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::ops::{self, ControlFlow, Deref, DerefMut};
494 use crate::{convert, fmt, hint};
496 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
498 /// See the [module documentation](self) for details.
499 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
500 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
501 #[rustc_diagnostic_item = "Result"]
502 #[stable(feature = "rust1", since = "1.0.0")]
503 pub enum Result<T, E> {
504 /// Contains the success value
506 #[stable(feature = "rust1", since = "1.0.0")]
507 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
509 /// Contains the error value
511 #[stable(feature = "rust1", since = "1.0.0")]
512 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
515 /////////////////////////////////////////////////////////////////////////////
516 // Type implementation
517 /////////////////////////////////////////////////////////////////////////////
519 impl<T, E> Result<T, E> {
520 /////////////////////////////////////////////////////////////////////////
521 // Querying the contained values
522 /////////////////////////////////////////////////////////////////////////
524 /// Returns `true` if the result is [`Ok`].
531 /// let x: Result<i32, &str> = Ok(-3);
532 /// assert_eq!(x.is_ok(), true);
534 /// let x: Result<i32, &str> = Err("Some error message");
535 /// assert_eq!(x.is_ok(), false);
537 #[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
538 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
540 #[stable(feature = "rust1", since = "1.0.0")]
541 pub const fn is_ok(&self) -> bool {
542 matches!(*self, Ok(_))
545 /// Returns `true` if the result is [`Ok`] wrapping a value matching the predicate.
550 /// #![feature(is_some_with)]
552 /// let x: Result<u32, &str> = Ok(2);
553 /// assert_eq!(x.is_ok_with(|&x| x > 1), true);
555 /// let x: Result<u32, &str> = Ok(0);
556 /// assert_eq!(x.is_ok_with(|&x| x > 1), false);
558 /// let x: Result<u32, &str> = Err("hey");
559 /// assert_eq!(x.is_ok_with(|&x| x > 1), false);
563 #[unstable(feature = "is_some_with", issue = "93050")]
564 pub fn is_ok_with(&self, f: impl FnOnce(&T) -> bool) -> bool {
565 matches!(self, Ok(x) if f(x))
568 /// Returns `true` if the result is [`Err`].
575 /// let x: Result<i32, &str> = Ok(-3);
576 /// assert_eq!(x.is_err(), false);
578 /// let x: Result<i32, &str> = Err("Some error message");
579 /// assert_eq!(x.is_err(), true);
581 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
582 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
584 #[stable(feature = "rust1", since = "1.0.0")]
585 pub const fn is_err(&self) -> bool {
589 /// Returns `true` if the result is [`Err`] wrapping a value matching the predicate.
594 /// #![feature(is_some_with)]
595 /// use std::io::{Error, ErrorKind};
597 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
598 /// assert_eq!(x.is_err_with(|x| x.kind() == ErrorKind::NotFound), true);
600 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
601 /// assert_eq!(x.is_err_with(|x| x.kind() == ErrorKind::NotFound), false);
603 /// let x: Result<u32, Error> = Ok(123);
604 /// assert_eq!(x.is_err_with(|x| x.kind() == ErrorKind::NotFound), false);
608 #[unstable(feature = "is_some_with", issue = "93050")]
609 pub fn is_err_with(&self, f: impl FnOnce(&E) -> bool) -> bool {
610 matches!(self, Err(x) if f(x))
613 /////////////////////////////////////////////////////////////////////////
614 // Adapter for each variant
615 /////////////////////////////////////////////////////////////////////////
617 /// Converts from `Result<T, E>` to [`Option<T>`].
619 /// Converts `self` into an [`Option<T>`], consuming `self`,
620 /// and discarding the error, if any.
627 /// let x: Result<u32, &str> = Ok(2);
628 /// assert_eq!(x.ok(), Some(2));
630 /// let x: Result<u32, &str> = Err("Nothing here");
631 /// assert_eq!(x.ok(), None);
634 #[stable(feature = "rust1", since = "1.0.0")]
635 pub fn ok(self) -> Option<T> {
642 /// Converts from `Result<T, E>` to [`Option<E>`].
644 /// Converts `self` into an [`Option<E>`], consuming `self`,
645 /// and discarding the success value, if any.
652 /// let x: Result<u32, &str> = Ok(2);
653 /// assert_eq!(x.err(), None);
655 /// let x: Result<u32, &str> = Err("Nothing here");
656 /// assert_eq!(x.err(), Some("Nothing here"));
659 #[stable(feature = "rust1", since = "1.0.0")]
660 pub fn err(self) -> Option<E> {
667 /////////////////////////////////////////////////////////////////////////
668 // Adapter for working with references
669 /////////////////////////////////////////////////////////////////////////
671 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
673 /// Produces a new `Result`, containing a reference
674 /// into the original, leaving the original in place.
681 /// let x: Result<u32, &str> = Ok(2);
682 /// assert_eq!(x.as_ref(), Ok(&2));
684 /// let x: Result<u32, &str> = Err("Error");
685 /// assert_eq!(x.as_ref(), Err(&"Error"));
688 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
689 #[stable(feature = "rust1", since = "1.0.0")]
690 pub const fn as_ref(&self) -> Result<&T, &E> {
693 Err(ref x) => Err(x),
697 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
704 /// fn mutate(r: &mut Result<i32, i32>) {
705 /// match r.as_mut() {
706 /// Ok(v) => *v = 42,
707 /// Err(e) => *e = 0,
711 /// let mut x: Result<i32, i32> = Ok(2);
713 /// assert_eq!(x.unwrap(), 42);
715 /// let mut x: Result<i32, i32> = Err(13);
717 /// assert_eq!(x.unwrap_err(), 0);
720 #[stable(feature = "rust1", since = "1.0.0")]
721 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
722 pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
724 Ok(ref mut x) => Ok(x),
725 Err(ref mut x) => Err(x),
729 /////////////////////////////////////////////////////////////////////////
730 // Transforming contained values
731 /////////////////////////////////////////////////////////////////////////
733 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
734 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
736 /// This function can be used to compose the results of two functions.
740 /// Print the numbers on each line of a string multiplied by two.
743 /// let line = "1\n2\n3\n4\n";
745 /// for num in line.lines() {
746 /// match num.parse::<i32>().map(|i| i * 2) {
747 /// Ok(n) => println!("{}", n),
753 #[stable(feature = "rust1", since = "1.0.0")]
754 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
761 /// Returns the provided default (if [`Err`]), or
762 /// applies a function to the contained value (if [`Ok`]),
764 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
765 /// the result of a function call, it is recommended to use [`map_or_else`],
766 /// which is lazily evaluated.
768 /// [`map_or_else`]: Result::map_or_else
773 /// let x: Result<_, &str> = Ok("foo");
774 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
776 /// let x: Result<&str, _> = Err("bar");
777 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
780 #[stable(feature = "result_map_or", since = "1.41.0")]
781 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
788 /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
789 /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
791 /// This function can be used to unpack a successful result
792 /// while handling an error.
802 /// let x : Result<_, &str> = Ok("foo");
803 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
805 /// let x : Result<&str, _> = Err("bar");
806 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
809 #[stable(feature = "result_map_or_else", since = "1.41.0")]
810 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
813 Err(e) => default(e),
817 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
818 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
820 /// This function can be used to pass through a successful result while handling
829 /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
831 /// let x: Result<u32, u32> = Ok(2);
832 /// assert_eq!(x.map_err(stringify), Ok(2));
834 /// let x: Result<u32, u32> = Err(13);
835 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
838 #[stable(feature = "rust1", since = "1.0.0")]
839 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
842 Err(e) => Err(op(e)),
846 /// Calls the provided closure with a reference to the contained value (if [`Ok`]).
851 /// #![feature(result_option_inspect)]
855 /// .inspect(|x| println!("original: {}", x))
856 /// .map(|x| x.pow(3))
857 /// .expect("failed to parse number");
860 #[unstable(feature = "result_option_inspect", issue = "91345")]
861 pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
862 if let Ok(ref t) = self {
869 /// Calls the provided closure with a reference to the contained error (if [`Err`]).
874 /// #![feature(result_option_inspect)]
876 /// use std::{fs, io};
878 /// fn read() -> io::Result<String> {
879 /// fs::read_to_string("address.txt")
880 /// .inspect_err(|e| eprintln!("failed to read file: {}", e))
884 #[unstable(feature = "result_option_inspect", issue = "91345")]
885 pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self {
886 if let Err(ref e) = self {
893 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
895 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
896 /// and returns the new [`Result`].
901 /// let x: Result<String, u32> = Ok("hello".to_string());
902 /// let y: Result<&str, &u32> = Ok("hello");
903 /// assert_eq!(x.as_deref(), y);
905 /// let x: Result<String, u32> = Err(42);
906 /// let y: Result<&str, &u32> = Err(&42);
907 /// assert_eq!(x.as_deref(), y);
909 #[stable(feature = "inner_deref", since = "1.47.0")]
910 pub fn as_deref(&self) -> Result<&T::Target, &E>
914 self.as_ref().map(|t| t.deref())
917 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
919 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
920 /// and returns the new [`Result`].
925 /// let mut s = "HELLO".to_string();
926 /// let mut x: Result<String, u32> = Ok("hello".to_string());
927 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
928 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
931 /// let mut x: Result<String, u32> = Err(42);
932 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
933 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
935 #[stable(feature = "inner_deref", since = "1.47.0")]
936 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
940 self.as_mut().map(|t| t.deref_mut())
943 /////////////////////////////////////////////////////////////////////////
944 // Iterator constructors
945 /////////////////////////////////////////////////////////////////////////
947 /// Returns an iterator over the possibly contained value.
949 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
956 /// let x: Result<u32, &str> = Ok(7);
957 /// assert_eq!(x.iter().next(), Some(&7));
959 /// let x: Result<u32, &str> = Err("nothing!");
960 /// assert_eq!(x.iter().next(), None);
963 #[stable(feature = "rust1", since = "1.0.0")]
964 pub fn iter(&self) -> Iter<'_, T> {
965 Iter { inner: self.as_ref().ok() }
968 /// Returns a mutable iterator over the possibly contained value.
970 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
977 /// let mut x: Result<u32, &str> = Ok(7);
978 /// match x.iter_mut().next() {
979 /// Some(v) => *v = 40,
982 /// assert_eq!(x, Ok(40));
984 /// let mut x: Result<u32, &str> = Err("nothing!");
985 /// assert_eq!(x.iter_mut().next(), None);
988 #[stable(feature = "rust1", since = "1.0.0")]
989 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
990 IterMut { inner: self.as_mut().ok() }
993 /////////////////////////////////////////////////////////////////////////
995 /////////////////////////////////////////////////////////////////////////
997 /// Returns the contained [`Ok`] value, consuming the `self` value.
1001 /// Panics if the value is an [`Err`], with a panic message including the
1002 /// passed message, and the content of the [`Err`].
1010 /// let x: Result<u32, &str> = Err("emergency failure");
1011 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1015 #[stable(feature = "result_expect", since = "1.4.0")]
1016 pub fn expect(self, msg: &str) -> T
1022 Err(e) => unwrap_failed(msg, &e),
1026 /// Returns the contained [`Ok`] value, consuming the `self` value.
1028 /// Because this function may panic, its use is generally discouraged.
1029 /// Instead, prefer to use pattern matching and handle the [`Err`]
1030 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1031 /// [`unwrap_or_default`].
1033 /// [`unwrap_or`]: Result::unwrap_or
1034 /// [`unwrap_or_else`]: Result::unwrap_or_else
1035 /// [`unwrap_or_default`]: Result::unwrap_or_default
1039 /// Panics if the value is an [`Err`], with a panic message provided by the
1040 /// [`Err`]'s value.
1048 /// let x: Result<u32, &str> = Ok(2);
1049 /// assert_eq!(x.unwrap(), 2);
1053 /// let x: Result<u32, &str> = Err("emergency failure");
1054 /// x.unwrap(); // panics with `emergency failure`
1058 #[stable(feature = "rust1", since = "1.0.0")]
1059 pub fn unwrap(self) -> T
1065 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1069 /// Returns the contained [`Ok`] value or a default
1071 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1072 /// value, otherwise if [`Err`], returns the default value for that
1077 /// Converts a string to an integer, turning poorly-formed strings
1078 /// into 0 (the default value for integers). [`parse`] converts
1079 /// a string to any other type that implements [`FromStr`], returning an
1080 /// [`Err`] on error.
1083 /// let good_year_from_input = "1909";
1084 /// let bad_year_from_input = "190blarg";
1085 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1086 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1088 /// assert_eq!(1909, good_year);
1089 /// assert_eq!(0, bad_year);
1092 /// [`parse`]: str::parse
1093 /// [`FromStr`]: crate::str::FromStr
1095 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1096 pub fn unwrap_or_default(self) -> T
1102 Err(_) => Default::default(),
1106 /// Returns the contained [`Err`] value, consuming the `self` value.
1110 /// Panics if the value is an [`Ok`], with a panic message including the
1111 /// passed message, and the content of the [`Ok`].
1119 /// let x: Result<u32, &str> = Ok(10);
1120 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1124 #[stable(feature = "result_expect_err", since = "1.17.0")]
1125 pub fn expect_err(self, msg: &str) -> E
1130 Ok(t) => unwrap_failed(msg, &t),
1135 /// Returns the contained [`Err`] value, consuming the `self` value.
1139 /// Panics if the value is an [`Ok`], with a custom panic message provided
1140 /// by the [`Ok`]'s value.
1145 /// let x: Result<u32, &str> = Ok(2);
1146 /// x.unwrap_err(); // panics with `2`
1150 /// let x: Result<u32, &str> = Err("emergency failure");
1151 /// assert_eq!(x.unwrap_err(), "emergency failure");
1155 #[stable(feature = "rust1", since = "1.0.0")]
1156 pub fn unwrap_err(self) -> E
1161 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1166 /// Returns the contained [`Ok`] value, but never panics.
1168 /// Unlike [`unwrap`], this method is known to never panic on the
1169 /// result types it is implemented for. Therefore, it can be used
1170 /// instead of `unwrap` as a maintainability safeguard that will fail
1171 /// to compile if the error type of the `Result` is later changed
1172 /// to an error that can actually occur.
1174 /// [`unwrap`]: Result::unwrap
1181 /// # #![feature(never_type)]
1182 /// # #![feature(unwrap_infallible)]
1184 /// fn only_good_news() -> Result<String, !> {
1185 /// Ok("this is fine".into())
1188 /// let s: String = only_good_news().into_ok();
1189 /// println!("{}", s);
1191 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1193 pub fn into_ok(self) -> T
1203 /// Returns the contained [`Err`] value, but never panics.
1205 /// Unlike [`unwrap_err`], this method is known to never panic on the
1206 /// result types it is implemented for. Therefore, it can be used
1207 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1208 /// to compile if the ok type of the `Result` is later changed
1209 /// to a type that can actually occur.
1211 /// [`unwrap_err`]: Result::unwrap_err
1218 /// # #![feature(never_type)]
1219 /// # #![feature(unwrap_infallible)]
1221 /// fn only_bad_news() -> Result<!, String> {
1222 /// Err("Oops, it failed".into())
1225 /// let error: String = only_bad_news().into_err();
1226 /// println!("{}", error);
1228 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1230 pub fn into_err(self) -> E
1240 ////////////////////////////////////////////////////////////////////////
1241 // Boolean operations on the values, eager and lazy
1242 /////////////////////////////////////////////////////////////////////////
1244 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1252 /// let x: Result<u32, &str> = Ok(2);
1253 /// let y: Result<&str, &str> = Err("late error");
1254 /// assert_eq!(x.and(y), Err("late error"));
1256 /// let x: Result<u32, &str> = Err("early error");
1257 /// let y: Result<&str, &str> = Ok("foo");
1258 /// assert_eq!(x.and(y), Err("early error"));
1260 /// let x: Result<u32, &str> = Err("not a 2");
1261 /// let y: Result<&str, &str> = Err("late error");
1262 /// assert_eq!(x.and(y), Err("not a 2"));
1264 /// let x: Result<u32, &str> = Ok(2);
1265 /// let y: Result<&str, &str> = Ok("different result type");
1266 /// assert_eq!(x.and(y), Ok("different result type"));
1269 #[stable(feature = "rust1", since = "1.0.0")]
1270 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
1277 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1280 /// This function can be used for control flow based on `Result` values.
1287 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1288 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1290 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
1291 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
1292 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
1293 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
1296 #[stable(feature = "rust1", since = "1.0.0")]
1297 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
1304 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1306 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1307 /// result of a function call, it is recommended to use [`or_else`], which is
1308 /// lazily evaluated.
1310 /// [`or_else`]: Result::or_else
1317 /// let x: Result<u32, &str> = Ok(2);
1318 /// let y: Result<u32, &str> = Err("late error");
1319 /// assert_eq!(x.or(y), Ok(2));
1321 /// let x: Result<u32, &str> = Err("early error");
1322 /// let y: Result<u32, &str> = Ok(2);
1323 /// assert_eq!(x.or(y), Ok(2));
1325 /// let x: Result<u32, &str> = Err("not a 2");
1326 /// let y: Result<u32, &str> = Err("late error");
1327 /// assert_eq!(x.or(y), Err("late error"));
1329 /// let x: Result<u32, &str> = Ok(2);
1330 /// let y: Result<u32, &str> = Ok(100);
1331 /// assert_eq!(x.or(y), Ok(2));
1334 #[stable(feature = "rust1", since = "1.0.0")]
1335 pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
1342 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1344 /// This function can be used for control flow based on result values.
1352 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1353 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1355 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1356 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1357 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1358 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1361 #[stable(feature = "rust1", since = "1.0.0")]
1362 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
1369 /// Returns the contained [`Ok`] value or a provided default.
1371 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1372 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1373 /// which is lazily evaluated.
1375 /// [`unwrap_or_else`]: Result::unwrap_or_else
1382 /// let default = 2;
1383 /// let x: Result<u32, &str> = Ok(9);
1384 /// assert_eq!(x.unwrap_or(default), 9);
1386 /// let x: Result<u32, &str> = Err("error");
1387 /// assert_eq!(x.unwrap_or(default), default);
1390 #[stable(feature = "rust1", since = "1.0.0")]
1391 pub fn unwrap_or(self, default: T) -> T {
1398 /// Returns the contained [`Ok`] value or computes it from a closure.
1406 /// fn count(x: &str) -> usize { x.len() }
1408 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1409 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1412 #[stable(feature = "rust1", since = "1.0.0")]
1413 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
1420 /// Returns the contained [`Ok`] value, consuming the `self` value,
1421 /// without checking that the value is not an [`Err`].
1425 /// Calling this method on an [`Err`] is *[undefined behavior]*.
1427 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1432 /// let x: Result<u32, &str> = Ok(2);
1433 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1437 /// let x: Result<u32, &str> = Err("emergency failure");
1438 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
1442 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1443 pub unsafe fn unwrap_unchecked(self) -> T {
1444 debug_assert!(self.is_ok());
1447 // SAFETY: the safety contract must be upheld by the caller.
1448 Err(_) => unsafe { hint::unreachable_unchecked() },
1452 /// Returns the contained [`Err`] value, consuming the `self` value,
1453 /// without checking that the value is not an [`Ok`].
1457 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1459 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1464 /// let x: Result<u32, &str> = Ok(2);
1465 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1469 /// let x: Result<u32, &str> = Err("emergency failure");
1470 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1474 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1475 pub unsafe fn unwrap_err_unchecked(self) -> E {
1476 debug_assert!(self.is_err());
1478 // SAFETY: the safety contract must be upheld by the caller.
1479 Ok(_) => unsafe { hint::unreachable_unchecked() },
1484 /////////////////////////////////////////////////////////////////////////
1486 /////////////////////////////////////////////////////////////////////////
1488 /// Returns `true` if the result is an [`Ok`] value containing the given value.
1493 /// #![feature(option_result_contains)]
1495 /// let x: Result<u32, &str> = Ok(2);
1496 /// assert_eq!(x.contains(&2), true);
1498 /// let x: Result<u32, &str> = Ok(3);
1499 /// assert_eq!(x.contains(&2), false);
1501 /// let x: Result<u32, &str> = Err("Some error message");
1502 /// assert_eq!(x.contains(&2), false);
1506 #[unstable(feature = "option_result_contains", issue = "62358")]
1507 pub fn contains<U>(&self, x: &U) -> bool
1517 /// Returns `true` if the result is an [`Err`] value containing the given value.
1522 /// #![feature(result_contains_err)]
1524 /// let x: Result<u32, &str> = Ok(2);
1525 /// assert_eq!(x.contains_err(&"Some error message"), false);
1527 /// let x: Result<u32, &str> = Err("Some error message");
1528 /// assert_eq!(x.contains_err(&"Some error message"), true);
1530 /// let x: Result<u32, &str> = Err("Some other error message");
1531 /// assert_eq!(x.contains_err(&"Some error message"), false);
1535 #[unstable(feature = "result_contains_err", issue = "62358")]
1536 pub fn contains_err<F>(&self, f: &F) -> bool
1547 impl<T, E> Result<&T, E> {
1548 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1555 /// let x: Result<&i32, i32> = Ok(&val);
1556 /// assert_eq!(x, Ok(&12));
1557 /// let copied = x.copied();
1558 /// assert_eq!(copied, Ok(12));
1561 #[stable(feature = "result_copied", since = "1.59.0")]
1562 pub fn copied(self) -> Result<T, E>
1569 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1576 /// let x: Result<&i32, i32> = Ok(&val);
1577 /// assert_eq!(x, Ok(&12));
1578 /// let cloned = x.cloned();
1579 /// assert_eq!(cloned, Ok(12));
1582 #[stable(feature = "result_cloned", since = "1.59.0")]
1583 pub fn cloned(self) -> Result<T, E>
1587 self.map(|t| t.clone())
1591 impl<T, E> Result<&mut T, E> {
1592 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1598 /// let mut val = 12;
1599 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1600 /// assert_eq!(x, Ok(&mut 12));
1601 /// let copied = x.copied();
1602 /// assert_eq!(copied, Ok(12));
1605 #[stable(feature = "result_copied", since = "1.59.0")]
1606 pub fn copied(self) -> Result<T, E>
1610 self.map(|&mut t| t)
1613 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1619 /// let mut val = 12;
1620 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1621 /// assert_eq!(x, Ok(&mut 12));
1622 /// let cloned = x.cloned();
1623 /// assert_eq!(cloned, Ok(12));
1626 #[stable(feature = "result_cloned", since = "1.59.0")]
1627 pub fn cloned(self) -> Result<T, E>
1631 self.map(|t| t.clone())
1635 impl<T, E> Result<Option<T>, E> {
1636 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1638 /// `Ok(None)` will be mapped to `None`.
1639 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1644 /// #[derive(Debug, Eq, PartialEq)]
1647 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1648 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1649 /// assert_eq!(x.transpose(), y);
1652 #[stable(feature = "transpose_result", since = "1.33.0")]
1653 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
1654 pub const fn transpose(self) -> Option<Result<T, E>> {
1656 Ok(Some(x)) => Some(Ok(x)),
1658 Err(e) => Some(Err(e)),
1663 impl<T, E> Result<Result<T, E>, E> {
1664 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1671 /// #![feature(result_flattening)]
1672 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1673 /// assert_eq!(Ok("hello"), x.flatten());
1675 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1676 /// assert_eq!(Err(6), x.flatten());
1678 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1679 /// assert_eq!(Err(6), x.flatten());
1682 /// Flattening only removes one level of nesting at a time:
1685 /// #![feature(result_flattening)]
1686 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1687 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1688 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1691 #[unstable(feature = "result_flattening", issue = "70142")]
1692 pub fn flatten(self) -> Result<T, E> {
1693 self.and_then(convert::identity)
1697 impl<T> Result<T, T> {
1698 /// Returns the [`Ok`] value if `self` is `Ok`, and the [`Err`] value if
1699 /// `self` is `Err`.
1701 /// In other words, this function returns the value (the `T`) of a
1702 /// `Result<T, T>`, regardless of whether or not that result is `Ok` or
1705 /// This can be useful in conjunction with APIs such as
1706 /// [`Atomic*::compare_exchange`], or [`slice::binary_search`], but only in
1707 /// cases where you don't care if the result was `Ok` or not.
1709 /// [`Atomic*::compare_exchange`]: crate::sync::atomic::AtomicBool::compare_exchange
1714 /// #![feature(result_into_ok_or_err)]
1715 /// let ok: Result<u32, u32> = Ok(3);
1716 /// let err: Result<u32, u32> = Err(4);
1718 /// assert_eq!(ok.into_ok_or_err(), 3);
1719 /// assert_eq!(err.into_ok_or_err(), 4);
1722 #[unstable(feature = "result_into_ok_or_err", reason = "newly added", issue = "82223")]
1723 pub const fn into_ok_or_err(self) -> T {
1731 // This is a separate function to reduce the code size of the methods
1732 #[cfg(not(feature = "panic_immediate_abort"))]
1736 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1737 panic!("{}: {:?}", msg, error)
1740 // This is a separate function to avoid constructing a `dyn Debug`
1741 // that gets immediately thrown away, since vtables don't get cleaned up
1742 // by dead code elimination if a trait object is constructed even if it goes
1744 #[cfg(feature = "panic_immediate_abort")]
1748 fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
1752 /////////////////////////////////////////////////////////////////////////////
1753 // Trait implementations
1754 /////////////////////////////////////////////////////////////////////////////
1756 #[stable(feature = "rust1", since = "1.0.0")]
1757 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1759 fn clone(&self) -> Self {
1761 Ok(x) => Ok(x.clone()),
1762 Err(x) => Err(x.clone()),
1767 fn clone_from(&mut self, source: &Self) {
1768 match (self, source) {
1769 (Ok(to), Ok(from)) => to.clone_from(from),
1770 (Err(to), Err(from)) => to.clone_from(from),
1771 (to, from) => *to = from.clone(),
1776 #[stable(feature = "rust1", since = "1.0.0")]
1777 impl<T, E> IntoIterator for Result<T, E> {
1779 type IntoIter = IntoIter<T>;
1781 /// Returns a consuming iterator over the possibly contained value.
1783 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1790 /// let x: Result<u32, &str> = Ok(5);
1791 /// let v: Vec<u32> = x.into_iter().collect();
1792 /// assert_eq!(v, [5]);
1794 /// let x: Result<u32, &str> = Err("nothing!");
1795 /// let v: Vec<u32> = x.into_iter().collect();
1796 /// assert_eq!(v, []);
1799 fn into_iter(self) -> IntoIter<T> {
1800 IntoIter { inner: self.ok() }
1804 #[stable(since = "1.4.0", feature = "result_iter")]
1805 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1807 type IntoIter = Iter<'a, T>;
1809 fn into_iter(self) -> Iter<'a, T> {
1814 #[stable(since = "1.4.0", feature = "result_iter")]
1815 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1816 type Item = &'a mut T;
1817 type IntoIter = IterMut<'a, T>;
1819 fn into_iter(self) -> IterMut<'a, T> {
1824 /////////////////////////////////////////////////////////////////////////////
1825 // The Result Iterators
1826 /////////////////////////////////////////////////////////////////////////////
1828 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1830 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1832 /// Created by [`Result::iter`].
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 pub struct Iter<'a, T: 'a> {
1836 inner: Option<&'a T>,
1839 #[stable(feature = "rust1", since = "1.0.0")]
1840 impl<'a, T> Iterator for Iter<'a, T> {
1844 fn next(&mut self) -> Option<&'a T> {
1848 fn size_hint(&self) -> (usize, Option<usize>) {
1849 let n = if self.inner.is_some() { 1 } else { 0 };
1854 #[stable(feature = "rust1", since = "1.0.0")]
1855 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1857 fn next_back(&mut self) -> Option<&'a T> {
1862 #[stable(feature = "rust1", since = "1.0.0")]
1863 impl<T> ExactSizeIterator for Iter<'_, T> {}
1865 #[stable(feature = "fused", since = "1.26.0")]
1866 impl<T> FusedIterator for Iter<'_, T> {}
1868 #[unstable(feature = "trusted_len", issue = "37572")]
1869 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1871 #[stable(feature = "rust1", since = "1.0.0")]
1872 impl<T> Clone for Iter<'_, T> {
1874 fn clone(&self) -> Self {
1875 Iter { inner: self.inner }
1879 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1881 /// Created by [`Result::iter_mut`].
1883 #[stable(feature = "rust1", since = "1.0.0")]
1884 pub struct IterMut<'a, T: 'a> {
1885 inner: Option<&'a mut T>,
1888 #[stable(feature = "rust1", since = "1.0.0")]
1889 impl<'a, T> Iterator for IterMut<'a, T> {
1890 type Item = &'a mut T;
1893 fn next(&mut self) -> Option<&'a mut T> {
1897 fn size_hint(&self) -> (usize, Option<usize>) {
1898 let n = if self.inner.is_some() { 1 } else { 0 };
1903 #[stable(feature = "rust1", since = "1.0.0")]
1904 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1906 fn next_back(&mut self) -> Option<&'a mut T> {
1911 #[stable(feature = "rust1", since = "1.0.0")]
1912 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1914 #[stable(feature = "fused", since = "1.26.0")]
1915 impl<T> FusedIterator for IterMut<'_, T> {}
1917 #[unstable(feature = "trusted_len", issue = "37572")]
1918 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1920 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1922 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1924 /// This struct is created by the [`into_iter`] method on
1925 /// [`Result`] (provided by the [`IntoIterator`] trait).
1927 /// [`into_iter`]: IntoIterator::into_iter
1928 #[derive(Clone, Debug)]
1929 #[stable(feature = "rust1", since = "1.0.0")]
1930 pub struct IntoIter<T> {
1934 #[stable(feature = "rust1", since = "1.0.0")]
1935 impl<T> Iterator for IntoIter<T> {
1939 fn next(&mut self) -> Option<T> {
1943 fn size_hint(&self) -> (usize, Option<usize>) {
1944 let n = if self.inner.is_some() { 1 } else { 0 };
1949 #[stable(feature = "rust1", since = "1.0.0")]
1950 impl<T> DoubleEndedIterator for IntoIter<T> {
1952 fn next_back(&mut self) -> Option<T> {
1957 #[stable(feature = "rust1", since = "1.0.0")]
1958 impl<T> ExactSizeIterator for IntoIter<T> {}
1960 #[stable(feature = "fused", since = "1.26.0")]
1961 impl<T> FusedIterator for IntoIter<T> {}
1963 #[unstable(feature = "trusted_len", issue = "37572")]
1964 unsafe impl<A> TrustedLen for IntoIter<A> {}
1966 /////////////////////////////////////////////////////////////////////////////
1968 /////////////////////////////////////////////////////////////////////////////
1970 #[stable(feature = "rust1", since = "1.0.0")]
1971 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1972 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1973 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1974 /// container with the values of each `Result` is returned.
1976 /// Here is an example which increments every integer in a vector,
1977 /// checking for overflow:
1980 /// let v = vec![1, 2];
1981 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1982 /// x.checked_add(1).ok_or("Overflow!")
1984 /// assert_eq!(res, Ok(vec![2, 3]));
1987 /// Here is another example that tries to subtract one from another list
1988 /// of integers, this time checking for underflow:
1991 /// let v = vec![1, 2, 0];
1992 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1993 /// x.checked_sub(1).ok_or("Underflow!")
1995 /// assert_eq!(res, Err("Underflow!"));
1998 /// Here is a variation on the previous example, showing that no
1999 /// further elements are taken from `iter` after the first `Err`.
2002 /// let v = vec![3, 2, 1, 10];
2003 /// let mut shared = 0;
2004 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
2006 /// x.checked_sub(2).ok_or("Underflow!")
2008 /// assert_eq!(res, Err("Underflow!"));
2009 /// assert_eq!(shared, 6);
2012 /// Since the third element caused an underflow, no further elements were taken,
2013 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2015 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
2016 // FIXME(#11084): This could be replaced with Iterator::scan when this
2017 // performance bug is closed.
2019 iter::process_results(iter.into_iter(), |i| i.collect())
2023 #[unstable(feature = "try_trait_v2", issue = "84277")]
2024 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2025 impl<T, E> const ops::Try for Result<T, E> {
2027 type Residual = Result<convert::Infallible, E>;
2030 fn from_output(output: Self::Output) -> Self {
2035 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2037 Ok(v) => ControlFlow::Continue(v),
2038 Err(e) => ControlFlow::Break(Err(e)),
2043 #[unstable(feature = "try_trait_v2", issue = "84277")]
2044 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2045 impl<T, E, F: ~const From<E>> const ops::FromResidual<Result<convert::Infallible, E>>
2050 fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
2052 Err(e) => Err(From::from(e)),
2057 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2058 impl<T, E> ops::Residual<T> for Result<convert::Infallible, E> {
2059 type TryType = Result<T, E>;