuable_data.txt").unwrap();
 129 //! file.write_all(b"important message").ok().expect("failed to write message");
 130 //! ```
 131 //!
 132 //! You might also simply assert success:
 133 //!
 134 //! ```{.no_run}
 135 //! # use std::fs::File;
 136 //! # use std::io::prelude::*;
 137 //! # let mut file = File::create("valuable_data.txt").unwrap();
 138 //! assert!(file.write_all(b"important message").is_ok());
 139 //! ```
 140 //!
 141 //! Or propagate the error up the call stack with `try!`:
 142 //!
 143 //! ```
 144 //! # use std::fs::File;
 145 //! # use std::io::prelude::*;
 146 //! # use std::io;
 147 //! fn write_message() -> io::Result<()> {
 148 //!     let mut file = try!(File::create("valuable_data.txt"));
 149 //!     try!(file.write_all(b"important message"));
 150 //!     Ok(())
 151 //! }
 152 //! ```
 153 //!
 154 //! # The `try!` macro
 155 //!
 156 //! When writing code that calls many functions that return the
 157 //! `Result` type, the error handling can be tedious.  The `try!`
 158 //! macro hides some of the boilerplate of propagating errors up the
 159 //! call stack.
 160 //!
 161 //! It replaces this:
 162 //!
 163 //! ```
 164 //! use std::fs::File;
 165 //! use std::io::prelude::*;
 166 //! use std::io;
 167 //!
 168 //! struct Info {
 169 //!     name: String,
 170 //!     age: i32,
 171 //!     rating: i32,
 172 //! }
 173 //!
 174 //! fn write_info(info: &Info) -> io::Result<()> {
 175 //!     let mut file = try!(File::create("my_best_friends.txt"));
 176 //!     // Early return on error
 177 //!     if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
 178 //!         return Err(e)
 179 //!     }
 180 //!     if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
 181 //!         return Err(e)
 182 //!     }
 183 //!     if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
 184 //!         return Err(e)
 185 //!     }
 186 //!     Ok(())
 187 //! }
 188 //! ```
 189 //!
 190 //! With this:
 191 //!
 192 //! ```
 193 //! use std::fs::File;
 194 //! use std::io::prelude::*;
 195 //! use std::io;
 196 //!
 197 //! struct Info {
 198 //!     name: String,
 199 //!     age: i32,
 200 //!     rating: i32,
 201 //! }
 202 //!
 203 //! fn write_info(info: &Info) -> io::Result<()> {
 204 //!     let mut file = try!(File::create("my_best_friends.txt"));
 205 //!     // Early return on error
 206 //!     try!(file.write_all(format!("name: {}\n", info.name).as_bytes()));
 207 //!     try!(file.write_all(format!("age: {}\n", info.age).as_bytes()));
 208 //!     try!(file.write_all(format!("rating: {}\n", info.rating).as_bytes()));
 209 //!     Ok(())
 210 //! }
 211 //! ```
 212 //!
 213 //! *It's much nicer!*
 214 //!
 215 //! Wrapping an expression in `try!` will result in the unwrapped
 216 //! success (`Ok`) value, unless the result is `Err`, in which case
 217 //! `Err` is returned early from the enclosing function. Its simple definition
 218 //! makes it clear:
 219 //!
 220 //! ```
 221 //! macro_rules! try {
 222 //!     ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
 223 //! }
 224 //! ```
 225 //!
 226 //! `try!` is imported by the prelude, and is available everywhere.
 227
 228 #![stable(feature = "rust1", since = "1.0.0")]
 229
 230 use self::Result::{Ok, Err};
 231
 232 use clone::Clone;
 233 use fmt;
 234 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSizeIterator, IntoIterator};
 235 use ops::{FnMut, FnOnce};
 236 use option::Option::{self, None, Some};
 237 use slice;
 238
 239 /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
 240 ///
 241 /// See the [`std::result`](index.html) module documentation for details.
 242 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
 243 #[must_use]
 244 #[stable(feature = "rust1", since = "1.0.0")]
 245 pub enum Result<T, E> {
 246     /// Contains the success value
 247     #[stable(feature = "rust1", since = "1.0.0")]
 248     Ok(T),
 249
 250     /// Contains the error value
 251     #[stable(feature = "rust1", since = "1.0.0")]
 252     Err(E)
 253 }
 254
 255 /////////////////////////////////////////////////////////////////////////////
 256 // Type implementation
 257 /////////////////////////////////////////////////////////////////////////////
 258
 259 #[stable(feature = "rust1", since = "1.0.0")]
 260 impl<T, E> Result<T, E> {
 261     /////////////////////////////////////////////////////////////////////////
 262     // Querying the contained values
 263     /////////////////////////////////////////////////////////////////////////
 264
 265     /// Returns true if the result is `Ok`
 266     ///
 267     /// # Examples
 268     ///
 269     /// ```
 270     /// let x: Result<i32, &str> = Ok(-3);
 271     /// assert_eq!(x.is_ok(), true);
 272     ///
 273     /// let x: Result<i32, &str> = Err("Some error message");
 274     /// assert_eq!(x.is_ok(), false);
 275     /// ```
 276     #[inline]
 277     #[stable(feature = "rust1", since = "1.0.0")]
 278     pub fn is_ok(&self) -> bool {
 279         match *self {
 280             Ok(_) => true,
 281             Err(_) => false
 282         }
 283     }
 284
 285     /// Returns true if the result is `Err`
 286     ///
 287     /// # Examples
 288     ///
 289     /// ```
 290     /// let x: Result<i32, &str> = Ok(-3);
 291     /// assert_eq!(x.is_err(), false);
 292     ///
 293     /// let x: Result<i32, &str> = Err("Some error message");
 294     /// assert_eq!(x.is_err(), true);
 295     /// ```
 296     #[inline]
 297     #[stable(feature = "rust1", since = "1.0.0")]
 298     pub fn is_err(&self) -> bool {
 299         !self.is_ok()
 300     }
 301
 302     /////////////////////////////////////////////////////////////////////////
 303     // Adapter for each variant
 304     /////////////////////////////////////////////////////////////////////////
 305
 306     /// Converts from `Result<T, E>` to `Option<T>`
 307     ///
 308     /// Converts `self` into an `Option<T>`, consuming `self`,
 309     /// and discarding the error, if any.
 310     ///
 311     /// # Examples
 312     ///
 313     /// ```
 314     /// let x: Result<u32, &str> = Ok(2);
 315     /// assert_eq!(x.ok(), Some(2));
 316     ///
 317     /// let x: Result<u32, &str> = Err("Nothing here");
 318     /// assert_eq!(x.ok(), None);
 319     /// ```
 320     #[inline]
 321     #[stable(feature = "rust1", since = "1.0.0")]
 322     pub fn ok(self) -> Option<T> {
 323         match self {
 324             Ok(x)  => Some(x),
 325             Err(_) => None,
 326         }
 327     }
 328
 329     /// Converts from `Result<T, E>` to `Option<E>`
 330     ///
 331     /// Converts `self` into an `Option<E>`, consuming `self`,
 332     /// and discarding the success value, if any.
 333     ///
 334     /// # Examples
 335     ///
 336     /// ```
 337     /// let x: Result<u32, &str> = Ok(2);
 338     /// assert_eq!(x.err(), None);
 339     ///
 340     /// let x: Result<u32, &str> = Err("Nothing here");
 341     /// assert_eq!(x.err(), Some("Nothing here"));
 342     /// ```
 343     #[inline]
 344     #[stable(feature = "rust1", since = "1.0.0")]
 345     pub fn err(self) -> Option<E> {
 346         match self {
 347             Ok(_)  => None,
 348             Err(x) => Some(x),
 349         }
 350     }
 351
 352     /////////////////////////////////////////////////////////////////////////
 353     // Adapter for working with references
 354     /////////////////////////////////////////////////////////////////////////
 355
 356     /// Converts from `Result<T, E>` to `Result<&T, &E>`
 357     ///
 358     /// Produces a new `Result`, containing a reference
 359     /// into the original, leaving the original in place.
 360     ///
 361     /// ```
 362     /// let x: Result<u32, &str> = Ok(2);
 363     /// assert_eq!(x.as_ref(), Ok(&2));
 364     ///
 365     /// let x: Result<u32, &str> = Err("Error");
 366     /// assert_eq!(x.as_ref(), Err(&"Error"));
 367     /// ```
 368     #[inline]
 369     #[stable(feature = "rust1", since = "1.0.0")]
 370     pub fn as_ref(&self) -> Result<&T, &E> {
 371         match *self {
 372             Ok(ref x) => Ok(x),
 373             Err(ref x) => Err(x),
 374         }
 375     }
 376
 377     /// Converts from `Result<T, E>` to `Result<&mut T, &mut E>`
 378     ///
 379     /// ```
 380     /// fn mutate(r: &mut Result<i32, i32>) {
 381     ///     match r.as_mut() {
 382     ///         Ok(&mut ref mut v) => *v = 42,
 383     ///         Err(&mut ref mut e) => *e = 0,
 384     ///     }
 385     /// }
 386     ///
 387     /// let mut x: Result<i32, i32> = Ok(2);
 388     /// mutate(&mut x);
 389     /// assert_eq!(x.unwrap(), 42);
 390     ///
 391     /// let mut x: Result<i32, i32> = Err(13);
 392     /// mutate(&mut x);
 393     /// assert_eq!(x.unwrap_err(), 0);
 394     /// ```
 395     #[inline]
 396     #[stable(feature = "rust1", since = "1.0.0")]
 397     pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
 398         match *self {
 399             Ok(ref mut x) => Ok(x),
 400             Err(ref mut x) => Err(x),
 401         }
 402     }
 403
 404     /// Converts from `Result<T, E>` to `&[T]` (without copying)
 405     #[inline]
 406     #[unstable(feature = "as_slice", since = "unsure of the utility here")]
 407     pub fn as_slice(&self) -> &[T] {
 408         match *self {
 409             Ok(ref x) => slice::ref_slice(x),
 410             Err(_) => {
 411                 // work around lack of implicit coercion from fixed-size array to slice
 412                 let emp: &[_] = &[];
 413                 emp
 414             }
 415         }
 416     }
 417
 418     /// Converts from `Result<T, E>` to `&mut [T]` (without copying)
 419     ///
 420     /// ```
 421     /// # #![feature(core)]
 422     /// let mut x: Result<&str, u32> = Ok("Gold");
 423     /// {
 424     ///     let v = x.as_mut_slice();
 425     ///     assert!(v == ["Gold"]);
 426     ///     v[0] = "Silver";
 427     ///     assert!(v == ["Silver"]);
 428     /// }
 429     /// assert_eq!(x, Ok("Silver"));
 430     ///
 431     /// let mut x: Result<&str, u32> = Err(45);
 432     /// assert!(x.as_mut_slice().is_empty());
 433     /// ```
 434     #[inline]
 435     #[unstable(feature = "core",
 436                reason = "waiting for mut conventions")]
 437     pub fn as_mut_slice(&mut self) -> &mut [T] {
 438         match *self {
 439             Ok(ref mut x) => slice::mut_ref_slice(x),
 440             Err(_) => {
 441                 // work around lack of implicit coercion from fixed-size array to slice
 442                 let emp: &mut [_] = &mut [];
 443                 emp
 444             }
 445         }
 446     }
 447
 448     /////////////////////////////////////////////////////////////////////////
 449     // Transforming contained values
 450     /////////////////////////////////////////////////////////////////////////
 451
 452     /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to an
 453     /// contained `Ok` value, leaving an `Err` value untouched.
 454     ///
 455     /// This function can be used to compose the results of two functions.
 456     ///
 457     /// # Examples
 458     ///
 459     /// Print the numbers on each line of a string multiplied by two.
 460     ///
 461     /// ```
 462     /// let line = "1\n2\n3\n4\n";
 463     ///
 464     /// for num in line.lines() {
 465     ///     match num.parse::<i32>().map(|i| i * 2) {
 466     ///         Ok(n) => println!("{}", n),
 467     ///         Err(..) => {}
 468     ///     }
 469     /// }
 470     /// ```
 471     #[inline]
 472     #[stable(feature = "rust1", since = "1.0.0")]
 473     pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
 474         match self {
 475             Ok(t) => Ok(op(t)),
 476             Err(e) => Err(e)
 477         }
 478     }
 479
 480     /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to an
 481     /// contained `Err` value, leaving an `Ok` value untouched.
 482     ///
 483     /// This function can be used to pass through a successful result while handling
 484     /// an error.
 485     ///
 486     /// # Examples
 487     ///
 488     /// ```
 489     /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
 490     ///
 491     /// let x: Result<u32, u32> = Ok(2);
 492     /// assert_eq!(x.map_err(stringify), Ok(2));
 493     ///
 494     /// let x: Result<u32, u32> = Err(13);
 495     /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
 496     /// ```
 497     #[inline]
 498     #[stable(feature = "rust1", since = "1.0.0")]
 499     pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
 500         match self {
 501             Ok(t) => Ok(t),
 502             Err(e) => Err(op(e))
 503         }
 504     }
 505
 506     /////////////////////////////////////////////////////////////////////////
 507     // Iterator constructors
 508     /////////////////////////////////////////////////////////////////////////
 509
 510     /// Returns an iterator over the possibly contained value.
 511     ///
 512     /// # Examples
 513     ///
 514     /// ```
 515     /// let x: Result<u32, &str> = Ok(7);
 516     /// assert_eq!(x.iter().next(), Some(&7));
 517     ///
 518     /// let x: Result<u32, &str> = Err("nothing!");
 519     /// assert_eq!(x.iter().next(), None);
 520     /// ```
 521     #[inline]
 522     #[stable(feature = "rust1", since = "1.0.0")]
 523     pub fn iter(&self) -> Iter<T> {
 524         Iter { inner: self.as_ref().ok() }
 525     }
 526
 527     /// Returns a mutable iterator over the possibly contained value.
 528     ///
 529     /// # Examples
 530     ///
 531     /// ```
 532     /// let mut x: Result<u32, &str> = Ok(7);
 533     /// match x.iter_mut().next() {
 534     ///     Some(&mut ref mut x) => *x = 40,
 535     ///     None => {},
 536     /// }
 537     /// assert_eq!(x, Ok(40));
 538     ///
 539     /// let mut x: Result<u32, &str> = Err("nothing!");
 540     /// assert_eq!(x.iter_mut().next(), None);
 541     /// ```
 542     #[inline]
 543     #[stable(feature = "rust1", since = "1.0.0")]
 544     pub fn iter_mut(&mut self) -> IterMut<T> {
 545         IterMut { inner: self.as_mut().ok() }
 546     }
 547
 548     ////////////////////////////////////////////////////////////////////////
 549     // Boolean operations on the values, eager and lazy
 550     /////////////////////////////////////////////////////////////////////////
 551
 552     /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
 553     ///
 554     /// # Examples
 555     ///
 556     /// ```
 557     /// let x: Result<u32, &str> = Ok(2);
 558     /// let y: Result<&str, &str> = Err("late error");
 559     /// assert_eq!(x.and(y), Err("late error"));
 560     ///
 561     /// let x: Result<u32, &str> = Err("early error");
 562     /// let y: Result<&str, &str> = Ok("foo");
 563     /// assert_eq!(x.and(y), Err("early error"));
 564     ///
 565     /// let x: Result<u32, &str> = Err("not a 2");
 566     /// let y: Result<&str, &str> = Err("late error");
 567     /// assert_eq!(x.and(y), Err("not a 2"));
 568     ///
 569     /// let x: Result<u32, &str> = Ok(2);
 570     /// let y: Result<&str, &str> = Ok("different result type");
 571     /// assert_eq!(x.and(y), Ok("different result type"));
 572     /// ```
 573     #[inline]
 574     #[stable(feature = "rust1", since = "1.0.0")]
 575     pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
 576         match self {
 577             Ok(_) => res,
 578             Err(e) => Err(e),
 579         }
 580     }
 581
 582     /// Calls `op` if the result is `Ok`, otherwise returns the `Err` value of `self`.
 583     ///
 584     /// This function can be used for control flow based on result values.
 585     ///
 586     /// # Examples
 587     ///
 588     /// ```
 589     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 590     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 591     ///
 592     /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
 593     /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
 594     /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
 595     /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
 596     /// ```
 597     #[inline]
 598     #[stable(feature = "rust1", since = "1.0.0")]
 599     pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
 600         match self {
 601             Ok(t) => op(t),
 602             Err(e) => Err(e),
 603         }
 604     }
 605
 606     /// Returns `res` if the result is `Err`, otherwise returns the `Ok` value of `self`.
 607     ///
 608     /// # Examples
 609     ///
 610     /// ```
 611     /// let x: Result<u32, &str> = Ok(2);
 612     /// let y: Result<u32, &str> = Err("late error");
 613     /// assert_eq!(x.or(y), Ok(2));
 614     ///
 615     /// let x: Result<u32, &str> = Err("early error");
 616     /// let y: Result<u32, &str> = Ok(2);
 617     /// assert_eq!(x.or(y), Ok(2));
 618     ///
 619     /// let x: Result<u32, &str> = Err("not a 2");
 620     /// let y: Result<u32, &str> = Err("late error");
 621     /// assert_eq!(x.or(y), Err("late error"));
 622     ///
 623     /// let x: Result<u32, &str> = Ok(2);
 624     /// let y: Result<u32, &str> = Ok(100);
 625     /// assert_eq!(x.or(y), Ok(2));
 626     /// ```
 627     #[inline]
 628     #[stable(feature = "rust1", since = "1.0.0")]
 629     pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
 630         match self {
 631             Ok(v) => Ok(v),
 632             Err(_) => res,
 633         }
 634     }
 635
 636     /// Calls `op` if the result is `Err`, otherwise returns the `Ok` value of `self`.
 637     ///
 638     /// This function can be used for control flow based on result values.
 639     ///
 640     /// # Examples
 641     ///
 642     /// ```
 643     /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
 644     /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
 645     ///
 646     /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
 647     /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
 648     /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
 649     /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
 650     /// ```
 651     #[inline]
 652     #[stable(feature = "rust1", since = "1.0.0")]
 653     pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
 654         match self {
 655             Ok(t) => Ok(t),
 656             Err(e) => op(e),
 657         }
 658     }
 659
 660     /// Unwraps a result, yielding the content of an `Ok`.
 661     /// Else it returns `optb`.
 662     ///
 663     /// # Examples
 664     ///
 665     /// ```
 666     /// let optb = 2;
 667     /// let x: Result<u32, &str> = Ok(9);
 668     /// assert_eq!(x.unwrap_or(optb), 9);
 669     ///
 670     /// let x: Result<u32, &str> = Err("error");
 671     /// assert_eq!(x.unwrap_or(optb), optb);
 672     /// ```
 673     #[inline]
 674     #[stable(feature = "rust1", since = "1.0.0")]
 675     pub fn unwrap_or(self, optb: T) -> T {
 676         match self {
 677             Ok(t) => t,
 678             Err(_) => optb
 679         }
 680     }
 681
 682     /// Unwraps a result, yielding the content of an `Ok`.
 683     /// If the value is an `Err` then it calls `op` with its value.
 684     ///
 685     /// # Examples
 686     ///
 687     /// ```
 688     /// fn count(x: &str) -> usize { x.len() }
 689     ///
 690     /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
 691     /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
 692     /// ```
 693     #[inline]
 694     #[stable(feature = "rust1", since = "1.0.0")]
 695     pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
 696         match self {
 697             Ok(t) => t,
 698             Err(e) => op(e)
 699         }
 700     }
 701 }
 702
 703 #[stable(feature = "rust1", since = "1.0.0")]
 704 impl<T, E: fmt::Debug> Result<T, E> {
 705     /// Unwraps a result, yielding the content of an `Ok`.
 706     ///
 707     /// # Panics
 708     ///
 709     /// Panics if the value is an `Err`, with a panic message provided by the
 710     /// `Err`'s value.
 711     ///
 712     /// # Examples
 713     ///
 714     /// ```
 715     /// let x: Result<u32, &str> = Ok(2);
 716     /// assert_eq!(x.unwrap(), 2);
 717     /// ```
 718     ///
 719     /// ```{.should_panic}
 720     /// let x: Result<u32, &str> = Err("emergency failure");
 721     /// x.unwrap(); // panics with `emergency failure`
 722     /// ```
 723     #[inline]
 724     #[stable(feature = "rust1", since = "1.0.0")]
 725     pub fn unwrap(self) -> T {
 726         match self {
 727             Ok(t) => t,
 728             Err(e) =>
 729                 panic!("called `Result::unwrap()` on an `Err` value: {:?}", e)
 730         }
 731     }
 732 }
 733
 734 #[stable(feature = "rust1", since = "1.0.0")]
 735 impl<T: fmt::Debug, E> Result<T, E> {
 736     /// Unwraps a result, yielding the content of an `Err`.
 737     ///
 738     /// # Panics
 739     ///
 740     /// Panics if the value is an `Ok`, with a custom panic message provided
 741     /// by the `Ok`'s value.
 742     ///
 743     /// # Examples
 744     ///
 745     /// ```{.should_panic}
 746     /// let x: Result<u32, &str> = Ok(2);
 747     /// x.unwrap_err(); // panics with `2`
 748     /// ```
 749     ///
 750     /// ```
 751     /// let x: Result<u32, &str> = Err("emergency failure");
 752     /// assert_eq!(x.unwrap_err(), "emergency failure");
 753     /// ```
 754     #[inline]
 755     #[stable(feature = "rust1", since = "1.0.0")]
 756     pub fn unwrap_err(self) -> E {
 757         match self {
 758             Ok(t) =>
 759                 panic!("called `Result::unwrap_err()` on an `Ok` value: {:?}", t),
 760             Err(e) => e
 761         }
 762     }
 763 }
 764
 765 /////////////////////////////////////////////////////////////////////////////
 766 // Trait implementations
 767 /////////////////////////////////////////////////////////////////////////////
 768
 769 #[stable(feature = "rust1", since = "1.0.0")]
 770 impl<T, E> IntoIterator for Result<T, E> {
 771     type Item = T;
 772     type IntoIter = IntoIter<T>;
 773
 774     /// Returns a consuming iterator over the possibly contained value.
 775     ///
 776     /// # Examples
 777     ///
 778     /// ```
 779     /// let x: Result<u32, &str> = Ok(5);
 780     /// let v: Vec<u32> = x.into_iter().collect();
 781     /// assert_eq!(v, [5]);
 782     ///
 783     /// let x: Result<u32, &str> = Err("nothing!");
 784     /// let v: Vec<u32> = x.into_iter().collect();
 785     /// assert_eq!(v, []);
 786     /// ```
 787     #[inline]
 788     fn into_iter(self) -> IntoIter<T> {
 789         IntoIter { inner: self.ok() }
 790     }
 791 }
 792
 793 /////////////////////////////////////////////////////////////////////////////
 794 // The Result Iterators
 795 /////////////////////////////////////////////////////////////////////////////
 796
 797 /// An iterator over a reference to the `Ok` variant of a `Result`.
 798 #[stable(feature = "rust1", since = "1.0.0")]
 799 pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }
 800
 801 #[stable(feature = "rust1", since = "1.0.0")]
 802 impl<'a, T> Iterator for Iter<'a, T> {
 803     type Item = &'a T;
 804
 805     #[inline]
 806     fn next(&mut self) -> Option<&'a T> { self.inner.take() }
 807     #[inline]
 808     fn size_hint(&self) -> (usize, Option<usize>) {
 809         let n = if self.inner.is_some() {1} else {0};
 810         (n, Some(n))
 811     }
 812 }
 813
 814 #[stable(feature = "rust1", since = "1.0.0")]
 815 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
 816     #[inline]
 817     fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
 818 }
 819
 820 #[stable(feature = "rust1", since = "1.0.0")]
 821 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
 822
 823 impl<'a, T> Clone for Iter<'a, T> {
 824     fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
 825 }
 826
 827 /// An iterator over a mutable reference to the `Ok` variant of a `Result`.
 828 #[stable(feature = "rust1", since = "1.0.0")]
 829 pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }
 830
 831 #[stable(feature = "rust1", since = "1.0.0")]
 832 impl<'a, T> Iterator for IterMut<'a, T> {
 833     type Item = &'a mut T;
 834
 835     #[inline]
 836     fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
 837     #[inline]
 838     fn size_hint(&self) -> (usize, Option<usize>) {
 839         let n = if self.inner.is_some() {1} else {0};
 840         (n, Some(n))
 841     }
 842 }
 843
 844 #[stable(feature = "rust1", since = "1.0.0")]
 845 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
 846     #[inline]
 847     fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
 848 }
 849
 850 #[stable(feature = "rust1", since = "1.0.0")]
 851 impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
 852
 853 /// An iterator over the value in a `Ok` variant of a `Result`.
 854 #[stable(feature = "rust1", since = "1.0.0")]
 855 pub struct IntoIter<T> { inner: Option<T> }
 856
 857 #[stable(feature = "rust1", since = "1.0.0")]
 858 impl<T> Iterator for IntoIter<T> {
 859     type Item = T;
 860
 861     #[inline]
 862     fn next(&mut self) -> Option<T> { self.inner.take() }
 863     #[inline]
 864     fn size_hint(&self) -> (usize, Option<usize>) {
 865         let n = if self.inner.is_some() {1} else {0};
 866         (n, Some(n))
 867     }
 868 }
 869
 870 #[stable(feature = "rust1", since = "1.0.0")]
 871 impl<T> DoubleEndedIterator for IntoIter<T> {
 872     #[inline]
 873     fn next_back(&mut self) -> Option<T> { self.inner.take() }
 874 }
 875
 876 #[stable(feature = "rust1", since = "1.0.0")]
 877 impl<T> ExactSizeIterator for IntoIter<T> {}
 878
 879 /////////////////////////////////////////////////////////////////////////////
 880 // FromIterator
 881 /////////////////////////////////////////////////////////////////////////////
 882
 883 #[stable(feature = "rust1", since = "1.0.0")]
 884 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
 885     /// Takes each element in the `Iterator`: if it is an `Err`, no further
 886     /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
 887     /// container with the values of each `Result` is returned.
 888     ///
 889     /// Here is an example which increments every integer in a vector,
 890     /// checking for overflow:
 891     ///
 892     /// ```
 893     /// use std::u32;
 894     ///
 895     /// let v = vec!(1, 2);
 896     /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32|
 897     ///     if x == u32::MAX { Err("Overflow!") }
 898     ///     else { Ok(x + 1) }
 899     /// ).collect();
 900     /// assert!(res == Ok(vec!(2, 3)));
 901     /// ```
 902     #[inline]
 903     fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
 904         // FIXME(#11084): This could be replaced with Iterator::scan when this
 905         // performance bug is closed.
 906
 907         struct Adapter<Iter, E> {
 908             iter: Iter,
 909             err: Option<E>,
 910         }
 911
 912         impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
 913             type Item = T;
 914
 915             #[inline]
 916             fn next(&mut self) -> Option<T> {
 917                 match self.iter.next() {
 918                     Some(Ok(value)) => Some(value),
 919                     Some(Err(err)) => {
 920                         self.err = Some(err);
 921                         None
 922                     }
 923                     None => None,
 924                 }
 925             }
 926         }
 927
 928         let mut adapter = Adapter { iter: iter.into_iter(), err: None };
 929         let v: V = FromIterator::from_iter(adapter.by_ref());
 930
 931         match adapter.err {
 932             Some(err) => Err(err),
 933             None => Ok(v),
 934         }
 935     }
 936 }
 937
 938 /////////////////////////////////////////////////////////////////////////////
 939 // FromIterator
 940 /////////////////////////////////////////////////////////////////////////////
 941
 942 /// Performs a fold operation over the result values from an iterator.
 943 ///
 944 /// If an `Err` is encountered, it is immediately returned.
 945 /// Otherwise, the folded value is returned.
 946 #[inline]
 947 #[unstable(feature = "core")]
 948 pub fn fold<T,
 949             V,
 950             E,
 951             F: FnMut(V, T) -> V,
 952             Iter: Iterator<Item=Result<T, E>>>(
 953             iterator: Iter,
 954             mut init: V,
 955             mut f: F)
 956             -> Result<V, E> {
 957     for t in iterator {
 958         match t {
 959             Ok(v) => init = f(init, v),
 960             Err(u) => return Err(u)
 961         }
 962     }
 963     Ok(init)
 964 }