1 //! Error handling with the `Result` type.
3 //! [`Result<T, E>`][`Result`] is the type used for returning and propagating
4 //! errors. It is an enum with the variants, [`Ok(T)`], representing
5 //! success and containing a value, and [`Err(E)`], representing error
6 //! and containing an error value.
9 //! # #[allow(dead_code)]
10 //! enum Result<T, E> {
16 //! Functions return [`Result`] whenever errors are expected and
17 //! recoverable. In the `std` crate, [`Result`] is most prominently used
18 //! for [I/O](../../std/io/index.html).
20 //! A simple function returning [`Result`] might be
21 //! defined and used like so:
25 //! enum Version { Version1, Version2 }
27 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
28 //! match header.get(0) {
29 //! None => Err("invalid header length"),
30 //! Some(&1) => Ok(Version::Version1),
31 //! Some(&2) => Ok(Version::Version2),
32 //! Some(_) => Err("invalid version"),
36 //! let version = parse_version(&[1, 2, 3, 4]);
38 //! Ok(v) => println!("working with version: {:?}", v),
39 //! Err(e) => println!("error parsing header: {:?}", e),
43 //! Pattern matching on [`Result`]s is clear and straightforward for
44 //! simple cases, but [`Result`] comes with some convenience methods
45 //! that make working with it more succinct.
48 //! let good_result: Result<i32, i32> = Ok(10);
49 //! let bad_result: Result<i32, i32> = Err(10);
51 //! // The `is_ok` and `is_err` methods do what they say.
52 //! assert!(good_result.is_ok() && !good_result.is_err());
53 //! assert!(bad_result.is_err() && !bad_result.is_ok());
55 //! // `map` consumes the `Result` and produces another.
56 //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
57 //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
59 //! // Use `and_then` to continue the computation.
60 //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
62 //! // Use `or_else` to handle the error.
63 //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
65 //! // Consume the result and return the contents with `unwrap`.
66 //! let final_awesome_result = good_result.unwrap();
69 //! # Results must be used
71 //! A common problem with using return values to indicate errors is
72 //! that it is easy to ignore the return value, thus failing to handle
73 //! the error. [`Result`] is annotated with the `#[must_use]` attribute,
74 //! which will cause the compiler to issue a warning when a Result
75 //! value is ignored. This makes [`Result`] especially useful with
76 //! functions that may encounter errors but don't otherwise return a
79 //! Consider the [`write_all`] method defined for I/O types
80 //! by the [`Write`] trait:
86 //! fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
90 //! *Note: The actual definition of [`Write`] uses [`io::Result`], which
91 //! is just a synonym for [`Result`]`<T, `[`io::Error`]`>`.*
93 //! This method doesn't produce a value, but the write may
94 //! fail. It's crucial to handle the error case, and *not* write
95 //! something like this:
98 //! # #![allow(unused_must_use)] // \o/
99 //! use std::fs::File;
100 //! use std::io::prelude::*;
102 //! let mut file = File::create("valuable_data.txt").unwrap();
103 //! // If `write_all` errors, then we'll never know, because the return
104 //! // value is ignored.
105 //! file.write_all(b"important message");
108 //! If you *do* write that in Rust, the compiler will give you a
109 //! warning (by default, controlled by the `unused_must_use` lint).
111 //! You might instead, if you don't want to handle the error, simply
112 //! assert success with [`expect`]. This will panic if the
113 //! write fails, providing a marginally useful message indicating why:
116 //! use std::fs::File;
117 //! use std::io::prelude::*;
119 //! let mut file = File::create("valuable_data.txt").unwrap();
120 //! file.write_all(b"important message").expect("failed to write message");
123 //! You might also simply assert success:
126 //! # use std::fs::File;
127 //! # use std::io::prelude::*;
128 //! # let mut file = File::create("valuable_data.txt").unwrap();
129 //! assert!(file.write_all(b"important message").is_ok());
132 //! Or propagate the error up the call stack with [`?`]:
135 //! # use std::fs::File;
136 //! # use std::io::prelude::*;
138 //! # #[allow(dead_code)]
139 //! fn write_message() -> io::Result<()> {
140 //! let mut file = File::create("valuable_data.txt")?;
141 //! file.write_all(b"important message")?;
146 //! # The question mark operator, `?`
148 //! When writing code that calls many functions that return the
149 //! [`Result`] type, the error handling can be tedious. The question mark
150 //! operator, [`?`], hides some of the boilerplate of propagating errors
151 //! up the call stack.
153 //! It replaces this:
156 //! # #![allow(dead_code)]
157 //! use std::fs::File;
158 //! use std::io::prelude::*;
167 //! fn write_info(info: &Info) -> io::Result<()> {
168 //! // Early return on error
169 //! let mut file = match File::create("my_best_friends.txt") {
170 //! Err(e) => return Err(e),
173 //! if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
176 //! if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
179 //! if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
189 //! # #![allow(dead_code)]
190 //! use std::fs::File;
191 //! use std::io::prelude::*;
200 //! fn write_info(info: &Info) -> io::Result<()> {
201 //! let mut file = File::create("my_best_friends.txt")?;
202 //! // Early return on error
203 //! file.write_all(format!("name: {}\n", info.name).as_bytes())?;
204 //! file.write_all(format!("age: {}\n", info.age).as_bytes())?;
205 //! file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
210 //! *It's much nicer!*
212 //! Ending the expression with [`?`] will result in the unwrapped
213 //! success ([`Ok`]) value, unless the result is [`Err`], in which case
214 //! [`Err`] is returned early from the enclosing function.
216 //! [`?`] can only be used in functions that return [`Result`] because of the
217 //! early return of [`Err`] that it provides.
219 //! [`expect`]: Result::expect
220 //! [`Write`]: ../../std/io/trait.Write.html
221 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
222 //! [`io::Result`]: ../../std/io/type.Result.html
223 //! [`?`]: ../../std/macro.try.html
226 //! [`io::Error`]: ../../std/io/struct.Error.html
228 #![stable(feature = "rust1", since = "1.0.0")]
230 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
231 use crate::ops::{self, Deref, DerefMut};
232 use crate::{convert, fmt};
234 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
236 /// See the [`std::result`](index.html) module documentation for details.
237 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
238 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
239 #[rustc_diagnostic_item = "result_type"]
240 #[stable(feature = "rust1", since = "1.0.0")]
241 pub enum Result<T, E> {
242 /// Contains the success value
243 #[cfg_attr(not(bootstrap), lang = "Ok")]
244 #[stable(feature = "rust1", since = "1.0.0")]
245 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
247 /// Contains the error value
248 #[cfg_attr(not(bootstrap), lang = "Err")]
249 #[stable(feature = "rust1", since = "1.0.0")]
250 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
253 /////////////////////////////////////////////////////////////////////////////
254 // Type implementation
255 /////////////////////////////////////////////////////////////////////////////
257 impl<T, E> Result<T, E> {
258 /////////////////////////////////////////////////////////////////////////
259 // Querying the contained values
260 /////////////////////////////////////////////////////////////////////////
262 /// Returns `true` if the result is [`Ok`].
269 /// let x: Result<i32, &str> = Ok(-3);
270 /// assert_eq!(x.is_ok(), true);
272 /// let x: Result<i32, &str> = Err("Some error message");
273 /// assert_eq!(x.is_ok(), false);
275 #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
276 #[rustc_const_unstable(feature = "const_result", issue = "67520")]
278 #[stable(feature = "rust1", since = "1.0.0")]
279 pub const fn is_ok(&self) -> bool {
280 matches!(*self, Ok(_))
283 /// Returns `true` if the result is [`Err`].
290 /// let x: Result<i32, &str> = Ok(-3);
291 /// assert_eq!(x.is_err(), false);
293 /// let x: Result<i32, &str> = Err("Some error message");
294 /// assert_eq!(x.is_err(), true);
296 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
297 #[rustc_const_unstable(feature = "const_result", issue = "67520")]
299 #[stable(feature = "rust1", since = "1.0.0")]
300 pub const fn is_err(&self) -> bool {
304 /// Returns `true` if the result is an [`Ok`] value containing the given value.
309 /// #![feature(option_result_contains)]
311 /// let x: Result<u32, &str> = Ok(2);
312 /// assert_eq!(x.contains(&2), true);
314 /// let x: Result<u32, &str> = Ok(3);
315 /// assert_eq!(x.contains(&2), false);
317 /// let x: Result<u32, &str> = Err("Some error message");
318 /// assert_eq!(x.contains(&2), false);
322 #[unstable(feature = "option_result_contains", issue = "62358")]
323 pub fn contains<U>(&self, x: &U) -> bool
333 /// Returns `true` if the result is an [`Err`] value containing the given value.
338 /// #![feature(result_contains_err)]
340 /// let x: Result<u32, &str> = Ok(2);
341 /// assert_eq!(x.contains_err(&"Some error message"), false);
343 /// let x: Result<u32, &str> = Err("Some error message");
344 /// assert_eq!(x.contains_err(&"Some error message"), true);
346 /// let x: Result<u32, &str> = Err("Some other error message");
347 /// assert_eq!(x.contains_err(&"Some error message"), false);
351 #[unstable(feature = "result_contains_err", issue = "62358")]
352 pub fn contains_err<F>(&self, f: &F) -> bool
362 /////////////////////////////////////////////////////////////////////////
363 // Adapter for each variant
364 /////////////////////////////////////////////////////////////////////////
366 /// Converts from `Result<T, E>` to [`Option<T>`].
368 /// Converts `self` into an [`Option<T>`], consuming `self`,
369 /// and discarding the error, if any.
371 /// [`Option<T>`]: Option
378 /// let x: Result<u32, &str> = Ok(2);
379 /// assert_eq!(x.ok(), Some(2));
381 /// let x: Result<u32, &str> = Err("Nothing here");
382 /// assert_eq!(x.ok(), None);
385 #[stable(feature = "rust1", since = "1.0.0")]
386 pub fn ok(self) -> Option<T> {
393 /// Converts from `Result<T, E>` to [`Option<E>`].
395 /// Converts `self` into an [`Option<E>`], consuming `self`,
396 /// and discarding the success value, if any.
398 /// [`Option<E>`]: Option
405 /// let x: Result<u32, &str> = Ok(2);
406 /// assert_eq!(x.err(), None);
408 /// let x: Result<u32, &str> = Err("Nothing here");
409 /// assert_eq!(x.err(), Some("Nothing here"));
412 #[stable(feature = "rust1", since = "1.0.0")]
413 pub fn err(self) -> Option<E> {
420 /////////////////////////////////////////////////////////////////////////
421 // Adapter for working with references
422 /////////////////////////////////////////////////////////////////////////
424 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
426 /// Produces a new `Result`, containing a reference
427 /// into the original, leaving the original in place.
434 /// let x: Result<u32, &str> = Ok(2);
435 /// assert_eq!(x.as_ref(), Ok(&2));
437 /// let x: Result<u32, &str> = Err("Error");
438 /// assert_eq!(x.as_ref(), Err(&"Error"));
441 #[rustc_const_unstable(feature = "const_result", issue = "67520")]
442 #[stable(feature = "rust1", since = "1.0.0")]
443 pub const fn as_ref(&self) -> Result<&T, &E> {
446 Err(ref x) => Err(x),
450 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
457 /// fn mutate(r: &mut Result<i32, i32>) {
458 /// match r.as_mut() {
459 /// Ok(v) => *v = 42,
460 /// Err(e) => *e = 0,
464 /// let mut x: Result<i32, i32> = Ok(2);
466 /// assert_eq!(x.unwrap(), 42);
468 /// let mut x: Result<i32, i32> = Err(13);
470 /// assert_eq!(x.unwrap_err(), 0);
473 #[stable(feature = "rust1", since = "1.0.0")]
474 pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
476 Ok(ref mut x) => Ok(x),
477 Err(ref mut x) => Err(x),
481 /////////////////////////////////////////////////////////////////////////
482 // Transforming contained values
483 /////////////////////////////////////////////////////////////////////////
485 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
486 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
488 /// This function can be used to compose the results of two functions.
492 /// Print the numbers on each line of a string multiplied by two.
495 /// let line = "1\n2\n3\n4\n";
497 /// for num in line.lines() {
498 /// match num.parse::<i32>().map(|i| i * 2) {
499 /// Ok(n) => println!("{}", n),
505 #[stable(feature = "rust1", since = "1.0.0")]
506 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
513 /// Applies a function to the contained value (if [`Ok`]),
514 /// or returns the provided default (if [`Err`]).
516 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
517 /// the result of a function call, it is recommended to use [`map_or_else`],
518 /// which is lazily evaluated.
520 /// [`map_or_else`]: Result::map_or_else
525 /// let x: Result<_, &str> = Ok("foo");
526 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
528 /// let x: Result<&str, _> = Err("bar");
529 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
532 #[stable(feature = "result_map_or", since = "1.41.0")]
533 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
540 /// Maps a `Result<T, E>` to `U` by applying a function to a
541 /// contained [`Ok`] value, or a fallback function to a
542 /// contained [`Err`] value.
544 /// This function can be used to unpack a successful result
545 /// while handling an error.
555 /// let x : Result<_, &str> = Ok("foo");
556 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
558 /// let x : Result<&str, _> = Err("bar");
559 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
562 #[stable(feature = "result_map_or_else", since = "1.41.0")]
563 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
566 Err(e) => default(e),
570 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
571 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
573 /// This function can be used to pass through a successful result while handling
582 /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
584 /// let x: Result<u32, u32> = Ok(2);
585 /// assert_eq!(x.map_err(stringify), Ok(2));
587 /// let x: Result<u32, u32> = Err(13);
588 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
591 #[stable(feature = "rust1", since = "1.0.0")]
592 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
595 Err(e) => Err(op(e)),
599 /////////////////////////////////////////////////////////////////////////
600 // Iterator constructors
601 /////////////////////////////////////////////////////////////////////////
603 /// Returns an iterator over the possibly contained value.
605 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
612 /// let x: Result<u32, &str> = Ok(7);
613 /// assert_eq!(x.iter().next(), Some(&7));
615 /// let x: Result<u32, &str> = Err("nothing!");
616 /// assert_eq!(x.iter().next(), None);
619 #[stable(feature = "rust1", since = "1.0.0")]
620 pub fn iter(&self) -> Iter<'_, T> {
621 Iter { inner: self.as_ref().ok() }
624 /// Returns a mutable iterator over the possibly contained value.
626 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
633 /// let mut x: Result<u32, &str> = Ok(7);
634 /// match x.iter_mut().next() {
635 /// Some(v) => *v = 40,
638 /// assert_eq!(x, Ok(40));
640 /// let mut x: Result<u32, &str> = Err("nothing!");
641 /// assert_eq!(x.iter_mut().next(), None);
644 #[stable(feature = "rust1", since = "1.0.0")]
645 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
646 IterMut { inner: self.as_mut().ok() }
649 ////////////////////////////////////////////////////////////////////////
650 // Boolean operations on the values, eager and lazy
651 /////////////////////////////////////////////////////////////////////////
653 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
661 /// let x: Result<u32, &str> = Ok(2);
662 /// let y: Result<&str, &str> = Err("late error");
663 /// assert_eq!(x.and(y), Err("late error"));
665 /// let x: Result<u32, &str> = Err("early error");
666 /// let y: Result<&str, &str> = Ok("foo");
667 /// assert_eq!(x.and(y), Err("early error"));
669 /// let x: Result<u32, &str> = Err("not a 2");
670 /// let y: Result<&str, &str> = Err("late error");
671 /// assert_eq!(x.and(y), Err("not a 2"));
673 /// let x: Result<u32, &str> = Ok(2);
674 /// let y: Result<&str, &str> = Ok("different result type");
675 /// assert_eq!(x.and(y), Ok("different result type"));
678 #[stable(feature = "rust1", since = "1.0.0")]
679 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
686 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
689 /// This function can be used for control flow based on `Result` values.
696 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
697 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
699 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
700 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
701 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
702 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
705 #[stable(feature = "rust1", since = "1.0.0")]
706 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
713 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
715 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
716 /// result of a function call, it is recommended to use [`or_else`], which is
717 /// lazily evaluated.
719 /// [`or_else`]: Result::or_else
726 /// let x: Result<u32, &str> = Ok(2);
727 /// let y: Result<u32, &str> = Err("late error");
728 /// assert_eq!(x.or(y), Ok(2));
730 /// let x: Result<u32, &str> = Err("early error");
731 /// let y: Result<u32, &str> = Ok(2);
732 /// assert_eq!(x.or(y), Ok(2));
734 /// let x: Result<u32, &str> = Err("not a 2");
735 /// let y: Result<u32, &str> = Err("late error");
736 /// assert_eq!(x.or(y), Err("late error"));
738 /// let x: Result<u32, &str> = Ok(2);
739 /// let y: Result<u32, &str> = Ok(100);
740 /// assert_eq!(x.or(y), Ok(2));
743 #[stable(feature = "rust1", since = "1.0.0")]
744 pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
751 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
753 /// This function can be used for control flow based on result values.
761 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
762 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
764 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
765 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
766 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
767 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
770 #[stable(feature = "rust1", since = "1.0.0")]
771 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
778 /// Returns the contained [`Ok`] value or a provided default.
780 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
781 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
782 /// which is lazily evaluated.
784 /// [`unwrap_or_else`]: Result::unwrap_or_else
792 /// let x: Result<u32, &str> = Ok(9);
793 /// assert_eq!(x.unwrap_or(default), 9);
795 /// let x: Result<u32, &str> = Err("error");
796 /// assert_eq!(x.unwrap_or(default), default);
799 #[stable(feature = "rust1", since = "1.0.0")]
800 pub fn unwrap_or(self, default: T) -> T {
807 /// Returns the contained [`Ok`] value or computes it from a closure.
815 /// fn count(x: &str) -> usize { x.len() }
817 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
818 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
821 #[stable(feature = "rust1", since = "1.0.0")]
822 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
830 impl<T: Copy, E> Result<&T, E> {
831 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
837 /// #![feature(result_copied)]
839 /// let x: Result<&i32, i32> = Ok(&val);
840 /// assert_eq!(x, Ok(&12));
841 /// let copied = x.copied();
842 /// assert_eq!(copied, Ok(12));
844 #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
845 pub fn copied(self) -> Result<T, E> {
850 impl<T: Copy, E> Result<&mut T, E> {
851 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
857 /// #![feature(result_copied)]
858 /// let mut val = 12;
859 /// let x: Result<&mut i32, i32> = Ok(&mut val);
860 /// assert_eq!(x, Ok(&mut 12));
861 /// let copied = x.copied();
862 /// assert_eq!(copied, Ok(12));
864 #[unstable(feature = "result_copied", reason = "newly added", issue = "63168")]
865 pub fn copied(self) -> Result<T, E> {
870 impl<T: Clone, E> Result<&T, E> {
871 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
877 /// #![feature(result_cloned)]
879 /// let x: Result<&i32, i32> = Ok(&val);
880 /// assert_eq!(x, Ok(&12));
881 /// let cloned = x.cloned();
882 /// assert_eq!(cloned, Ok(12));
884 #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
885 pub fn cloned(self) -> Result<T, E> {
886 self.map(|t| t.clone())
890 impl<T: Clone, E> Result<&mut T, E> {
891 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
897 /// #![feature(result_cloned)]
898 /// let mut val = 12;
899 /// let x: Result<&mut i32, i32> = Ok(&mut val);
900 /// assert_eq!(x, Ok(&mut 12));
901 /// let cloned = x.cloned();
902 /// assert_eq!(cloned, Ok(12));
904 #[unstable(feature = "result_cloned", reason = "newly added", issue = "63168")]
905 pub fn cloned(self) -> Result<T, E> {
906 self.map(|t| t.clone())
910 impl<T, E: fmt::Debug> Result<T, E> {
911 /// Returns the contained [`Ok`] value, consuming the `self` value.
915 /// Panics if the value is an [`Err`], with a panic message including the
916 /// passed message, and the content of the [`Err`].
923 /// ```{.should_panic}
924 /// let x: Result<u32, &str> = Err("emergency failure");
925 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
929 #[stable(feature = "result_expect", since = "1.4.0")]
930 pub fn expect(self, msg: &str) -> T {
933 Err(e) => unwrap_failed(msg, &e),
937 /// Returns the contained [`Ok`] value, consuming the `self` value.
939 /// Because this function may panic, its use is generally discouraged.
940 /// Instead, prefer to use pattern matching and handle the [`Err`]
941 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
942 /// [`unwrap_or_default`].
944 /// [`unwrap_or`]: Result::unwrap_or
945 /// [`unwrap_or_else`]: Result::unwrap_or_else
946 /// [`unwrap_or_default`]: Result::unwrap_or_default
950 /// Panics if the value is an [`Err`], with a panic message provided by the
959 /// let x: Result<u32, &str> = Ok(2);
960 /// assert_eq!(x.unwrap(), 2);
963 /// ```{.should_panic}
964 /// let x: Result<u32, &str> = Err("emergency failure");
965 /// x.unwrap(); // panics with `emergency failure`
969 #[stable(feature = "rust1", since = "1.0.0")]
970 pub fn unwrap(self) -> T {
973 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
978 impl<T: fmt::Debug, E> Result<T, E> {
979 /// Returns the contained [`Err`] value, consuming the `self` value.
983 /// Panics if the value is an [`Ok`], with a panic message including the
984 /// passed message, and the content of the [`Ok`].
991 /// ```{.should_panic}
992 /// let x: Result<u32, &str> = Ok(10);
993 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
997 #[stable(feature = "result_expect_err", since = "1.17.0")]
998 pub fn expect_err(self, msg: &str) -> E {
1000 Ok(t) => unwrap_failed(msg, &t),
1005 /// Returns the contained [`Err`] value, consuming the `self` value.
1009 /// Panics if the value is an [`Ok`], with a custom panic message provided
1010 /// by the [`Ok`]'s value.
1016 /// ```{.should_panic}
1017 /// let x: Result<u32, &str> = Ok(2);
1018 /// x.unwrap_err(); // panics with `2`
1022 /// let x: Result<u32, &str> = Err("emergency failure");
1023 /// assert_eq!(x.unwrap_err(), "emergency failure");
1027 #[stable(feature = "rust1", since = "1.0.0")]
1028 pub fn unwrap_err(self) -> E {
1030 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1036 impl<T: Default, E> Result<T, E> {
1037 /// Returns the contained [`Ok`] value or a default
1039 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1040 /// value, otherwise if [`Err`], returns the default value for that
1045 /// Converts a string to an integer, turning poorly-formed strings
1046 /// into 0 (the default value for integers). [`parse`] converts
1047 /// a string to any other type that implements [`FromStr`], returning an
1048 /// [`Err`] on error.
1051 /// let good_year_from_input = "1909";
1052 /// let bad_year_from_input = "190blarg";
1053 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1054 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1056 /// assert_eq!(1909, good_year);
1057 /// assert_eq!(0, bad_year);
1060 /// [`parse`]: str::parse
1061 /// [`FromStr`]: ../../std/str/trait.FromStr.html
1063 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1064 pub fn unwrap_or_default(self) -> T {
1067 Err(_) => Default::default(),
1072 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1073 impl<T, E: Into<!>> Result<T, E> {
1074 /// Returns the contained [`Ok`] value, but never panics.
1076 /// Unlike [`unwrap`], this method is known to never panic on the
1077 /// result types it is implemented for. Therefore, it can be used
1078 /// instead of `unwrap` as a maintainability safeguard that will fail
1079 /// to compile if the error type of the `Result` is later changed
1080 /// to an error that can actually occur.
1082 /// [`unwrap`]: Result::unwrap
1089 /// # #![feature(never_type)]
1090 /// # #![feature(unwrap_infallible)]
1092 /// fn only_good_news() -> Result<String, !> {
1093 /// Ok("this is fine".into())
1096 /// let s: String = only_good_news().into_ok();
1097 /// println!("{}", s);
1100 pub fn into_ok(self) -> T {
1108 impl<T: Deref, E> Result<T, E> {
1109 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
1111 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
1112 /// and returns the new [`Result`].
1117 /// let x: Result<String, u32> = Ok("hello".to_string());
1118 /// let y: Result<&str, &u32> = Ok("hello");
1119 /// assert_eq!(x.as_deref(), y);
1121 /// let x: Result<String, u32> = Err(42);
1122 /// let y: Result<&str, &u32> = Err(&42);
1123 /// assert_eq!(x.as_deref(), y);
1125 #[stable(feature = "inner_deref", since = "1.47.0")]
1126 pub fn as_deref(&self) -> Result<&T::Target, &E> {
1127 self.as_ref().map(|t| t.deref())
1131 impl<T: DerefMut, E> Result<T, E> {
1132 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
1134 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
1135 /// and returns the new [`Result`].
1140 /// let mut s = "HELLO".to_string();
1141 /// let mut x: Result<String, u32> = Ok("hello".to_string());
1142 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
1143 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1146 /// let mut x: Result<String, u32> = Err(42);
1147 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
1148 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
1150 #[stable(feature = "inner_deref", since = "1.47.0")]
1151 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E> {
1152 self.as_mut().map(|t| t.deref_mut())
1156 impl<T, E> Result<Option<T>, E> {
1157 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1159 /// `Ok(None)` will be mapped to `None`.
1160 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1165 /// #[derive(Debug, Eq, PartialEq)]
1168 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1169 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1170 /// assert_eq!(x.transpose(), y);
1173 #[stable(feature = "transpose_result", since = "1.33.0")]
1174 pub fn transpose(self) -> Option<Result<T, E>> {
1176 Ok(Some(x)) => Some(Ok(x)),
1178 Err(e) => Some(Err(e)),
1183 impl<T, E> Result<Result<T, E>, E> {
1184 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1189 /// #![feature(result_flattening)]
1190 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1191 /// assert_eq!(Ok("hello"), x.flatten());
1193 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1194 /// assert_eq!(Err(6), x.flatten());
1196 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1197 /// assert_eq!(Err(6), x.flatten());
1200 /// Flattening once only removes one level of nesting:
1203 /// #![feature(result_flattening)]
1204 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1205 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1206 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1209 #[unstable(feature = "result_flattening", issue = "70142")]
1210 pub fn flatten(self) -> Result<T, E> {
1211 self.and_then(convert::identity)
1215 // This is a separate function to reduce the code size of the methods
1219 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1220 panic!("{}: {:?}", msg, error)
1223 /////////////////////////////////////////////////////////////////////////////
1224 // Trait implementations
1225 /////////////////////////////////////////////////////////////////////////////
1227 #[stable(feature = "rust1", since = "1.0.0")]
1228 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1230 fn clone(&self) -> Self {
1232 Ok(x) => Ok(x.clone()),
1233 Err(x) => Err(x.clone()),
1238 fn clone_from(&mut self, source: &Self) {
1239 match (self, source) {
1240 (Ok(to), Ok(from)) => to.clone_from(from),
1241 (Err(to), Err(from)) => to.clone_from(from),
1242 (to, from) => *to = from.clone(),
1247 #[stable(feature = "rust1", since = "1.0.0")]
1248 impl<T, E> IntoIterator for Result<T, E> {
1250 type IntoIter = IntoIter<T>;
1252 /// Returns a consuming iterator over the possibly contained value.
1254 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1261 /// let x: Result<u32, &str> = Ok(5);
1262 /// let v: Vec<u32> = x.into_iter().collect();
1263 /// assert_eq!(v, [5]);
1265 /// let x: Result<u32, &str> = Err("nothing!");
1266 /// let v: Vec<u32> = x.into_iter().collect();
1267 /// assert_eq!(v, []);
1270 fn into_iter(self) -> IntoIter<T> {
1271 IntoIter { inner: self.ok() }
1275 #[stable(since = "1.4.0", feature = "result_iter")]
1276 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1278 type IntoIter = Iter<'a, T>;
1280 fn into_iter(self) -> Iter<'a, T> {
1285 #[stable(since = "1.4.0", feature = "result_iter")]
1286 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1287 type Item = &'a mut T;
1288 type IntoIter = IterMut<'a, T>;
1290 fn into_iter(self) -> IterMut<'a, T> {
1295 /////////////////////////////////////////////////////////////////////////////
1296 // The Result Iterators
1297 /////////////////////////////////////////////////////////////////////////////
1299 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1301 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1303 /// Created by [`Result::iter`].
1305 #[stable(feature = "rust1", since = "1.0.0")]
1306 pub struct Iter<'a, T: 'a> {
1307 inner: Option<&'a T>,
1310 #[stable(feature = "rust1", since = "1.0.0")]
1311 impl<'a, T> Iterator for Iter<'a, T> {
1315 fn next(&mut self) -> Option<&'a T> {
1319 fn size_hint(&self) -> (usize, Option<usize>) {
1320 let n = if self.inner.is_some() { 1 } else { 0 };
1325 #[stable(feature = "rust1", since = "1.0.0")]
1326 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1328 fn next_back(&mut self) -> Option<&'a T> {
1333 #[stable(feature = "rust1", since = "1.0.0")]
1334 impl<T> ExactSizeIterator for Iter<'_, T> {}
1336 #[stable(feature = "fused", since = "1.26.0")]
1337 impl<T> FusedIterator for Iter<'_, T> {}
1339 #[unstable(feature = "trusted_len", issue = "37572")]
1340 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1342 #[stable(feature = "rust1", since = "1.0.0")]
1343 impl<T> Clone for Iter<'_, T> {
1345 fn clone(&self) -> Self {
1346 Iter { inner: self.inner }
1350 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1352 /// Created by [`Result::iter_mut`].
1354 #[stable(feature = "rust1", since = "1.0.0")]
1355 pub struct IterMut<'a, T: 'a> {
1356 inner: Option<&'a mut T>,
1359 #[stable(feature = "rust1", since = "1.0.0")]
1360 impl<'a, T> Iterator for IterMut<'a, T> {
1361 type Item = &'a mut T;
1364 fn next(&mut self) -> Option<&'a mut T> {
1368 fn size_hint(&self) -> (usize, Option<usize>) {
1369 let n = if self.inner.is_some() { 1 } else { 0 };
1374 #[stable(feature = "rust1", since = "1.0.0")]
1375 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1377 fn next_back(&mut self) -> Option<&'a mut T> {
1382 #[stable(feature = "rust1", since = "1.0.0")]
1383 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1385 #[stable(feature = "fused", since = "1.26.0")]
1386 impl<T> FusedIterator for IterMut<'_, T> {}
1388 #[unstable(feature = "trusted_len", issue = "37572")]
1389 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1391 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1393 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1395 /// This struct is created by the [`into_iter`] method on
1396 /// [`Result`] (provided by the [`IntoIterator`] trait).
1398 /// [`into_iter`]: IntoIterator::into_iter
1399 #[derive(Clone, Debug)]
1400 #[stable(feature = "rust1", since = "1.0.0")]
1401 pub struct IntoIter<T> {
1405 #[stable(feature = "rust1", since = "1.0.0")]
1406 impl<T> Iterator for IntoIter<T> {
1410 fn next(&mut self) -> Option<T> {
1414 fn size_hint(&self) -> (usize, Option<usize>) {
1415 let n = if self.inner.is_some() { 1 } else { 0 };
1420 #[stable(feature = "rust1", since = "1.0.0")]
1421 impl<T> DoubleEndedIterator for IntoIter<T> {
1423 fn next_back(&mut self) -> Option<T> {
1428 #[stable(feature = "rust1", since = "1.0.0")]
1429 impl<T> ExactSizeIterator for IntoIter<T> {}
1431 #[stable(feature = "fused", since = "1.26.0")]
1432 impl<T> FusedIterator for IntoIter<T> {}
1434 #[unstable(feature = "trusted_len", issue = "37572")]
1435 unsafe impl<A> TrustedLen for IntoIter<A> {}
1437 /////////////////////////////////////////////////////////////////////////////
1439 /////////////////////////////////////////////////////////////////////////////
1441 #[stable(feature = "rust1", since = "1.0.0")]
1442 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1443 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1444 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1445 /// container with the values of each `Result` is returned.
1447 /// Here is an example which increments every integer in a vector,
1448 /// checking for overflow:
1451 /// let v = vec![1, 2];
1452 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1453 /// x.checked_add(1).ok_or("Overflow!")
1455 /// assert_eq!(res, Ok(vec![2, 3]));
1458 /// Here is another example that tries to subtract one from another list
1459 /// of integers, this time checking for underflow:
1462 /// let v = vec![1, 2, 0];
1463 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1464 /// x.checked_sub(1).ok_or("Underflow!")
1466 /// assert_eq!(res, Err("Underflow!"));
1469 /// Here is a variation on the previous example, showing that no
1470 /// further elements are taken from `iter` after the first `Err`.
1473 /// let v = vec![3, 2, 1, 10];
1474 /// let mut shared = 0;
1475 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1477 /// x.checked_sub(2).ok_or("Underflow!")
1479 /// assert_eq!(res, Err("Underflow!"));
1480 /// assert_eq!(shared, 6);
1483 /// Since the third element caused an underflow, no further elements were taken,
1484 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1486 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
1487 // FIXME(#11084): This could be replaced with Iterator::scan when this
1488 // performance bug is closed.
1490 iter::process_results(iter.into_iter(), |i| i.collect())
1494 #[unstable(feature = "try_trait", issue = "42327")]
1495 impl<T, E> ops::Try for Result<T, E> {
1500 fn into_result(self) -> Self {
1505 fn from_ok(v: T) -> Self {
1510 fn from_error(v: E) -> Self {