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 <code>[Result]<T, [io::Error]></code>.*
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 "io::Write"
221 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all "io::Write::write_all"
222 //! [`io::Result`]: ../../std/io/type.Result.html "io::Result"
223 //! [`?`]: crate::ops::Try
226 //! [io::Error]: ../../std/io/struct.Error.html "io::Error"
228 //! # Method overview
230 //! In addition to working with pattern matching, [`Result`] provides a
231 //! wide variety of different methods.
233 //! ## Querying the variant
235 //! The [`is_ok`] and [`is_err`] methods return [`true`] if the [`Result`]
236 //! is [`Ok`] or [`Err`], respectively.
238 //! [`is_err`]: Result::is_err
239 //! [`is_ok`]: Result::is_ok
241 //! ## Adapters for working with references
243 //! * [`as_ref`] converts from `&Result<T, E>` to `Result<&T, &E>`
244 //! * [`as_mut`] converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`
245 //! * [`as_deref`] converts from `&Result<T, E>` to `Result<&T::Target, &E>`
246 //! * [`as_deref_mut`] converts from `&mut Result<T, E>` to
247 //! `Result<&mut T::Target, &mut E>`
249 //! [`as_deref`]: Result::as_deref
250 //! [`as_deref_mut`]: Result::as_deref_mut
251 //! [`as_mut`]: Result::as_mut
252 //! [`as_ref`]: Result::as_ref
254 //! ## Extracting contained values
256 //! These methods extract the contained value in a [`Result<T, E>`] when it
257 //! is the [`Ok`] variant. If the [`Result`] is [`Err`]:
259 //! * [`expect`] panics with a provided custom message
260 //! * [`unwrap`] panics with a generic message
261 //! * [`unwrap_or`] returns the provided default value
262 //! * [`unwrap_or_default`] returns the default value of the type `T`
263 //! (which must implement the [`Default`] trait)
264 //! * [`unwrap_or_else`] returns the result of evaluating the provided
267 //! The panicking methods [`expect`] and [`unwrap`] require `E` to
268 //! implement the [`Debug`] trait.
270 //! [`Debug`]: crate::fmt::Debug
271 //! [`expect`]: Result::expect
272 //! [`unwrap`]: Result::unwrap
273 //! [`unwrap_or`]: Result::unwrap_or
274 //! [`unwrap_or_default`]: Result::unwrap_or_default
275 //! [`unwrap_or_else`]: Result::unwrap_or_else
277 //! These methods extract the contained value in a [`Result<T, E>`] when it
278 //! is the [`Err`] variant. They require `T` to implement the [`Debug`]
279 //! trait. If the [`Result`] is [`Ok`]:
281 //! * [`expect_err`] panics with a provided custom message
282 //! * [`unwrap_err`] panics with a generic message
284 //! [`Debug`]: crate::fmt::Debug
285 //! [`expect_err`]: Result::expect_err
286 //! [`unwrap_err`]: Result::unwrap_err
288 //! ## Transforming contained values
290 //! These methods transform [`Result`] to [`Option`]:
292 //! * [`err`][Result::err] transforms [`Result<T, E>`] into [`Option<E>`],
293 //! mapping [`Err(e)`] to [`Some(e)`] and [`Ok(v)`] to [`None`]
294 //! * [`ok`][Result::ok] transforms [`Result<T, E>`] into [`Option<T>`],
295 //! mapping [`Ok(v)`] to [`Some(v)`] and [`Err(e)`] to [`None`]
296 //! * [`transpose`] transposes a [`Result`] of an [`Option`] into an
297 //! [`Option`] of a [`Result`]
299 // Do NOT add link reference definitions for `err` or `ok`, because they
300 // will generate numerous incorrect URLs for `Err` and `Ok` elsewhere, due
305 //! [`Some(e)`]: Option::Some
306 //! [`Some(v)`]: Option::Some
307 //! [`transpose`]: Result::transpose
309 //! This method transforms the contained value of the [`Ok`] variant:
311 //! * [`map`] transforms [`Result<T, E>`] into [`Result<U, E>`] by applying
312 //! the provided function to the contained value of [`Ok`] and leaving
313 //! [`Err`] values unchanged
315 //! [`map`]: Result::map
317 //! This method transforms the contained value of the [`Err`] variant:
319 //! * [`map_err`] transforms [`Result<T, E>`] into [`Result<T, F>`] by
320 //! applying the provided function to the contained value of [`Err`] and
321 //! leaving [`Ok`] values unchanged
323 //! [`map_err`]: Result::map_err
325 //! These methods transform a [`Result<T, E>`] into a value of a possibly
326 //! different type `U`:
328 //! * [`map_or`] applies the provided function to the contained value of
329 //! [`Ok`], or returns the provided default value if the [`Result`] is
331 //! * [`map_or_else`] applies the provided function to the contained value
332 //! of [`Ok`], or applies the provided default fallback function to the
333 //! contained value of [`Err`]
335 //! [`map_or`]: Result::map_or
336 //! [`map_or_else`]: Result::map_or_else
338 //! ## Boolean operators
340 //! These methods treat the [`Result`] as a boolean value, where [`Ok`]
341 //! acts like [`true`] and [`Err`] acts like [`false`]. There are two
342 //! categories of these methods: ones that take a [`Result`] as input, and
343 //! ones that take a function as input (to be lazily evaluated).
345 //! The [`and`] and [`or`] methods take another [`Result`] as input, and
346 //! produce a [`Result`] as output. The [`and`] method can produce a
347 //! [`Result<U, E>`] value having a different inner type `U` than
348 //! [`Result<T, E>`]. The [`or`] method can produce a [`Result<T, F>`]
349 //! value having a different error type `F` than [`Result<T, E>`].
351 //! | method | self | input | output |
352 //! |---------|----------|-----------|----------|
353 //! | [`and`] | `Err(e)` | (ignored) | `Err(e)` |
354 //! | [`and`] | `Ok(x)` | `Err(d)` | `Err(d)` |
355 //! | [`and`] | `Ok(x)` | `Ok(y)` | `Ok(y)` |
356 //! | [`or`] | `Err(e)` | `Err(d)` | `Err(d)` |
357 //! | [`or`] | `Err(e)` | `Ok(y)` | `Ok(y)` |
358 //! | [`or`] | `Ok(x)` | (ignored) | `Ok(x)` |
360 //! [`and`]: Result::and
361 //! [`or`]: Result::or
363 //! The [`and_then`] and [`or_else`] methods take a function as input, and
364 //! only evaluate the function when they need to produce a new value. The
365 //! [`and_then`] method can produce a [`Result<U, E>`] value having a
366 //! different inner type `U` than [`Result<T, E>`]. The [`or_else`] method
367 //! can produce a [`Result<T, F>`] value having a different error type `F`
368 //! than [`Result<T, E>`].
370 //! | method | self | function input | function result | output |
371 //! |--------------|----------|----------------|-----------------|----------|
372 //! | [`and_then`] | `Err(e)` | (not provided) | (not evaluated) | `Err(e)` |
373 //! | [`and_then`] | `Ok(x)` | `x` | `Err(d)` | `Err(d)` |
374 //! | [`and_then`] | `Ok(x)` | `x` | `Ok(y)` | `Ok(y)` |
375 //! | [`or_else`] | `Err(e)` | `e` | `Err(d)` | `Err(d)` |
376 //! | [`or_else`] | `Err(e)` | `e` | `Ok(y)` | `Ok(y)` |
377 //! | [`or_else`] | `Ok(x)` | (not provided) | (not evaluated) | `Ok(x)` |
379 //! [`and_then`]: Result::and_then
380 //! [`or_else`]: Result::or_else
382 //! ## Comparison operators
384 //! If `T` and `E` both implement [`PartialOrd`] then [`Result<T, E>`] will
385 //! derive its [`PartialOrd`] implementation. With this order, an [`Ok`]
386 //! compares as less than any [`Err`], while two [`Ok`] or two [`Err`]
387 //! compare as their contained values would in `T` or `E` respectively. If `T`
388 //! and `E` both also implement [`Ord`], then so does [`Result<T, E>`].
391 //! assert!(Ok(1) < Err(0));
392 //! let x: Result<i32, ()> = Ok(0);
395 //! let x: Result<(), i32> = Err(0);
400 //! ## Iterating over `Result`
402 //! A [`Result`] can be iterated over. This can be helpful if you need an
403 //! iterator that is conditionally empty. The iterator will either produce
404 //! a single value (when the [`Result`] is [`Ok`]), or produce no values
405 //! (when the [`Result`] is [`Err`]). For example, [`into_iter`] acts like
406 //! [`once(v)`] if the [`Result`] is [`Ok(v)`], and like [`empty()`] if the
407 //! [`Result`] is [`Err`].
410 //! [`empty()`]: crate::iter::empty
411 //! [`once(v)`]: crate::iter::once
413 //! Iterators over [`Result<T, E>`] come in three types:
415 //! * [`into_iter`] consumes the [`Result`] and produces the contained
417 //! * [`iter`] produces an immutable reference of type `&T` to the
419 //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
422 //! See [Iterating over `Option`] for examples of how this can be useful.
424 //! [Iterating over `Option`]: crate::option#iterating-over-option
425 //! [`into_iter`]: Result::into_iter
426 //! [`iter`]: Result::iter
427 //! [`iter_mut`]: Result::iter_mut
429 //! You might want to use an iterator chain to do multiple instances of an
430 //! operation that can fail, but would like to ignore failures while
431 //! continuing to process the successful results. In this example, we take
432 //! advantage of the iterable nature of [`Result`] to select only the
433 //! [`Ok`] values using [`flatten`][Iterator::flatten].
436 //! # use std::str::FromStr;
437 //! let mut results = vec![];
438 //! let mut errs = vec![];
439 //! let nums: Vec<_> = ["17", "not a number", "99", "-27", "768"]
441 //! .map(u8::from_str)
442 //! // Save clones of the raw `Result` values to inspect
443 //! .inspect(|x| results.push(x.clone()))
444 //! // Challenge: explain how this captures only the `Err` values
445 //! .inspect(|x| errs.extend(x.clone().err()))
448 //! assert_eq!(errs.len(), 3);
449 //! assert_eq!(nums, [17, 99]);
450 //! println!("results {results:?}");
451 //! println!("errs {errs:?}");
452 //! println!("nums {nums:?}");
455 //! ## Collecting into `Result`
457 //! [`Result`] implements the [`FromIterator`][impl-FromIterator] trait,
458 //! which allows an iterator over [`Result`] values to be collected into a
459 //! [`Result`] of a collection of each contained value of the original
460 //! [`Result`] values, or [`Err`] if any of the elements was [`Err`].
462 //! [impl-FromIterator]: Result#impl-FromIterator%3CResult%3CA%2C%20E%3E%3E
465 //! let v = [Ok(2), Ok(4), Err("err!"), Ok(8)];
466 //! let res: Result<Vec<_>, &str> = v.into_iter().collect();
467 //! assert_eq!(res, Err("err!"));
468 //! let v = [Ok(2), Ok(4), Ok(8)];
469 //! let res: Result<Vec<_>, &str> = v.into_iter().collect();
470 //! assert_eq!(res, Ok(vec![2, 4, 8]));
473 //! [`Result`] also implements the [`Product`][impl-Product] and
474 //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Result`] values
475 //! to provide the [`product`][Iterator::product] and
476 //! [`sum`][Iterator::sum] methods.
478 //! [impl-Product]: Result#impl-Product%3CResult%3CU%2C%20E%3E%3E
479 //! [impl-Sum]: Result#impl-Sum%3CResult%3CU%2C%20E%3E%3E
482 //! let v = [Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
483 //! let res: Result<i32, &str> = v.into_iter().sum();
484 //! assert_eq!(res, Err("error!"));
485 //! let v = [Ok(1), Ok(2), Ok(21)];
486 //! let res: Result<i32, &str> = v.into_iter().product();
487 //! assert_eq!(res, Ok(42));
490 #![stable(feature = "rust1", since = "1.0.0")]
492 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
493 use crate::marker::Destruct;
494 use crate::ops::{self, ControlFlow, Deref, DerefMut};
495 use crate::{convert, fmt, hint};
497 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
499 /// See the [module documentation](self) for details.
500 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
501 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
502 #[rustc_diagnostic_item = "Result"]
503 #[stable(feature = "rust1", since = "1.0.0")]
504 pub enum Result<T, E> {
505 /// Contains the success value
507 #[stable(feature = "rust1", since = "1.0.0")]
508 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
510 /// Contains the error value
512 #[stable(feature = "rust1", since = "1.0.0")]
513 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
516 /////////////////////////////////////////////////////////////////////////////
517 // Type implementation
518 /////////////////////////////////////////////////////////////////////////////
520 impl<T, E> Result<T, E> {
521 /////////////////////////////////////////////////////////////////////////
522 // Querying the contained values
523 /////////////////////////////////////////////////////////////////////////
525 /// Returns `true` if the result is [`Ok`].
532 /// let x: Result<i32, &str> = Ok(-3);
533 /// assert_eq!(x.is_ok(), true);
535 /// let x: Result<i32, &str> = Err("Some error message");
536 /// assert_eq!(x.is_ok(), false);
538 #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
539 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
541 #[stable(feature = "rust1", since = "1.0.0")]
542 pub const fn is_ok(&self) -> bool {
543 matches!(*self, Ok(_))
546 /// Returns `true` if the result is [`Ok`] and the value inside of it matches a predicate.
551 /// #![feature(is_some_with)]
553 /// let x: Result<u32, &str> = Ok(2);
554 /// assert_eq!(x.is_ok_and(|&x| x > 1), true);
556 /// let x: Result<u32, &str> = Ok(0);
557 /// assert_eq!(x.is_ok_and(|&x| x > 1), false);
559 /// let x: Result<u32, &str> = Err("hey");
560 /// assert_eq!(x.is_ok_and(|&x| x > 1), false);
564 #[unstable(feature = "is_some_with", issue = "93050")]
565 pub fn is_ok_and(&self, f: impl FnOnce(&T) -> bool) -> bool {
566 matches!(self, Ok(x) if f(x))
569 /// Returns `true` if the result is [`Err`].
576 /// let x: Result<i32, &str> = Ok(-3);
577 /// assert_eq!(x.is_err(), false);
579 /// let x: Result<i32, &str> = Err("Some error message");
580 /// assert_eq!(x.is_err(), true);
582 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
583 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
585 #[stable(feature = "rust1", since = "1.0.0")]
586 pub const fn is_err(&self) -> bool {
590 /// Returns `true` if the result is [`Err`] and the value inside of it matches a predicate.
595 /// #![feature(is_some_with)]
596 /// use std::io::{Error, ErrorKind};
598 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
599 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), true);
601 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
602 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
604 /// let x: Result<u32, Error> = Ok(123);
605 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
609 #[unstable(feature = "is_some_with", issue = "93050")]
610 pub fn is_err_and(&self, f: impl FnOnce(&E) -> bool) -> bool {
611 matches!(self, Err(x) if f(x))
614 /////////////////////////////////////////////////////////////////////////
615 // Adapter for each variant
616 /////////////////////////////////////////////////////////////////////////
618 /// Converts from `Result<T, E>` to [`Option<T>`].
620 /// Converts `self` into an [`Option<T>`], consuming `self`,
621 /// and discarding the error, if any.
628 /// let x: Result<u32, &str> = Ok(2);
629 /// assert_eq!(x.ok(), Some(2));
631 /// let x: Result<u32, &str> = Err("Nothing here");
632 /// assert_eq!(x.ok(), None);
635 #[stable(feature = "rust1", since = "1.0.0")]
636 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
637 pub const fn ok(self) -> Option<T>
643 // FIXME: ~const Drop doesn't quite work right yet
644 #[allow(unused_variables)]
649 /// Converts from `Result<T, E>` to [`Option<E>`].
651 /// Converts `self` into an [`Option<E>`], consuming `self`,
652 /// and discarding the success value, if any.
659 /// let x: Result<u32, &str> = Ok(2);
660 /// assert_eq!(x.err(), None);
662 /// let x: Result<u32, &str> = Err("Nothing here");
663 /// assert_eq!(x.err(), Some("Nothing here"));
666 #[stable(feature = "rust1", since = "1.0.0")]
667 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
668 pub const fn err(self) -> Option<E>
673 // FIXME: ~const Drop doesn't quite work right yet
674 #[allow(unused_variables)]
680 /////////////////////////////////////////////////////////////////////////
681 // Adapter for working with references
682 /////////////////////////////////////////////////////////////////////////
684 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
686 /// Produces a new `Result`, containing a reference
687 /// into the original, leaving the original in place.
694 /// let x: Result<u32, &str> = Ok(2);
695 /// assert_eq!(x.as_ref(), Ok(&2));
697 /// let x: Result<u32, &str> = Err("Error");
698 /// assert_eq!(x.as_ref(), Err(&"Error"));
701 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
702 #[stable(feature = "rust1", since = "1.0.0")]
703 pub const fn as_ref(&self) -> Result<&T, &E> {
706 Err(ref x) => Err(x),
710 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
717 /// fn mutate(r: &mut Result<i32, i32>) {
718 /// match r.as_mut() {
719 /// Ok(v) => *v = 42,
720 /// Err(e) => *e = 0,
724 /// let mut x: Result<i32, i32> = Ok(2);
726 /// assert_eq!(x.unwrap(), 42);
728 /// let mut x: Result<i32, i32> = Err(13);
730 /// assert_eq!(x.unwrap_err(), 0);
733 #[stable(feature = "rust1", since = "1.0.0")]
734 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
735 pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
737 Ok(ref mut x) => Ok(x),
738 Err(ref mut x) => Err(x),
742 /////////////////////////////////////////////////////////////////////////
743 // Transforming contained values
744 /////////////////////////////////////////////////////////////////////////
746 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
747 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
749 /// This function can be used to compose the results of two functions.
753 /// Print the numbers on each line of a string multiplied by two.
756 /// let line = "1\n2\n3\n4\n";
758 /// for num in line.lines() {
759 /// match num.parse::<i32>().map(|i| i * 2) {
760 /// Ok(n) => println!("{n}"),
766 #[stable(feature = "rust1", since = "1.0.0")]
767 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
774 /// Returns the provided default (if [`Err`]), or
775 /// applies a function to the contained value (if [`Ok`]),
777 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
778 /// the result of a function call, it is recommended to use [`map_or_else`],
779 /// which is lazily evaluated.
781 /// [`map_or_else`]: Result::map_or_else
786 /// let x: Result<_, &str> = Ok("foo");
787 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
789 /// let x: Result<&str, _> = Err("bar");
790 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
793 #[stable(feature = "result_map_or", since = "1.41.0")]
794 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
801 /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
802 /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
804 /// This function can be used to unpack a successful result
805 /// while handling an error.
815 /// let x : Result<_, &str> = Ok("foo");
816 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
818 /// let x : Result<&str, _> = Err("bar");
819 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
822 #[stable(feature = "result_map_or_else", since = "1.41.0")]
823 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
826 Err(e) => default(e),
830 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
831 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
833 /// This function can be used to pass through a successful result while handling
842 /// fn stringify(x: u32) -> String { format!("error code: {x}") }
844 /// let x: Result<u32, u32> = Ok(2);
845 /// assert_eq!(x.map_err(stringify), Ok(2));
847 /// let x: Result<u32, u32> = Err(13);
848 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
851 #[stable(feature = "rust1", since = "1.0.0")]
852 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
855 Err(e) => Err(op(e)),
859 /// Calls the provided closure with a reference to the contained value (if [`Ok`]).
864 /// #![feature(result_option_inspect)]
868 /// .inspect(|x| println!("original: {x}"))
869 /// .map(|x| x.pow(3))
870 /// .expect("failed to parse number");
873 #[unstable(feature = "result_option_inspect", issue = "91345")]
874 pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
875 if let Ok(ref t) = self {
882 /// Calls the provided closure with a reference to the contained error (if [`Err`]).
887 /// #![feature(result_option_inspect)]
889 /// use std::{fs, io};
891 /// fn read() -> io::Result<String> {
892 /// fs::read_to_string("address.txt")
893 /// .inspect_err(|e| eprintln!("failed to read file: {e}"))
897 #[unstable(feature = "result_option_inspect", issue = "91345")]
898 pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self {
899 if let Err(ref e) = self {
906 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
908 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
909 /// and returns the new [`Result`].
914 /// let x: Result<String, u32> = Ok("hello".to_string());
915 /// let y: Result<&str, &u32> = Ok("hello");
916 /// assert_eq!(x.as_deref(), y);
918 /// let x: Result<String, u32> = Err(42);
919 /// let y: Result<&str, &u32> = Err(&42);
920 /// assert_eq!(x.as_deref(), y);
922 #[stable(feature = "inner_deref", since = "1.47.0")]
923 pub fn as_deref(&self) -> Result<&T::Target, &E>
927 self.as_ref().map(|t| t.deref())
930 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
932 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
933 /// and returns the new [`Result`].
938 /// let mut s = "HELLO".to_string();
939 /// let mut x: Result<String, u32> = Ok("hello".to_string());
940 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
941 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
944 /// let mut x: Result<String, u32> = Err(42);
945 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
946 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
948 #[stable(feature = "inner_deref", since = "1.47.0")]
949 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
953 self.as_mut().map(|t| t.deref_mut())
956 /////////////////////////////////////////////////////////////////////////
957 // Iterator constructors
958 /////////////////////////////////////////////////////////////////////////
960 /// Returns an iterator over the possibly contained value.
962 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
969 /// let x: Result<u32, &str> = Ok(7);
970 /// assert_eq!(x.iter().next(), Some(&7));
972 /// let x: Result<u32, &str> = Err("nothing!");
973 /// assert_eq!(x.iter().next(), None);
976 #[stable(feature = "rust1", since = "1.0.0")]
977 pub fn iter(&self) -> Iter<'_, T> {
978 Iter { inner: self.as_ref().ok() }
981 /// Returns a mutable iterator over the possibly contained value.
983 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
990 /// let mut x: Result<u32, &str> = Ok(7);
991 /// match x.iter_mut().next() {
992 /// Some(v) => *v = 40,
995 /// assert_eq!(x, Ok(40));
997 /// let mut x: Result<u32, &str> = Err("nothing!");
998 /// assert_eq!(x.iter_mut().next(), None);
1001 #[stable(feature = "rust1", since = "1.0.0")]
1002 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1003 IterMut { inner: self.as_mut().ok() }
1006 /////////////////////////////////////////////////////////////////////////
1008 /////////////////////////////////////////////////////////////////////////
1010 /// Returns the contained [`Ok`] value, consuming the `self` value.
1014 /// Panics if the value is an [`Err`], with a panic message including the
1015 /// passed message, and the content of the [`Err`].
1023 /// let x: Result<u32, &str> = Err("emergency failure");
1024 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1028 #[stable(feature = "result_expect", since = "1.4.0")]
1029 pub fn expect(self, msg: &str) -> T
1035 Err(e) => unwrap_failed(msg, &e),
1039 /// Returns the contained [`Ok`] value, consuming the `self` value.
1041 /// Because this function may panic, its use is generally discouraged.
1042 /// Instead, prefer to use pattern matching and handle the [`Err`]
1043 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1044 /// [`unwrap_or_default`].
1046 /// [`unwrap_or`]: Result::unwrap_or
1047 /// [`unwrap_or_else`]: Result::unwrap_or_else
1048 /// [`unwrap_or_default`]: Result::unwrap_or_default
1052 /// Panics if the value is an [`Err`], with a panic message provided by the
1053 /// [`Err`]'s value.
1061 /// let x: Result<u32, &str> = Ok(2);
1062 /// assert_eq!(x.unwrap(), 2);
1066 /// let x: Result<u32, &str> = Err("emergency failure");
1067 /// x.unwrap(); // panics with `emergency failure`
1071 #[stable(feature = "rust1", since = "1.0.0")]
1072 pub fn unwrap(self) -> T
1078 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1082 /// Returns the contained [`Ok`] value or a default
1084 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1085 /// value, otherwise if [`Err`], returns the default value for that
1090 /// Converts a string to an integer, turning poorly-formed strings
1091 /// into 0 (the default value for integers). [`parse`] converts
1092 /// a string to any other type that implements [`FromStr`], returning an
1093 /// [`Err`] on error.
1096 /// let good_year_from_input = "1909";
1097 /// let bad_year_from_input = "190blarg";
1098 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1099 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1101 /// assert_eq!(1909, good_year);
1102 /// assert_eq!(0, bad_year);
1105 /// [`parse`]: str::parse
1106 /// [`FromStr`]: crate::str::FromStr
1108 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1109 pub fn unwrap_or_default(self) -> T
1115 Err(_) => Default::default(),
1119 /// Returns the contained [`Err`] value, consuming the `self` value.
1123 /// Panics if the value is an [`Ok`], with a panic message including the
1124 /// passed message, and the content of the [`Ok`].
1132 /// let x: Result<u32, &str> = Ok(10);
1133 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1137 #[stable(feature = "result_expect_err", since = "1.17.0")]
1138 pub fn expect_err(self, msg: &str) -> E
1143 Ok(t) => unwrap_failed(msg, &t),
1148 /// Returns the contained [`Err`] value, consuming the `self` value.
1152 /// Panics if the value is an [`Ok`], with a custom panic message provided
1153 /// by the [`Ok`]'s value.
1158 /// let x: Result<u32, &str> = Ok(2);
1159 /// x.unwrap_err(); // panics with `2`
1163 /// let x: Result<u32, &str> = Err("emergency failure");
1164 /// assert_eq!(x.unwrap_err(), "emergency failure");
1168 #[stable(feature = "rust1", since = "1.0.0")]
1169 pub fn unwrap_err(self) -> E
1174 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1179 /// Returns the contained [`Ok`] value, but never panics.
1181 /// Unlike [`unwrap`], this method is known to never panic on the
1182 /// result types it is implemented for. Therefore, it can be used
1183 /// instead of `unwrap` as a maintainability safeguard that will fail
1184 /// to compile if the error type of the `Result` is later changed
1185 /// to an error that can actually occur.
1187 /// [`unwrap`]: Result::unwrap
1194 /// # #![feature(never_type)]
1195 /// # #![feature(unwrap_infallible)]
1197 /// fn only_good_news() -> Result<String, !> {
1198 /// Ok("this is fine".into())
1201 /// let s: String = only_good_news().into_ok();
1202 /// println!("{s}");
1204 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1206 pub fn into_ok(self) -> T
1216 /// Returns the contained [`Err`] value, but never panics.
1218 /// Unlike [`unwrap_err`], this method is known to never panic on the
1219 /// result types it is implemented for. Therefore, it can be used
1220 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1221 /// to compile if the ok type of the `Result` is later changed
1222 /// to a type that can actually occur.
1224 /// [`unwrap_err`]: Result::unwrap_err
1231 /// # #![feature(never_type)]
1232 /// # #![feature(unwrap_infallible)]
1234 /// fn only_bad_news() -> Result<!, String> {
1235 /// Err("Oops, it failed".into())
1238 /// let error: String = only_bad_news().into_err();
1239 /// println!("{error}");
1241 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1243 pub fn into_err(self) -> E
1253 ////////////////////////////////////////////////////////////////////////
1254 // Boolean operations on the values, eager and lazy
1255 /////////////////////////////////////////////////////////////////////////
1257 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1265 /// let x: Result<u32, &str> = Ok(2);
1266 /// let y: Result<&str, &str> = Err("late error");
1267 /// assert_eq!(x.and(y), Err("late error"));
1269 /// let x: Result<u32, &str> = Err("early error");
1270 /// let y: Result<&str, &str> = Ok("foo");
1271 /// assert_eq!(x.and(y), Err("early error"));
1273 /// let x: Result<u32, &str> = Err("not a 2");
1274 /// let y: Result<&str, &str> = Err("late error");
1275 /// assert_eq!(x.and(y), Err("not a 2"));
1277 /// let x: Result<u32, &str> = Ok(2);
1278 /// let y: Result<&str, &str> = Ok("different result type");
1279 /// assert_eq!(x.and(y), Ok("different result type"));
1282 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
1283 #[stable(feature = "rust1", since = "1.0.0")]
1284 pub const fn and<U>(self, res: Result<U, E>) -> Result<U, E>
1291 // FIXME: ~const Drop doesn't quite work right yet
1292 #[allow(unused_variables)]
1298 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1301 /// This function can be used for control flow based on `Result` values.
1306 /// fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
1307 /// x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
1310 /// assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
1311 /// assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
1312 /// assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));
1315 /// Often used to chain fallible operations that may return [`Err`].
1318 /// use std::{io::ErrorKind, path::Path};
1320 /// // Note: on Windows "/" maps to "C:\"
1321 /// let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
1322 /// assert!(root_modified_time.is_ok());
1324 /// let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
1325 /// assert!(should_fail.is_err());
1326 /// assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);
1329 #[stable(feature = "rust1", since = "1.0.0")]
1330 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
1337 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1339 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1340 /// result of a function call, it is recommended to use [`or_else`], which is
1341 /// lazily evaluated.
1343 /// [`or_else`]: Result::or_else
1350 /// let x: Result<u32, &str> = Ok(2);
1351 /// let y: Result<u32, &str> = Err("late error");
1352 /// assert_eq!(x.or(y), Ok(2));
1354 /// let x: Result<u32, &str> = Err("early error");
1355 /// let y: Result<u32, &str> = Ok(2);
1356 /// assert_eq!(x.or(y), Ok(2));
1358 /// let x: Result<u32, &str> = Err("not a 2");
1359 /// let y: Result<u32, &str> = Err("late error");
1360 /// assert_eq!(x.or(y), Err("late error"));
1362 /// let x: Result<u32, &str> = Ok(2);
1363 /// let y: Result<u32, &str> = Ok(100);
1364 /// assert_eq!(x.or(y), Ok(2));
1367 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
1368 #[stable(feature = "rust1", since = "1.0.0")]
1369 pub const fn or<F>(self, res: Result<T, F>) -> Result<T, F>
1377 // FIXME: ~const Drop doesn't quite work right yet
1378 #[allow(unused_variables)]
1383 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1385 /// This function can be used for control flow based on result values.
1393 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1394 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1396 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1397 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1398 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1399 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1402 #[stable(feature = "rust1", since = "1.0.0")]
1403 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
1410 /// Returns the contained [`Ok`] value or a provided default.
1412 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1413 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1414 /// which is lazily evaluated.
1416 /// [`unwrap_or_else`]: Result::unwrap_or_else
1423 /// let default = 2;
1424 /// let x: Result<u32, &str> = Ok(9);
1425 /// assert_eq!(x.unwrap_or(default), 9);
1427 /// let x: Result<u32, &str> = Err("error");
1428 /// assert_eq!(x.unwrap_or(default), default);
1431 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
1432 #[stable(feature = "rust1", since = "1.0.0")]
1433 pub const fn unwrap_or(self, default: T) -> T
1440 // FIXME: ~const Drop doesn't quite work right yet
1441 #[allow(unused_variables)]
1446 /// Returns the contained [`Ok`] value or computes it from a closure.
1454 /// fn count(x: &str) -> usize { x.len() }
1456 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1457 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1460 #[stable(feature = "rust1", since = "1.0.0")]
1461 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
1468 /// Returns the contained [`Ok`] value, consuming the `self` value,
1469 /// without checking that the value is not an [`Err`].
1473 /// Calling this method on an [`Err`] is *[undefined behavior]*.
1475 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1480 /// let x: Result<u32, &str> = Ok(2);
1481 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1485 /// let x: Result<u32, &str> = Err("emergency failure");
1486 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
1490 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1491 pub unsafe fn unwrap_unchecked(self) -> T {
1492 debug_assert!(self.is_ok());
1495 // SAFETY: the safety contract must be upheld by the caller.
1496 Err(_) => unsafe { hint::unreachable_unchecked() },
1500 /// Returns the contained [`Err`] value, consuming the `self` value,
1501 /// without checking that the value is not an [`Ok`].
1505 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1507 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1512 /// let x: Result<u32, &str> = Ok(2);
1513 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1517 /// let x: Result<u32, &str> = Err("emergency failure");
1518 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1522 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1523 pub unsafe fn unwrap_err_unchecked(self) -> E {
1524 debug_assert!(self.is_err());
1526 // SAFETY: the safety contract must be upheld by the caller.
1527 Ok(_) => unsafe { hint::unreachable_unchecked() },
1532 /////////////////////////////////////////////////////////////////////////
1534 /////////////////////////////////////////////////////////////////////////
1536 /// Returns `true` if the result is an [`Ok`] value containing the given value.
1541 /// #![feature(option_result_contains)]
1543 /// let x: Result<u32, &str> = Ok(2);
1544 /// assert_eq!(x.contains(&2), true);
1546 /// let x: Result<u32, &str> = Ok(3);
1547 /// assert_eq!(x.contains(&2), false);
1549 /// let x: Result<u32, &str> = Err("Some error message");
1550 /// assert_eq!(x.contains(&2), false);
1554 #[unstable(feature = "option_result_contains", issue = "62358")]
1555 pub fn contains<U>(&self, x: &U) -> bool
1565 /// Returns `true` if the result is an [`Err`] value containing the given value.
1570 /// #![feature(result_contains_err)]
1572 /// let x: Result<u32, &str> = Ok(2);
1573 /// assert_eq!(x.contains_err(&"Some error message"), false);
1575 /// let x: Result<u32, &str> = Err("Some error message");
1576 /// assert_eq!(x.contains_err(&"Some error message"), true);
1578 /// let x: Result<u32, &str> = Err("Some other error message");
1579 /// assert_eq!(x.contains_err(&"Some error message"), false);
1583 #[unstable(feature = "result_contains_err", issue = "62358")]
1584 pub fn contains_err<F>(&self, f: &F) -> bool
1595 impl<T, E> Result<&T, E> {
1596 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1603 /// let x: Result<&i32, i32> = Ok(&val);
1604 /// assert_eq!(x, Ok(&12));
1605 /// let copied = x.copied();
1606 /// assert_eq!(copied, Ok(12));
1609 #[stable(feature = "result_copied", since = "1.59.0")]
1610 pub fn copied(self) -> Result<T, E>
1617 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1624 /// let x: Result<&i32, i32> = Ok(&val);
1625 /// assert_eq!(x, Ok(&12));
1626 /// let cloned = x.cloned();
1627 /// assert_eq!(cloned, Ok(12));
1630 #[stable(feature = "result_cloned", since = "1.59.0")]
1631 pub fn cloned(self) -> Result<T, E>
1635 self.map(|t| t.clone())
1639 impl<T, E> Result<&mut T, E> {
1640 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1646 /// let mut val = 12;
1647 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1648 /// assert_eq!(x, Ok(&mut 12));
1649 /// let copied = x.copied();
1650 /// assert_eq!(copied, Ok(12));
1653 #[stable(feature = "result_copied", since = "1.59.0")]
1654 pub fn copied(self) -> Result<T, E>
1658 self.map(|&mut t| t)
1661 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1667 /// let mut val = 12;
1668 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1669 /// assert_eq!(x, Ok(&mut 12));
1670 /// let cloned = x.cloned();
1671 /// assert_eq!(cloned, Ok(12));
1674 #[stable(feature = "result_cloned", since = "1.59.0")]
1675 pub fn cloned(self) -> Result<T, E>
1679 self.map(|t| t.clone())
1683 impl<T, E> Result<Option<T>, E> {
1684 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1686 /// `Ok(None)` will be mapped to `None`.
1687 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1692 /// #[derive(Debug, Eq, PartialEq)]
1695 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1696 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1697 /// assert_eq!(x.transpose(), y);
1700 #[stable(feature = "transpose_result", since = "1.33.0")]
1701 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
1702 pub const fn transpose(self) -> Option<Result<T, E>> {
1704 Ok(Some(x)) => Some(Ok(x)),
1706 Err(e) => Some(Err(e)),
1711 impl<T, E> Result<Result<T, E>, E> {
1712 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1719 /// #![feature(result_flattening)]
1720 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1721 /// assert_eq!(Ok("hello"), x.flatten());
1723 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1724 /// assert_eq!(Err(6), x.flatten());
1726 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1727 /// assert_eq!(Err(6), x.flatten());
1730 /// Flattening only removes one level of nesting at a time:
1733 /// #![feature(result_flattening)]
1734 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1735 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1736 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1739 #[unstable(feature = "result_flattening", issue = "70142")]
1740 pub fn flatten(self) -> Result<T, E> {
1741 self.and_then(convert::identity)
1745 impl<T> Result<T, T> {
1746 /// Returns the [`Ok`] value if `self` is `Ok`, and the [`Err`] value if
1747 /// `self` is `Err`.
1749 /// In other words, this function returns the value (the `T`) of a
1750 /// `Result<T, T>`, regardless of whether or not that result is `Ok` or
1753 /// This can be useful in conjunction with APIs such as
1754 /// [`Atomic*::compare_exchange`], or [`slice::binary_search`], but only in
1755 /// cases where you don't care if the result was `Ok` or not.
1757 /// [`Atomic*::compare_exchange`]: crate::sync::atomic::AtomicBool::compare_exchange
1762 /// #![feature(result_into_ok_or_err)]
1763 /// let ok: Result<u32, u32> = Ok(3);
1764 /// let err: Result<u32, u32> = Err(4);
1766 /// assert_eq!(ok.into_ok_or_err(), 3);
1767 /// assert_eq!(err.into_ok_or_err(), 4);
1770 #[unstable(feature = "result_into_ok_or_err", reason = "newly added", issue = "82223")]
1771 pub const fn into_ok_or_err(self) -> T {
1779 // This is a separate function to reduce the code size of the methods
1780 #[cfg(not(feature = "panic_immediate_abort"))]
1784 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1785 panic!("{msg}: {error:?}")
1788 // This is a separate function to avoid constructing a `dyn Debug`
1789 // that gets immediately thrown away, since vtables don't get cleaned up
1790 // by dead code elimination if a trait object is constructed even if it goes
1792 #[cfg(feature = "panic_immediate_abort")]
1796 fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
1800 /////////////////////////////////////////////////////////////////////////////
1801 // Trait implementations
1802 /////////////////////////////////////////////////////////////////////////////
1804 #[stable(feature = "rust1", since = "1.0.0")]
1805 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
1806 impl<T, E> const Clone for Result<T, E>
1808 T: ~const Clone + ~const Destruct,
1809 E: ~const Clone + ~const Destruct,
1812 fn clone(&self) -> Self {
1814 Ok(x) => Ok(x.clone()),
1815 Err(x) => Err(x.clone()),
1820 fn clone_from(&mut self, source: &Self) {
1821 match (self, source) {
1822 (Ok(to), Ok(from)) => to.clone_from(from),
1823 (Err(to), Err(from)) => to.clone_from(from),
1824 (to, from) => *to = from.clone(),
1829 #[stable(feature = "rust1", since = "1.0.0")]
1830 impl<T, E> IntoIterator for Result<T, E> {
1832 type IntoIter = IntoIter<T>;
1834 /// Returns a consuming iterator over the possibly contained value.
1836 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1843 /// let x: Result<u32, &str> = Ok(5);
1844 /// let v: Vec<u32> = x.into_iter().collect();
1845 /// assert_eq!(v, [5]);
1847 /// let x: Result<u32, &str> = Err("nothing!");
1848 /// let v: Vec<u32> = x.into_iter().collect();
1849 /// assert_eq!(v, []);
1852 fn into_iter(self) -> IntoIter<T> {
1853 IntoIter { inner: self.ok() }
1857 #[stable(since = "1.4.0", feature = "result_iter")]
1858 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1860 type IntoIter = Iter<'a, T>;
1862 fn into_iter(self) -> Iter<'a, T> {
1867 #[stable(since = "1.4.0", feature = "result_iter")]
1868 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1869 type Item = &'a mut T;
1870 type IntoIter = IterMut<'a, T>;
1872 fn into_iter(self) -> IterMut<'a, T> {
1877 /////////////////////////////////////////////////////////////////////////////
1878 // The Result Iterators
1879 /////////////////////////////////////////////////////////////////////////////
1881 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1883 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1885 /// Created by [`Result::iter`].
1887 #[stable(feature = "rust1", since = "1.0.0")]
1888 pub struct Iter<'a, T: 'a> {
1889 inner: Option<&'a T>,
1892 #[stable(feature = "rust1", since = "1.0.0")]
1893 impl<'a, T> Iterator for Iter<'a, T> {
1897 fn next(&mut self) -> Option<&'a T> {
1901 fn size_hint(&self) -> (usize, Option<usize>) {
1902 let n = if self.inner.is_some() { 1 } else { 0 };
1907 #[stable(feature = "rust1", since = "1.0.0")]
1908 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1910 fn next_back(&mut self) -> Option<&'a T> {
1915 #[stable(feature = "rust1", since = "1.0.0")]
1916 impl<T> ExactSizeIterator for Iter<'_, T> {}
1918 #[stable(feature = "fused", since = "1.26.0")]
1919 impl<T> FusedIterator for Iter<'_, T> {}
1921 #[unstable(feature = "trusted_len", issue = "37572")]
1922 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1924 #[stable(feature = "rust1", since = "1.0.0")]
1925 impl<T> Clone for Iter<'_, T> {
1927 fn clone(&self) -> Self {
1928 Iter { inner: self.inner }
1932 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1934 /// Created by [`Result::iter_mut`].
1936 #[stable(feature = "rust1", since = "1.0.0")]
1937 pub struct IterMut<'a, T: 'a> {
1938 inner: Option<&'a mut T>,
1941 #[stable(feature = "rust1", since = "1.0.0")]
1942 impl<'a, T> Iterator for IterMut<'a, T> {
1943 type Item = &'a mut T;
1946 fn next(&mut self) -> Option<&'a mut T> {
1950 fn size_hint(&self) -> (usize, Option<usize>) {
1951 let n = if self.inner.is_some() { 1 } else { 0 };
1956 #[stable(feature = "rust1", since = "1.0.0")]
1957 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1959 fn next_back(&mut self) -> Option<&'a mut T> {
1964 #[stable(feature = "rust1", since = "1.0.0")]
1965 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1967 #[stable(feature = "fused", since = "1.26.0")]
1968 impl<T> FusedIterator for IterMut<'_, T> {}
1970 #[unstable(feature = "trusted_len", issue = "37572")]
1971 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1973 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1975 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1977 /// This struct is created by the [`into_iter`] method on
1978 /// [`Result`] (provided by the [`IntoIterator`] trait).
1980 /// [`into_iter`]: IntoIterator::into_iter
1981 #[derive(Clone, Debug)]
1982 #[stable(feature = "rust1", since = "1.0.0")]
1983 pub struct IntoIter<T> {
1987 #[stable(feature = "rust1", since = "1.0.0")]
1988 impl<T> Iterator for IntoIter<T> {
1992 fn next(&mut self) -> Option<T> {
1996 fn size_hint(&self) -> (usize, Option<usize>) {
1997 let n = if self.inner.is_some() { 1 } else { 0 };
2002 #[stable(feature = "rust1", since = "1.0.0")]
2003 impl<T> DoubleEndedIterator for IntoIter<T> {
2005 fn next_back(&mut self) -> Option<T> {
2010 #[stable(feature = "rust1", since = "1.0.0")]
2011 impl<T> ExactSizeIterator for IntoIter<T> {}
2013 #[stable(feature = "fused", since = "1.26.0")]
2014 impl<T> FusedIterator for IntoIter<T> {}
2016 #[unstable(feature = "trusted_len", issue = "37572")]
2017 unsafe impl<A> TrustedLen for IntoIter<A> {}
2019 /////////////////////////////////////////////////////////////////////////////
2021 /////////////////////////////////////////////////////////////////////////////
2023 #[stable(feature = "rust1", since = "1.0.0")]
2024 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
2025 /// Takes each element in the `Iterator`: if it is an `Err`, no further
2026 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
2027 /// container with the values of each `Result` is returned.
2029 /// Here is an example which increments every integer in a vector,
2030 /// checking for overflow:
2033 /// let v = vec![1, 2];
2034 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
2035 /// x.checked_add(1).ok_or("Overflow!")
2037 /// assert_eq!(res, Ok(vec![2, 3]));
2040 /// Here is another example that tries to subtract one from another list
2041 /// of integers, this time checking for underflow:
2044 /// let v = vec![1, 2, 0];
2045 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
2046 /// x.checked_sub(1).ok_or("Underflow!")
2048 /// assert_eq!(res, Err("Underflow!"));
2051 /// Here is a variation on the previous example, showing that no
2052 /// further elements are taken from `iter` after the first `Err`.
2055 /// let v = vec![3, 2, 1, 10];
2056 /// let mut shared = 0;
2057 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
2059 /// x.checked_sub(2).ok_or("Underflow!")
2061 /// assert_eq!(res, Err("Underflow!"));
2062 /// assert_eq!(shared, 6);
2065 /// Since the third element caused an underflow, no further elements were taken,
2066 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2068 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
2069 // FIXME(#11084): This could be replaced with Iterator::scan when this
2070 // performance bug is closed.
2072 iter::try_process(iter.into_iter(), |i| i.collect())
2076 #[unstable(feature = "try_trait_v2", issue = "84277")]
2077 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2078 impl<T, E> const ops::Try for Result<T, E> {
2080 type Residual = Result<convert::Infallible, E>;
2083 fn from_output(output: Self::Output) -> Self {
2088 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2090 Ok(v) => ControlFlow::Continue(v),
2091 Err(e) => ControlFlow::Break(Err(e)),
2096 #[unstable(feature = "try_trait_v2", issue = "84277")]
2097 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2098 impl<T, E, F: ~const From<E>> const ops::FromResidual<Result<convert::Infallible, E>>
2103 fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
2105 Err(e) => Err(From::from(e)),
2110 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2111 impl<T, E> ops::Residual<T> for Result<convert::Infallible, E> {
2112 type TryType = Result<T, E>;