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 /// let x: Result<u32, &str> = Ok(2);
551 /// assert_eq!(x.is_ok_with(|x| x > 1), true);
553 /// let x: Result<u32, &str> = Ok(0);
554 /// assert_eq!(x.is_ok_with(|x| x > 1), false);
556 /// let x: Result<u32, &str> = Err("hey");
557 /// assert_eq!(x.is_ok_with(|x| x > 1), false);
561 #[unstable(feature = "is_some_with", issue = "93050")]
562 pub fn is_ok_with(&self, f: impl FnOnce(&T) -> bool) -> bool {
563 matches!(self, Ok(x) if f(x))
566 /// Returns `true` if the result is [`Err`].
573 /// let x: Result<i32, &str> = Ok(-3);
574 /// assert_eq!(x.is_err(), false);
576 /// let x: Result<i32, &str> = Err("Some error message");
577 /// assert_eq!(x.is_err(), true);
579 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
580 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
582 #[stable(feature = "rust1", since = "1.0.0")]
583 pub const fn is_err(&self) -> bool {
587 /// Returns `true` if the result is [`Err`] wrapping a value matching the predicate.
592 /// let x: Result<u32, &str> = Err("abc");
593 /// assert_eq!(x.is_err_with(|x| x.len() > 1), true);
595 /// let x: Result<u32, &str> = Err("");
596 /// assert_eq!(x.is_ok_with(|x| x.len() > 1), false);
598 /// let x: Result<u32, &str> = Ok(123);
599 /// assert_eq!(x.is_ok_with(|x| x.len() > 1), false);
603 #[unstable(feature = "is_some_with", issue = "93050")]
604 pub fn is_err_with(&self, f: impl FnOnce(&E) -> bool) -> bool {
605 matches!(self, Err(x) if f(x))
608 /////////////////////////////////////////////////////////////////////////
609 // Adapter for each variant
610 /////////////////////////////////////////////////////////////////////////
612 /// Converts from `Result<T, E>` to [`Option<T>`].
614 /// Converts `self` into an [`Option<T>`], consuming `self`,
615 /// and discarding the error, if any.
622 /// let x: Result<u32, &str> = Ok(2);
623 /// assert_eq!(x.ok(), Some(2));
625 /// let x: Result<u32, &str> = Err("Nothing here");
626 /// assert_eq!(x.ok(), None);
629 #[stable(feature = "rust1", since = "1.0.0")]
630 pub fn ok(self) -> Option<T> {
637 /// Converts from `Result<T, E>` to [`Option<E>`].
639 /// Converts `self` into an [`Option<E>`], consuming `self`,
640 /// and discarding the success value, if any.
647 /// let x: Result<u32, &str> = Ok(2);
648 /// assert_eq!(x.err(), None);
650 /// let x: Result<u32, &str> = Err("Nothing here");
651 /// assert_eq!(x.err(), Some("Nothing here"));
654 #[stable(feature = "rust1", since = "1.0.0")]
655 pub fn err(self) -> Option<E> {
662 /////////////////////////////////////////////////////////////////////////
663 // Adapter for working with references
664 /////////////////////////////////////////////////////////////////////////
666 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
668 /// Produces a new `Result`, containing a reference
669 /// into the original, leaving the original in place.
676 /// let x: Result<u32, &str> = Ok(2);
677 /// assert_eq!(x.as_ref(), Ok(&2));
679 /// let x: Result<u32, &str> = Err("Error");
680 /// assert_eq!(x.as_ref(), Err(&"Error"));
683 #[rustc_const_stable(feature = "const_result", since = "1.48.0")]
684 #[stable(feature = "rust1", since = "1.0.0")]
685 pub const fn as_ref(&self) -> Result<&T, &E> {
688 Err(ref x) => Err(x),
692 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
699 /// fn mutate(r: &mut Result<i32, i32>) {
700 /// match r.as_mut() {
701 /// Ok(v) => *v = 42,
702 /// Err(e) => *e = 0,
706 /// let mut x: Result<i32, i32> = Ok(2);
708 /// assert_eq!(x.unwrap(), 42);
710 /// let mut x: Result<i32, i32> = Err(13);
712 /// assert_eq!(x.unwrap_err(), 0);
715 #[stable(feature = "rust1", since = "1.0.0")]
716 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
717 pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
719 Ok(ref mut x) => Ok(x),
720 Err(ref mut x) => Err(x),
724 /////////////////////////////////////////////////////////////////////////
725 // Transforming contained values
726 /////////////////////////////////////////////////////////////////////////
728 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
729 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
731 /// This function can be used to compose the results of two functions.
735 /// Print the numbers on each line of a string multiplied by two.
738 /// let line = "1\n2\n3\n4\n";
740 /// for num in line.lines() {
741 /// match num.parse::<i32>().map(|i| i * 2) {
742 /// Ok(n) => println!("{}", n),
748 #[stable(feature = "rust1", since = "1.0.0")]
749 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
756 /// Returns the provided default (if [`Err`]), or
757 /// applies a function to the contained value (if [`Ok`]),
759 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
760 /// the result of a function call, it is recommended to use [`map_or_else`],
761 /// which is lazily evaluated.
763 /// [`map_or_else`]: Result::map_or_else
768 /// let x: Result<_, &str> = Ok("foo");
769 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
771 /// let x: Result<&str, _> = Err("bar");
772 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
775 #[stable(feature = "result_map_or", since = "1.41.0")]
776 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
783 /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
784 /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
786 /// This function can be used to unpack a successful result
787 /// while handling an error.
797 /// let x : Result<_, &str> = Ok("foo");
798 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
800 /// let x : Result<&str, _> = Err("bar");
801 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
804 #[stable(feature = "result_map_or_else", since = "1.41.0")]
805 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
808 Err(e) => default(e),
812 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
813 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
815 /// This function can be used to pass through a successful result while handling
824 /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
826 /// let x: Result<u32, u32> = Ok(2);
827 /// assert_eq!(x.map_err(stringify), Ok(2));
829 /// let x: Result<u32, u32> = Err(13);
830 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
833 #[stable(feature = "rust1", since = "1.0.0")]
834 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
837 Err(e) => Err(op(e)),
841 /// Calls the provided closure with a reference to the contained value (if [`Ok`]).
846 /// #![feature(result_option_inspect)]
850 /// .inspect(|x| println!("original: {}", x))
851 /// .map(|x| x.pow(3))
852 /// .expect("failed to parse number");
855 #[unstable(feature = "result_option_inspect", issue = "91345")]
856 pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
857 if let Ok(ref t) = self {
864 /// Calls the provided closure with a reference to the contained error (if [`Err`]).
869 /// #![feature(result_option_inspect)]
871 /// use std::{fs, io};
873 /// fn read() -> io::Result<String> {
874 /// fs::read_to_string("address.txt")
875 /// .inspect_err(|e| eprintln!("failed to read file: {}", e))
879 #[unstable(feature = "result_option_inspect", issue = "91345")]
880 pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self {
881 if let Err(ref e) = self {
888 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
890 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
891 /// and returns the new [`Result`].
896 /// let x: Result<String, u32> = Ok("hello".to_string());
897 /// let y: Result<&str, &u32> = Ok("hello");
898 /// assert_eq!(x.as_deref(), y);
900 /// let x: Result<String, u32> = Err(42);
901 /// let y: Result<&str, &u32> = Err(&42);
902 /// assert_eq!(x.as_deref(), y);
904 #[stable(feature = "inner_deref", since = "1.47.0")]
905 pub fn as_deref(&self) -> Result<&T::Target, &E>
909 self.as_ref().map(|t| t.deref())
912 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
914 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
915 /// and returns the new [`Result`].
920 /// let mut s = "HELLO".to_string();
921 /// let mut x: Result<String, u32> = Ok("hello".to_string());
922 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
923 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
926 /// let mut x: Result<String, u32> = Err(42);
927 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
928 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
930 #[stable(feature = "inner_deref", since = "1.47.0")]
931 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
935 self.as_mut().map(|t| t.deref_mut())
938 /////////////////////////////////////////////////////////////////////////
939 // Iterator constructors
940 /////////////////////////////////////////////////////////////////////////
942 /// Returns an iterator over the possibly contained value.
944 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
951 /// let x: Result<u32, &str> = Ok(7);
952 /// assert_eq!(x.iter().next(), Some(&7));
954 /// let x: Result<u32, &str> = Err("nothing!");
955 /// assert_eq!(x.iter().next(), None);
958 #[stable(feature = "rust1", since = "1.0.0")]
959 pub fn iter(&self) -> Iter<'_, T> {
960 Iter { inner: self.as_ref().ok() }
963 /// Returns a mutable iterator over the possibly contained value.
965 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
972 /// let mut x: Result<u32, &str> = Ok(7);
973 /// match x.iter_mut().next() {
974 /// Some(v) => *v = 40,
977 /// assert_eq!(x, Ok(40));
979 /// let mut x: Result<u32, &str> = Err("nothing!");
980 /// assert_eq!(x.iter_mut().next(), None);
983 #[stable(feature = "rust1", since = "1.0.0")]
984 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
985 IterMut { inner: self.as_mut().ok() }
988 /////////////////////////////////////////////////////////////////////////
990 /////////////////////////////////////////////////////////////////////////
992 /// Returns the contained [`Ok`] value, consuming the `self` value.
996 /// Panics if the value is an [`Err`], with a panic message including the
997 /// passed message, and the content of the [`Err`].
1005 /// let x: Result<u32, &str> = Err("emergency failure");
1006 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1010 #[stable(feature = "result_expect", since = "1.4.0")]
1011 pub fn expect(self, msg: &str) -> T
1017 Err(e) => unwrap_failed(msg, &e),
1021 /// Returns the contained [`Ok`] value, consuming the `self` value.
1023 /// Because this function may panic, its use is generally discouraged.
1024 /// Instead, prefer to use pattern matching and handle the [`Err`]
1025 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1026 /// [`unwrap_or_default`].
1028 /// [`unwrap_or`]: Result::unwrap_or
1029 /// [`unwrap_or_else`]: Result::unwrap_or_else
1030 /// [`unwrap_or_default`]: Result::unwrap_or_default
1034 /// Panics if the value is an [`Err`], with a panic message provided by the
1035 /// [`Err`]'s value.
1043 /// let x: Result<u32, &str> = Ok(2);
1044 /// assert_eq!(x.unwrap(), 2);
1048 /// let x: Result<u32, &str> = Err("emergency failure");
1049 /// x.unwrap(); // panics with `emergency failure`
1053 #[stable(feature = "rust1", since = "1.0.0")]
1054 pub fn unwrap(self) -> T
1060 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1064 /// Returns the contained [`Ok`] value or a default
1066 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1067 /// value, otherwise if [`Err`], returns the default value for that
1072 /// Converts a string to an integer, turning poorly-formed strings
1073 /// into 0 (the default value for integers). [`parse`] converts
1074 /// a string to any other type that implements [`FromStr`], returning an
1075 /// [`Err`] on error.
1078 /// let good_year_from_input = "1909";
1079 /// let bad_year_from_input = "190blarg";
1080 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1081 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1083 /// assert_eq!(1909, good_year);
1084 /// assert_eq!(0, bad_year);
1087 /// [`parse`]: str::parse
1088 /// [`FromStr`]: crate::str::FromStr
1090 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1091 pub fn unwrap_or_default(self) -> T
1097 Err(_) => Default::default(),
1101 /// Returns the contained [`Err`] value, consuming the `self` value.
1105 /// Panics if the value is an [`Ok`], with a panic message including the
1106 /// passed message, and the content of the [`Ok`].
1114 /// let x: Result<u32, &str> = Ok(10);
1115 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1119 #[stable(feature = "result_expect_err", since = "1.17.0")]
1120 pub fn expect_err(self, msg: &str) -> E
1125 Ok(t) => unwrap_failed(msg, &t),
1130 /// Returns the contained [`Err`] value, consuming the `self` value.
1134 /// Panics if the value is an [`Ok`], with a custom panic message provided
1135 /// by the [`Ok`]'s value.
1140 /// let x: Result<u32, &str> = Ok(2);
1141 /// x.unwrap_err(); // panics with `2`
1145 /// let x: Result<u32, &str> = Err("emergency failure");
1146 /// assert_eq!(x.unwrap_err(), "emergency failure");
1150 #[stable(feature = "rust1", since = "1.0.0")]
1151 pub fn unwrap_err(self) -> E
1156 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1161 /// Returns the contained [`Ok`] value, but never panics.
1163 /// Unlike [`unwrap`], this method is known to never panic on the
1164 /// result types it is implemented for. Therefore, it can be used
1165 /// instead of `unwrap` as a maintainability safeguard that will fail
1166 /// to compile if the error type of the `Result` is later changed
1167 /// to an error that can actually occur.
1169 /// [`unwrap`]: Result::unwrap
1176 /// # #![feature(never_type)]
1177 /// # #![feature(unwrap_infallible)]
1179 /// fn only_good_news() -> Result<String, !> {
1180 /// Ok("this is fine".into())
1183 /// let s: String = only_good_news().into_ok();
1184 /// println!("{}", s);
1186 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1188 pub fn into_ok(self) -> T
1198 /// Returns the contained [`Err`] value, but never panics.
1200 /// Unlike [`unwrap_err`], this method is known to never panic on the
1201 /// result types it is implemented for. Therefore, it can be used
1202 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1203 /// to compile if the ok type of the `Result` is later changed
1204 /// to a type that can actually occur.
1206 /// [`unwrap_err`]: Result::unwrap_err
1213 /// # #![feature(never_type)]
1214 /// # #![feature(unwrap_infallible)]
1216 /// fn only_bad_news() -> Result<!, String> {
1217 /// Err("Oops, it failed".into())
1220 /// let error: String = only_bad_news().into_err();
1221 /// println!("{}", error);
1223 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1225 pub fn into_err(self) -> E
1235 ////////////////////////////////////////////////////////////////////////
1236 // Boolean operations on the values, eager and lazy
1237 /////////////////////////////////////////////////////////////////////////
1239 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1247 /// let x: Result<u32, &str> = Ok(2);
1248 /// let y: Result<&str, &str> = Err("late error");
1249 /// assert_eq!(x.and(y), Err("late error"));
1251 /// let x: Result<u32, &str> = Err("early error");
1252 /// let y: Result<&str, &str> = Ok("foo");
1253 /// assert_eq!(x.and(y), Err("early error"));
1255 /// let x: Result<u32, &str> = Err("not a 2");
1256 /// let y: Result<&str, &str> = Err("late error");
1257 /// assert_eq!(x.and(y), Err("not a 2"));
1259 /// let x: Result<u32, &str> = Ok(2);
1260 /// let y: Result<&str, &str> = Ok("different result type");
1261 /// assert_eq!(x.and(y), Ok("different result type"));
1264 #[stable(feature = "rust1", since = "1.0.0")]
1265 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
1272 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1275 /// This function can be used for control flow based on `Result` values.
1282 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1283 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1285 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
1286 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
1287 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
1288 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
1291 #[stable(feature = "rust1", since = "1.0.0")]
1292 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
1299 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1301 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1302 /// result of a function call, it is recommended to use [`or_else`], which is
1303 /// lazily evaluated.
1305 /// [`or_else`]: Result::or_else
1312 /// let x: Result<u32, &str> = Ok(2);
1313 /// let y: Result<u32, &str> = Err("late error");
1314 /// assert_eq!(x.or(y), Ok(2));
1316 /// let x: Result<u32, &str> = Err("early error");
1317 /// let y: Result<u32, &str> = Ok(2);
1318 /// assert_eq!(x.or(y), Ok(2));
1320 /// let x: Result<u32, &str> = Err("not a 2");
1321 /// let y: Result<u32, &str> = Err("late error");
1322 /// assert_eq!(x.or(y), Err("late error"));
1324 /// let x: Result<u32, &str> = Ok(2);
1325 /// let y: Result<u32, &str> = Ok(100);
1326 /// assert_eq!(x.or(y), Ok(2));
1329 #[stable(feature = "rust1", since = "1.0.0")]
1330 pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
1337 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1339 /// This function can be used for control flow based on result values.
1347 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1348 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1350 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1351 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1352 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1353 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1356 #[stable(feature = "rust1", since = "1.0.0")]
1357 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
1364 /// Returns the contained [`Ok`] value or a provided default.
1366 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1367 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1368 /// which is lazily evaluated.
1370 /// [`unwrap_or_else`]: Result::unwrap_or_else
1377 /// let default = 2;
1378 /// let x: Result<u32, &str> = Ok(9);
1379 /// assert_eq!(x.unwrap_or(default), 9);
1381 /// let x: Result<u32, &str> = Err("error");
1382 /// assert_eq!(x.unwrap_or(default), default);
1385 #[stable(feature = "rust1", since = "1.0.0")]
1386 pub fn unwrap_or(self, default: T) -> T {
1393 /// Returns the contained [`Ok`] value or computes it from a closure.
1401 /// fn count(x: &str) -> usize { x.len() }
1403 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1404 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1407 #[stable(feature = "rust1", since = "1.0.0")]
1408 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
1415 /// Returns the contained [`Ok`] value, consuming the `self` value,
1416 /// without checking that the value is not an [`Err`].
1420 /// Calling this method on an [`Err`] is *[undefined behavior]*.
1422 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1427 /// let x: Result<u32, &str> = Ok(2);
1428 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1432 /// let x: Result<u32, &str> = Err("emergency failure");
1433 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
1437 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1438 pub unsafe fn unwrap_unchecked(self) -> T {
1439 debug_assert!(self.is_ok());
1442 // SAFETY: the safety contract must be upheld by the caller.
1443 Err(_) => unsafe { hint::unreachable_unchecked() },
1447 /// Returns the contained [`Err`] value, consuming the `self` value,
1448 /// without checking that the value is not an [`Ok`].
1452 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1454 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1459 /// let x: Result<u32, &str> = Ok(2);
1460 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1464 /// let x: Result<u32, &str> = Err("emergency failure");
1465 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1469 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1470 pub unsafe fn unwrap_err_unchecked(self) -> E {
1471 debug_assert!(self.is_err());
1473 // SAFETY: the safety contract must be upheld by the caller.
1474 Ok(_) => unsafe { hint::unreachable_unchecked() },
1479 /////////////////////////////////////////////////////////////////////////
1481 /////////////////////////////////////////////////////////////////////////
1483 /// Returns `true` if the result is an [`Ok`] value containing the given value.
1488 /// #![feature(option_result_contains)]
1490 /// let x: Result<u32, &str> = Ok(2);
1491 /// assert_eq!(x.contains(&2), true);
1493 /// let x: Result<u32, &str> = Ok(3);
1494 /// assert_eq!(x.contains(&2), false);
1496 /// let x: Result<u32, &str> = Err("Some error message");
1497 /// assert_eq!(x.contains(&2), false);
1501 #[unstable(feature = "option_result_contains", issue = "62358")]
1502 pub fn contains<U>(&self, x: &U) -> bool
1512 /// Returns `true` if the result is an [`Err`] value containing the given value.
1517 /// #![feature(result_contains_err)]
1519 /// let x: Result<u32, &str> = Ok(2);
1520 /// assert_eq!(x.contains_err(&"Some error message"), false);
1522 /// let x: Result<u32, &str> = Err("Some error message");
1523 /// assert_eq!(x.contains_err(&"Some error message"), true);
1525 /// let x: Result<u32, &str> = Err("Some other error message");
1526 /// assert_eq!(x.contains_err(&"Some error message"), false);
1530 #[unstable(feature = "result_contains_err", issue = "62358")]
1531 pub fn contains_err<F>(&self, f: &F) -> bool
1542 impl<T, E> Result<&T, E> {
1543 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1550 /// let x: Result<&i32, i32> = Ok(&val);
1551 /// assert_eq!(x, Ok(&12));
1552 /// let copied = x.copied();
1553 /// assert_eq!(copied, Ok(12));
1556 #[stable(feature = "result_copied", since = "1.59.0")]
1557 pub fn copied(self) -> Result<T, E>
1564 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1571 /// let x: Result<&i32, i32> = Ok(&val);
1572 /// assert_eq!(x, Ok(&12));
1573 /// let cloned = x.cloned();
1574 /// assert_eq!(cloned, Ok(12));
1577 #[stable(feature = "result_cloned", since = "1.59.0")]
1578 pub fn cloned(self) -> Result<T, E>
1582 self.map(|t| t.clone())
1586 impl<T, E> Result<&mut T, E> {
1587 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1593 /// let mut val = 12;
1594 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1595 /// assert_eq!(x, Ok(&mut 12));
1596 /// let copied = x.copied();
1597 /// assert_eq!(copied, Ok(12));
1600 #[stable(feature = "result_copied", since = "1.59.0")]
1601 pub fn copied(self) -> Result<T, E>
1605 self.map(|&mut t| t)
1608 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1614 /// let mut val = 12;
1615 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1616 /// assert_eq!(x, Ok(&mut 12));
1617 /// let cloned = x.cloned();
1618 /// assert_eq!(cloned, Ok(12));
1621 #[stable(feature = "result_cloned", since = "1.59.0")]
1622 pub fn cloned(self) -> Result<T, E>
1626 self.map(|t| t.clone())
1630 impl<T, E> Result<Option<T>, E> {
1631 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1633 /// `Ok(None)` will be mapped to `None`.
1634 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1639 /// #[derive(Debug, Eq, PartialEq)]
1642 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1643 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1644 /// assert_eq!(x.transpose(), y);
1647 #[stable(feature = "transpose_result", since = "1.33.0")]
1648 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
1649 pub const fn transpose(self) -> Option<Result<T, E>> {
1651 Ok(Some(x)) => Some(Ok(x)),
1653 Err(e) => Some(Err(e)),
1658 impl<T, E> Result<Result<T, E>, E> {
1659 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1666 /// #![feature(result_flattening)]
1667 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1668 /// assert_eq!(Ok("hello"), x.flatten());
1670 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1671 /// assert_eq!(Err(6), x.flatten());
1673 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1674 /// assert_eq!(Err(6), x.flatten());
1677 /// Flattening only removes one level of nesting at a time:
1680 /// #![feature(result_flattening)]
1681 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1682 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1683 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1686 #[unstable(feature = "result_flattening", issue = "70142")]
1687 pub fn flatten(self) -> Result<T, E> {
1688 self.and_then(convert::identity)
1692 impl<T> Result<T, T> {
1693 /// Returns the [`Ok`] value if `self` is `Ok`, and the [`Err`] value if
1694 /// `self` is `Err`.
1696 /// In other words, this function returns the value (the `T`) of a
1697 /// `Result<T, T>`, regardless of whether or not that result is `Ok` or
1700 /// This can be useful in conjunction with APIs such as
1701 /// [`Atomic*::compare_exchange`], or [`slice::binary_search`], but only in
1702 /// cases where you don't care if the result was `Ok` or not.
1704 /// [`Atomic*::compare_exchange`]: crate::sync::atomic::AtomicBool::compare_exchange
1709 /// #![feature(result_into_ok_or_err)]
1710 /// let ok: Result<u32, u32> = Ok(3);
1711 /// let err: Result<u32, u32> = Err(4);
1713 /// assert_eq!(ok.into_ok_or_err(), 3);
1714 /// assert_eq!(err.into_ok_or_err(), 4);
1717 #[unstable(feature = "result_into_ok_or_err", reason = "newly added", issue = "82223")]
1718 pub const fn into_ok_or_err(self) -> T {
1726 // This is a separate function to reduce the code size of the methods
1727 #[cfg(not(feature = "panic_immediate_abort"))]
1731 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1732 panic!("{}: {:?}", msg, error)
1735 // This is a separate function to avoid constructing a `dyn Debug`
1736 // that gets immediately thrown away, since vtables don't get cleaned up
1737 // by dead code elimination if a trait object is constructed even if it goes
1739 #[cfg(feature = "panic_immediate_abort")]
1743 fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
1747 /////////////////////////////////////////////////////////////////////////////
1748 // Trait implementations
1749 /////////////////////////////////////////////////////////////////////////////
1751 #[stable(feature = "rust1", since = "1.0.0")]
1752 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1754 fn clone(&self) -> Self {
1756 Ok(x) => Ok(x.clone()),
1757 Err(x) => Err(x.clone()),
1762 fn clone_from(&mut self, source: &Self) {
1763 match (self, source) {
1764 (Ok(to), Ok(from)) => to.clone_from(from),
1765 (Err(to), Err(from)) => to.clone_from(from),
1766 (to, from) => *to = from.clone(),
1771 #[stable(feature = "rust1", since = "1.0.0")]
1772 impl<T, E> IntoIterator for Result<T, E> {
1774 type IntoIter = IntoIter<T>;
1776 /// Returns a consuming iterator over the possibly contained value.
1778 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1785 /// let x: Result<u32, &str> = Ok(5);
1786 /// let v: Vec<u32> = x.into_iter().collect();
1787 /// assert_eq!(v, [5]);
1789 /// let x: Result<u32, &str> = Err("nothing!");
1790 /// let v: Vec<u32> = x.into_iter().collect();
1791 /// assert_eq!(v, []);
1794 fn into_iter(self) -> IntoIter<T> {
1795 IntoIter { inner: self.ok() }
1799 #[stable(since = "1.4.0", feature = "result_iter")]
1800 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1802 type IntoIter = Iter<'a, T>;
1804 fn into_iter(self) -> Iter<'a, T> {
1809 #[stable(since = "1.4.0", feature = "result_iter")]
1810 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1811 type Item = &'a mut T;
1812 type IntoIter = IterMut<'a, T>;
1814 fn into_iter(self) -> IterMut<'a, T> {
1819 /////////////////////////////////////////////////////////////////////////////
1820 // The Result Iterators
1821 /////////////////////////////////////////////////////////////////////////////
1823 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1825 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1827 /// Created by [`Result::iter`].
1829 #[stable(feature = "rust1", since = "1.0.0")]
1830 pub struct Iter<'a, T: 'a> {
1831 inner: Option<&'a T>,
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 impl<'a, T> Iterator for Iter<'a, T> {
1839 fn next(&mut self) -> Option<&'a T> {
1843 fn size_hint(&self) -> (usize, Option<usize>) {
1844 let n = if self.inner.is_some() { 1 } else { 0 };
1849 #[stable(feature = "rust1", since = "1.0.0")]
1850 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1852 fn next_back(&mut self) -> Option<&'a T> {
1857 #[stable(feature = "rust1", since = "1.0.0")]
1858 impl<T> ExactSizeIterator for Iter<'_, T> {}
1860 #[stable(feature = "fused", since = "1.26.0")]
1861 impl<T> FusedIterator for Iter<'_, T> {}
1863 #[unstable(feature = "trusted_len", issue = "37572")]
1864 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1866 #[stable(feature = "rust1", since = "1.0.0")]
1867 impl<T> Clone for Iter<'_, T> {
1869 fn clone(&self) -> Self {
1870 Iter { inner: self.inner }
1874 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1876 /// Created by [`Result::iter_mut`].
1878 #[stable(feature = "rust1", since = "1.0.0")]
1879 pub struct IterMut<'a, T: 'a> {
1880 inner: Option<&'a mut T>,
1883 #[stable(feature = "rust1", since = "1.0.0")]
1884 impl<'a, T> Iterator for IterMut<'a, T> {
1885 type Item = &'a mut T;
1888 fn next(&mut self) -> Option<&'a mut T> {
1892 fn size_hint(&self) -> (usize, Option<usize>) {
1893 let n = if self.inner.is_some() { 1 } else { 0 };
1898 #[stable(feature = "rust1", since = "1.0.0")]
1899 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1901 fn next_back(&mut self) -> Option<&'a mut T> {
1906 #[stable(feature = "rust1", since = "1.0.0")]
1907 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1909 #[stable(feature = "fused", since = "1.26.0")]
1910 impl<T> FusedIterator for IterMut<'_, T> {}
1912 #[unstable(feature = "trusted_len", issue = "37572")]
1913 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1915 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1917 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1919 /// This struct is created by the [`into_iter`] method on
1920 /// [`Result`] (provided by the [`IntoIterator`] trait).
1922 /// [`into_iter`]: IntoIterator::into_iter
1923 #[derive(Clone, Debug)]
1924 #[stable(feature = "rust1", since = "1.0.0")]
1925 pub struct IntoIter<T> {
1929 #[stable(feature = "rust1", since = "1.0.0")]
1930 impl<T> Iterator for IntoIter<T> {
1934 fn next(&mut self) -> Option<T> {
1938 fn size_hint(&self) -> (usize, Option<usize>) {
1939 let n = if self.inner.is_some() { 1 } else { 0 };
1944 #[stable(feature = "rust1", since = "1.0.0")]
1945 impl<T> DoubleEndedIterator for IntoIter<T> {
1947 fn next_back(&mut self) -> Option<T> {
1952 #[stable(feature = "rust1", since = "1.0.0")]
1953 impl<T> ExactSizeIterator for IntoIter<T> {}
1955 #[stable(feature = "fused", since = "1.26.0")]
1956 impl<T> FusedIterator for IntoIter<T> {}
1958 #[unstable(feature = "trusted_len", issue = "37572")]
1959 unsafe impl<A> TrustedLen for IntoIter<A> {}
1961 /////////////////////////////////////////////////////////////////////////////
1963 /////////////////////////////////////////////////////////////////////////////
1965 #[stable(feature = "rust1", since = "1.0.0")]
1966 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1967 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1968 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1969 /// container with the values of each `Result` is returned.
1971 /// Here is an example which increments every integer in a vector,
1972 /// checking for overflow:
1975 /// let v = vec![1, 2];
1976 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1977 /// x.checked_add(1).ok_or("Overflow!")
1979 /// assert_eq!(res, Ok(vec![2, 3]));
1982 /// Here is another example that tries to subtract one from another list
1983 /// of integers, this time checking for underflow:
1986 /// let v = vec![1, 2, 0];
1987 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1988 /// x.checked_sub(1).ok_or("Underflow!")
1990 /// assert_eq!(res, Err("Underflow!"));
1993 /// Here is a variation on the previous example, showing that no
1994 /// further elements are taken from `iter` after the first `Err`.
1997 /// let v = vec![3, 2, 1, 10];
1998 /// let mut shared = 0;
1999 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
2001 /// x.checked_sub(2).ok_or("Underflow!")
2003 /// assert_eq!(res, Err("Underflow!"));
2004 /// assert_eq!(shared, 6);
2007 /// Since the third element caused an underflow, no further elements were taken,
2008 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2010 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
2011 // FIXME(#11084): This could be replaced with Iterator::scan when this
2012 // performance bug is closed.
2014 iter::process_results(iter.into_iter(), |i| i.collect())
2018 #[unstable(feature = "try_trait_v2", issue = "84277")]
2019 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2020 impl<T, E> const ops::Try for Result<T, E> {
2022 type Residual = Result<convert::Infallible, E>;
2025 fn from_output(output: Self::Output) -> Self {
2030 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2032 Ok(v) => ControlFlow::Continue(v),
2033 Err(e) => ControlFlow::Break(Err(e)),
2038 #[unstable(feature = "try_trait_v2", issue = "84277")]
2039 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2040 impl<T, E, F: ~const From<E>> const ops::FromResidual<Result<convert::Infallible, E>>
2045 fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
2047 Err(e) => Err(From::from(e)),
2052 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2053 impl<T, E> ops::Residual<T> for Result<convert::Infallible, E> {
2054 type TryType = Result<T, E>;