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 [`Ok`]'s unwrapped value, unless the result
213 //! is [`Err`], in which case [`Err`] is returned early from the enclosing function.
215 //! [`?`] can be used in functions that return [`Result`] because of the
216 //! early return of [`Err`] that it provides.
218 //! [`expect`]: Result::expect
219 //! [`Write`]: ../../std/io/trait.Write.html "io::Write"
220 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all "io::Write::write_all"
221 //! [`io::Result`]: ../../std/io/type.Result.html "io::Result"
222 //! [`?`]: crate::ops::Try
225 //! [io::Error]: ../../std/io/struct.Error.html "io::Error"
227 //! # Method overview
229 //! In addition to working with pattern matching, [`Result`] provides a
230 //! wide variety of different methods.
232 //! ## Querying the variant
234 //! The [`is_ok`] and [`is_err`] methods return [`true`] if the [`Result`]
235 //! is [`Ok`] or [`Err`], respectively.
237 //! [`is_err`]: Result::is_err
238 //! [`is_ok`]: Result::is_ok
240 //! ## Adapters for working with references
242 //! * [`as_ref`] converts from `&Result<T, E>` to `Result<&T, &E>`
243 //! * [`as_mut`] converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`
244 //! * [`as_deref`] converts from `&Result<T, E>` to `Result<&T::Target, &E>`
245 //! * [`as_deref_mut`] converts from `&mut Result<T, E>` to
246 //! `Result<&mut T::Target, &mut E>`
248 //! [`as_deref`]: Result::as_deref
249 //! [`as_deref_mut`]: Result::as_deref_mut
250 //! [`as_mut`]: Result::as_mut
251 //! [`as_ref`]: Result::as_ref
253 //! ## Extracting contained values
255 //! These methods extract the contained value in a [`Result<T, E>`] when it
256 //! is the [`Ok`] variant. If the [`Result`] is [`Err`]:
258 //! * [`expect`] panics with a provided custom message
259 //! * [`unwrap`] panics with a generic message
260 //! * [`unwrap_or`] returns the provided default value
261 //! * [`unwrap_or_default`] returns the default value of the type `T`
262 //! (which must implement the [`Default`] trait)
263 //! * [`unwrap_or_else`] returns the result of evaluating the provided
266 //! The panicking methods [`expect`] and [`unwrap`] require `E` to
267 //! implement the [`Debug`] trait.
269 //! [`Debug`]: crate::fmt::Debug
270 //! [`expect`]: Result::expect
271 //! [`unwrap`]: Result::unwrap
272 //! [`unwrap_or`]: Result::unwrap_or
273 //! [`unwrap_or_default`]: Result::unwrap_or_default
274 //! [`unwrap_or_else`]: Result::unwrap_or_else
276 //! These methods extract the contained value in a [`Result<T, E>`] when it
277 //! is the [`Err`] variant. They require `T` to implement the [`Debug`]
278 //! trait. If the [`Result`] is [`Ok`]:
280 //! * [`expect_err`] panics with a provided custom message
281 //! * [`unwrap_err`] panics with a generic message
283 //! [`Debug`]: crate::fmt::Debug
284 //! [`expect_err`]: Result::expect_err
285 //! [`unwrap_err`]: Result::unwrap_err
287 //! ## Transforming contained values
289 //! These methods transform [`Result`] to [`Option`]:
291 //! * [`err`][Result::err] transforms [`Result<T, E>`] into [`Option<E>`],
292 //! mapping [`Err(e)`] to [`Some(e)`] and [`Ok(v)`] to [`None`]
293 //! * [`ok`][Result::ok] transforms [`Result<T, E>`] into [`Option<T>`],
294 //! mapping [`Ok(v)`] to [`Some(v)`] and [`Err(e)`] to [`None`]
295 //! * [`transpose`] transposes a [`Result`] of an [`Option`] into an
296 //! [`Option`] of a [`Result`]
298 // Do NOT add link reference definitions for `err` or `ok`, because they
299 // will generate numerous incorrect URLs for `Err` and `Ok` elsewhere, due
304 //! [`Some(e)`]: Option::Some
305 //! [`Some(v)`]: Option::Some
306 //! [`transpose`]: Result::transpose
308 //! This method transforms the contained value of the [`Ok`] variant:
310 //! * [`map`] transforms [`Result<T, E>`] into [`Result<U, E>`] by applying
311 //! the provided function to the contained value of [`Ok`] and leaving
312 //! [`Err`] values unchanged
314 //! [`map`]: Result::map
316 //! This method transforms the contained value of the [`Err`] variant:
318 //! * [`map_err`] transforms [`Result<T, E>`] into [`Result<T, F>`] by
319 //! applying the provided function to the contained value of [`Err`] and
320 //! leaving [`Ok`] values unchanged
322 //! [`map_err`]: Result::map_err
324 //! These methods transform a [`Result<T, E>`] into a value of a possibly
325 //! different type `U`:
327 //! * [`map_or`] applies the provided function to the contained value of
328 //! [`Ok`], or returns the provided default value if the [`Result`] is
330 //! * [`map_or_else`] applies the provided function to the contained value
331 //! of [`Ok`], or applies the provided default fallback function to the
332 //! contained value of [`Err`]
334 //! [`map_or`]: Result::map_or
335 //! [`map_or_else`]: Result::map_or_else
337 //! ## Boolean operators
339 //! These methods treat the [`Result`] as a boolean value, where [`Ok`]
340 //! acts like [`true`] and [`Err`] acts like [`false`]. There are two
341 //! categories of these methods: ones that take a [`Result`] as input, and
342 //! ones that take a function as input (to be lazily evaluated).
344 //! The [`and`] and [`or`] methods take another [`Result`] as input, and
345 //! produce a [`Result`] as output. The [`and`] method can produce a
346 //! [`Result<U, E>`] value having a different inner type `U` than
347 //! [`Result<T, E>`]. The [`or`] method can produce a [`Result<T, F>`]
348 //! value having a different error type `F` than [`Result<T, E>`].
350 //! | method | self | input | output |
351 //! |---------|----------|-----------|----------|
352 //! | [`and`] | `Err(e)` | (ignored) | `Err(e)` |
353 //! | [`and`] | `Ok(x)` | `Err(d)` | `Err(d)` |
354 //! | [`and`] | `Ok(x)` | `Ok(y)` | `Ok(y)` |
355 //! | [`or`] | `Err(e)` | `Err(d)` | `Err(d)` |
356 //! | [`or`] | `Err(e)` | `Ok(y)` | `Ok(y)` |
357 //! | [`or`] | `Ok(x)` | (ignored) | `Ok(x)` |
359 //! [`and`]: Result::and
360 //! [`or`]: Result::or
362 //! The [`and_then`] and [`or_else`] methods take a function as input, and
363 //! only evaluate the function when they need to produce a new value. The
364 //! [`and_then`] method can produce a [`Result<U, E>`] value having a
365 //! different inner type `U` than [`Result<T, E>`]. The [`or_else`] method
366 //! can produce a [`Result<T, F>`] value having a different error type `F`
367 //! than [`Result<T, E>`].
369 //! | method | self | function input | function result | output |
370 //! |--------------|----------|----------------|-----------------|----------|
371 //! | [`and_then`] | `Err(e)` | (not provided) | (not evaluated) | `Err(e)` |
372 //! | [`and_then`] | `Ok(x)` | `x` | `Err(d)` | `Err(d)` |
373 //! | [`and_then`] | `Ok(x)` | `x` | `Ok(y)` | `Ok(y)` |
374 //! | [`or_else`] | `Err(e)` | `e` | `Err(d)` | `Err(d)` |
375 //! | [`or_else`] | `Err(e)` | `e` | `Ok(y)` | `Ok(y)` |
376 //! | [`or_else`] | `Ok(x)` | (not provided) | (not evaluated) | `Ok(x)` |
378 //! [`and_then`]: Result::and_then
379 //! [`or_else`]: Result::or_else
381 //! ## Comparison operators
383 //! If `T` and `E` both implement [`PartialOrd`] then [`Result<T, E>`] will
384 //! derive its [`PartialOrd`] implementation. With this order, an [`Ok`]
385 //! compares as less than any [`Err`], while two [`Ok`] or two [`Err`]
386 //! compare as their contained values would in `T` or `E` respectively. If `T`
387 //! and `E` both also implement [`Ord`], then so does [`Result<T, E>`].
390 //! assert!(Ok(1) < Err(0));
391 //! let x: Result<i32, ()> = Ok(0);
394 //! let x: Result<(), i32> = Err(0);
399 //! ## Iterating over `Result`
401 //! A [`Result`] can be iterated over. This can be helpful if you need an
402 //! iterator that is conditionally empty. The iterator will either produce
403 //! a single value (when the [`Result`] is [`Ok`]), or produce no values
404 //! (when the [`Result`] is [`Err`]). For example, [`into_iter`] acts like
405 //! [`once(v)`] if the [`Result`] is [`Ok(v)`], and like [`empty()`] if the
406 //! [`Result`] is [`Err`].
409 //! [`empty()`]: crate::iter::empty
410 //! [`once(v)`]: crate::iter::once
412 //! Iterators over [`Result<T, E>`] come in three types:
414 //! * [`into_iter`] consumes the [`Result`] and produces the contained
416 //! * [`iter`] produces an immutable reference of type `&T` to the
418 //! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
421 //! See [Iterating over `Option`] for examples of how this can be useful.
423 //! [Iterating over `Option`]: crate::option#iterating-over-option
424 //! [`into_iter`]: Result::into_iter
425 //! [`iter`]: Result::iter
426 //! [`iter_mut`]: Result::iter_mut
428 //! You might want to use an iterator chain to do multiple instances of an
429 //! operation that can fail, but would like to ignore failures while
430 //! continuing to process the successful results. In this example, we take
431 //! advantage of the iterable nature of [`Result`] to select only the
432 //! [`Ok`] values using [`flatten`][Iterator::flatten].
435 //! # use std::str::FromStr;
436 //! let mut results = vec![];
437 //! let mut errs = vec![];
438 //! let nums: Vec<_> = ["17", "not a number", "99", "-27", "768"]
440 //! .map(u8::from_str)
441 //! // Save clones of the raw `Result` values to inspect
442 //! .inspect(|x| results.push(x.clone()))
443 //! // Challenge: explain how this captures only the `Err` values
444 //! .inspect(|x| errs.extend(x.clone().err()))
447 //! assert_eq!(errs.len(), 3);
448 //! assert_eq!(nums, [17, 99]);
449 //! println!("results {results:?}");
450 //! println!("errs {errs:?}");
451 //! println!("nums {nums:?}");
454 //! ## Collecting into `Result`
456 //! [`Result`] implements the [`FromIterator`][impl-FromIterator] trait,
457 //! which allows an iterator over [`Result`] values to be collected into a
458 //! [`Result`] of a collection of each contained value of the original
459 //! [`Result`] values, or [`Err`] if any of the elements was [`Err`].
461 //! [impl-FromIterator]: Result#impl-FromIterator%3CResult%3CA%2C%20E%3E%3E-for-Result%3CV%2C%20E%3E
464 //! let v = [Ok(2), Ok(4), Err("err!"), Ok(8)];
465 //! let res: Result<Vec<_>, &str> = v.into_iter().collect();
466 //! assert_eq!(res, Err("err!"));
467 //! let v = [Ok(2), Ok(4), Ok(8)];
468 //! let res: Result<Vec<_>, &str> = v.into_iter().collect();
469 //! assert_eq!(res, Ok(vec![2, 4, 8]));
472 //! [`Result`] also implements the [`Product`][impl-Product] and
473 //! [`Sum`][impl-Sum] traits, allowing an iterator over [`Result`] values
474 //! to provide the [`product`][Iterator::product] and
475 //! [`sum`][Iterator::sum] methods.
477 //! [impl-Product]: Result#impl-Product%3CResult%3CU%2C%20E%3E%3E-for-Result%3CT%2C%20E%3E
478 //! [impl-Sum]: Result#impl-Sum%3CResult%3CU%2C%20E%3E%3E-for-Result%3CT%2C%20E%3E
481 //! let v = [Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
482 //! let res: Result<i32, &str> = v.into_iter().sum();
483 //! assert_eq!(res, Err("error!"));
484 //! let v = [Ok(1), Ok(2), Ok(21)];
485 //! let res: Result<i32, &str> = v.into_iter().product();
486 //! assert_eq!(res, Ok(42));
489 #![stable(feature = "rust1", since = "1.0.0")]
491 use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
492 use crate::marker::Destruct;
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_basics", 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`] and the value inside of it matches a predicate.
550 /// #![feature(is_some_and)]
552 /// let x: Result<u32, &str> = Ok(2);
553 /// assert_eq!(x.is_ok_and(|x| x > 1), true);
555 /// let x: Result<u32, &str> = Ok(0);
556 /// assert_eq!(x.is_ok_and(|x| x > 1), false);
558 /// let x: Result<u32, &str> = Err("hey");
559 /// assert_eq!(x.is_ok_and(|x| x > 1), false);
563 #[unstable(feature = "is_some_and", issue = "93050")]
564 pub fn is_ok_and(self, f: impl FnOnce(T) -> bool) -> bool {
571 /// Returns `true` if the result is [`Err`].
578 /// let x: Result<i32, &str> = Ok(-3);
579 /// assert_eq!(x.is_err(), false);
581 /// let x: Result<i32, &str> = Err("Some error message");
582 /// assert_eq!(x.is_err(), true);
584 #[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
585 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
587 #[stable(feature = "rust1", since = "1.0.0")]
588 pub const fn is_err(&self) -> bool {
592 /// Returns `true` if the result is [`Err`] and the value inside of it matches a predicate.
597 /// #![feature(is_some_and)]
598 /// use std::io::{Error, ErrorKind};
600 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
601 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), true);
603 /// let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
604 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
606 /// let x: Result<u32, Error> = Ok(123);
607 /// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
611 #[unstable(feature = "is_some_and", issue = "93050")]
612 pub fn is_err_and(self, f: impl FnOnce(E) -> bool) -> bool {
619 /////////////////////////////////////////////////////////////////////////
620 // Adapter for each variant
621 /////////////////////////////////////////////////////////////////////////
623 /// Converts from `Result<T, E>` to [`Option<T>`].
625 /// Converts `self` into an [`Option<T>`], consuming `self`,
626 /// and discarding the error, if any.
633 /// let x: Result<u32, &str> = Ok(2);
634 /// assert_eq!(x.ok(), Some(2));
636 /// let x: Result<u32, &str> = Err("Nothing here");
637 /// assert_eq!(x.ok(), None);
640 #[stable(feature = "rust1", since = "1.0.0")]
641 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
642 pub const fn ok(self) -> Option<T>
648 // FIXME: ~const Drop doesn't quite work right yet
649 #[allow(unused_variables)]
654 /// Converts from `Result<T, E>` to [`Option<E>`].
656 /// Converts `self` into an [`Option<E>`], consuming `self`,
657 /// and discarding the success value, if any.
664 /// let x: Result<u32, &str> = Ok(2);
665 /// assert_eq!(x.err(), None);
667 /// let x: Result<u32, &str> = Err("Nothing here");
668 /// assert_eq!(x.err(), Some("Nothing here"));
671 #[stable(feature = "rust1", since = "1.0.0")]
672 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
673 pub const fn err(self) -> Option<E>
678 // FIXME: ~const Drop doesn't quite work right yet
679 #[allow(unused_variables)]
685 /////////////////////////////////////////////////////////////////////////
686 // Adapter for working with references
687 /////////////////////////////////////////////////////////////////////////
689 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
691 /// Produces a new `Result`, containing a reference
692 /// into the original, leaving the original in place.
699 /// let x: Result<u32, &str> = Ok(2);
700 /// assert_eq!(x.as_ref(), Ok(&2));
702 /// let x: Result<u32, &str> = Err("Error");
703 /// assert_eq!(x.as_ref(), Err(&"Error"));
706 #[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
707 #[stable(feature = "rust1", since = "1.0.0")]
708 pub const fn as_ref(&self) -> Result<&T, &E> {
711 Err(ref x) => Err(x),
715 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
722 /// fn mutate(r: &mut Result<i32, i32>) {
723 /// match r.as_mut() {
724 /// Ok(v) => *v = 42,
725 /// Err(e) => *e = 0,
729 /// let mut x: Result<i32, i32> = Ok(2);
731 /// assert_eq!(x.unwrap(), 42);
733 /// let mut x: Result<i32, i32> = Err(13);
735 /// assert_eq!(x.unwrap_err(), 0);
738 #[stable(feature = "rust1", since = "1.0.0")]
739 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
740 pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
742 Ok(ref mut x) => Ok(x),
743 Err(ref mut x) => Err(x),
747 /////////////////////////////////////////////////////////////////////////
748 // Transforming contained values
749 /////////////////////////////////////////////////////////////////////////
751 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
752 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
754 /// This function can be used to compose the results of two functions.
758 /// Print the numbers on each line of a string multiplied by two.
761 /// let line = "1\n2\n3\n4\n";
763 /// for num in line.lines() {
764 /// match num.parse::<i32>().map(|i| i * 2) {
765 /// Ok(n) => println!("{n}"),
771 #[stable(feature = "rust1", since = "1.0.0")]
772 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
779 /// Returns the provided default (if [`Err`]), or
780 /// applies a function to the contained value (if [`Ok`]),
782 /// Arguments passed to `map_or` are eagerly evaluated; if you are passing
783 /// the result of a function call, it is recommended to use [`map_or_else`],
784 /// which is lazily evaluated.
786 /// [`map_or_else`]: Result::map_or_else
791 /// let x: Result<_, &str> = Ok("foo");
792 /// assert_eq!(x.map_or(42, |v| v.len()), 3);
794 /// let x: Result<&str, _> = Err("bar");
795 /// assert_eq!(x.map_or(42, |v| v.len()), 42);
798 #[stable(feature = "result_map_or", since = "1.41.0")]
799 pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
806 /// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
807 /// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
809 /// This function can be used to unpack a successful result
810 /// while handling an error.
820 /// let x : Result<_, &str> = Ok("foo");
821 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
823 /// let x : Result<&str, _> = Err("bar");
824 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
827 #[stable(feature = "result_map_or_else", since = "1.41.0")]
828 pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
831 Err(e) => default(e),
835 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
836 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
838 /// This function can be used to pass through a successful result while handling
847 /// fn stringify(x: u32) -> String { format!("error code: {x}") }
849 /// let x: Result<u32, u32> = Ok(2);
850 /// assert_eq!(x.map_err(stringify), Ok(2));
852 /// let x: Result<u32, u32> = Err(13);
853 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
856 #[stable(feature = "rust1", since = "1.0.0")]
857 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
860 Err(e) => Err(op(e)),
864 /// Calls the provided closure with a reference to the contained value (if [`Ok`]).
869 /// #![feature(result_option_inspect)]
873 /// .inspect(|x| println!("original: {x}"))
874 /// .map(|x| x.pow(3))
875 /// .expect("failed to parse number");
878 #[unstable(feature = "result_option_inspect", issue = "91345")]
879 pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
880 if let Ok(ref t) = self {
887 /// Calls the provided closure with a reference to the contained error (if [`Err`]).
892 /// #![feature(result_option_inspect)]
894 /// use std::{fs, io};
896 /// fn read() -> io::Result<String> {
897 /// fs::read_to_string("address.txt")
898 /// .inspect_err(|e| eprintln!("failed to read file: {e}"))
902 #[unstable(feature = "result_option_inspect", issue = "91345")]
903 pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self {
904 if let Err(ref e) = self {
911 /// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
913 /// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
914 /// and returns the new [`Result`].
919 /// let x: Result<String, u32> = Ok("hello".to_string());
920 /// let y: Result<&str, &u32> = Ok("hello");
921 /// assert_eq!(x.as_deref(), y);
923 /// let x: Result<String, u32> = Err(42);
924 /// let y: Result<&str, &u32> = Err(&42);
925 /// assert_eq!(x.as_deref(), y);
927 #[stable(feature = "inner_deref", since = "1.47.0")]
928 pub fn as_deref(&self) -> Result<&T::Target, &E>
932 self.as_ref().map(|t| t.deref())
935 /// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
937 /// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
938 /// and returns the new [`Result`].
943 /// let mut s = "HELLO".to_string();
944 /// let mut x: Result<String, u32> = Ok("hello".to_string());
945 /// let y: Result<&mut str, &mut u32> = Ok(&mut s);
946 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
949 /// let mut x: Result<String, u32> = Err(42);
950 /// let y: Result<&mut str, &mut u32> = Err(&mut i);
951 /// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
953 #[stable(feature = "inner_deref", since = "1.47.0")]
954 pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
958 self.as_mut().map(|t| t.deref_mut())
961 /////////////////////////////////////////////////////////////////////////
962 // Iterator constructors
963 /////////////////////////////////////////////////////////////////////////
965 /// Returns an iterator over the possibly contained value.
967 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
974 /// let x: Result<u32, &str> = Ok(7);
975 /// assert_eq!(x.iter().next(), Some(&7));
977 /// let x: Result<u32, &str> = Err("nothing!");
978 /// assert_eq!(x.iter().next(), None);
981 #[stable(feature = "rust1", since = "1.0.0")]
982 pub fn iter(&self) -> Iter<'_, T> {
983 Iter { inner: self.as_ref().ok() }
986 /// Returns a mutable iterator over the possibly contained value.
988 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
995 /// let mut x: Result<u32, &str> = Ok(7);
996 /// match x.iter_mut().next() {
997 /// Some(v) => *v = 40,
1000 /// assert_eq!(x, Ok(40));
1002 /// let mut x: Result<u32, &str> = Err("nothing!");
1003 /// assert_eq!(x.iter_mut().next(), None);
1006 #[stable(feature = "rust1", since = "1.0.0")]
1007 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
1008 IterMut { inner: self.as_mut().ok() }
1011 /////////////////////////////////////////////////////////////////////////
1013 /////////////////////////////////////////////////////////////////////////
1015 /// Returns the contained [`Ok`] value, consuming the `self` value.
1017 /// Because this function may panic, its use is generally discouraged.
1018 /// Instead, prefer to use pattern matching and handle the [`Err`]
1019 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1020 /// [`unwrap_or_default`].
1022 /// [`unwrap_or`]: Result::unwrap_or
1023 /// [`unwrap_or_else`]: Result::unwrap_or_else
1024 /// [`unwrap_or_default`]: Result::unwrap_or_default
1028 /// Panics if the value is an [`Err`], with a panic message including the
1029 /// passed message, and the content of the [`Err`].
1037 /// let x: Result<u32, &str> = Err("emergency failure");
1038 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
1041 /// # Recommended Message Style
1043 /// We recommend that `expect` messages are used to describe the reason you
1044 /// _expect_ the `Result` should be `Ok`.
1047 /// let path = std::env::var("IMPORTANT_PATH")
1048 /// .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");
1051 /// **Hint**: If you're having trouble remembering how to phrase expect
1052 /// error messages remember to focus on the word "should" as in "env
1053 /// variable should be set by blah" or "the given binary should be available
1054 /// and executable by the current user".
1056 /// For more detail on expect message styles and the reasoning behind our recommendation please
1057 /// refer to the section on ["Common Message
1058 /// Styles"](../../std/error/index.html#common-message-styles) in the
1059 /// [`std::error`](../../std/error/index.html) module docs.
1062 #[stable(feature = "result_expect", since = "1.4.0")]
1063 pub fn expect(self, msg: &str) -> T
1069 Err(e) => unwrap_failed(msg, &e),
1073 /// Returns the contained [`Ok`] value, consuming the `self` value.
1075 /// Because this function may panic, its use is generally discouraged.
1076 /// Instead, prefer to use pattern matching and handle the [`Err`]
1077 /// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
1078 /// [`unwrap_or_default`].
1080 /// [`unwrap_or`]: Result::unwrap_or
1081 /// [`unwrap_or_else`]: Result::unwrap_or_else
1082 /// [`unwrap_or_default`]: Result::unwrap_or_default
1086 /// Panics if the value is an [`Err`], with a panic message provided by the
1087 /// [`Err`]'s value.
1095 /// let x: Result<u32, &str> = Ok(2);
1096 /// assert_eq!(x.unwrap(), 2);
1100 /// let x: Result<u32, &str> = Err("emergency failure");
1101 /// x.unwrap(); // panics with `emergency failure`
1105 #[stable(feature = "rust1", since = "1.0.0")]
1106 pub fn unwrap(self) -> T
1112 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
1116 /// Returns the contained [`Ok`] value or a default
1118 /// Consumes the `self` argument then, if [`Ok`], returns the contained
1119 /// value, otherwise if [`Err`], returns the default value for that
1124 /// Converts a string to an integer, turning poorly-formed strings
1125 /// into 0 (the default value for integers). [`parse`] converts
1126 /// a string to any other type that implements [`FromStr`], returning an
1127 /// [`Err`] on error.
1130 /// let good_year_from_input = "1909";
1131 /// let bad_year_from_input = "190blarg";
1132 /// let good_year = good_year_from_input.parse().unwrap_or_default();
1133 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
1135 /// assert_eq!(1909, good_year);
1136 /// assert_eq!(0, bad_year);
1139 /// [`parse`]: str::parse
1140 /// [`FromStr`]: crate::str::FromStr
1142 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
1143 pub fn unwrap_or_default(self) -> T
1149 Err(_) => Default::default(),
1153 /// Returns the contained [`Err`] value, consuming the `self` value.
1157 /// Panics if the value is an [`Ok`], with a panic message including the
1158 /// passed message, and the content of the [`Ok`].
1166 /// let x: Result<u32, &str> = Ok(10);
1167 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
1171 #[stable(feature = "result_expect_err", since = "1.17.0")]
1172 pub fn expect_err(self, msg: &str) -> E
1177 Ok(t) => unwrap_failed(msg, &t),
1182 /// Returns the contained [`Err`] value, consuming the `self` value.
1186 /// Panics if the value is an [`Ok`], with a custom panic message provided
1187 /// by the [`Ok`]'s value.
1192 /// let x: Result<u32, &str> = Ok(2);
1193 /// x.unwrap_err(); // panics with `2`
1197 /// let x: Result<u32, &str> = Err("emergency failure");
1198 /// assert_eq!(x.unwrap_err(), "emergency failure");
1202 #[stable(feature = "rust1", since = "1.0.0")]
1203 pub fn unwrap_err(self) -> E
1208 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
1213 /// Returns the contained [`Ok`] value, but never panics.
1215 /// Unlike [`unwrap`], this method is known to never panic on the
1216 /// result types it is implemented for. Therefore, it can be used
1217 /// instead of `unwrap` as a maintainability safeguard that will fail
1218 /// to compile if the error type of the `Result` is later changed
1219 /// to an error that can actually occur.
1221 /// [`unwrap`]: Result::unwrap
1228 /// # #![feature(never_type)]
1229 /// # #![feature(unwrap_infallible)]
1231 /// fn only_good_news() -> Result<String, !> {
1232 /// Ok("this is fine".into())
1235 /// let s: String = only_good_news().into_ok();
1236 /// println!("{s}");
1238 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1240 pub fn into_ok(self) -> T
1250 /// Returns the contained [`Err`] value, but never panics.
1252 /// Unlike [`unwrap_err`], this method is known to never panic on the
1253 /// result types it is implemented for. Therefore, it can be used
1254 /// instead of `unwrap_err` as a maintainability safeguard that will fail
1255 /// to compile if the ok type of the `Result` is later changed
1256 /// to a type that can actually occur.
1258 /// [`unwrap_err`]: Result::unwrap_err
1265 /// # #![feature(never_type)]
1266 /// # #![feature(unwrap_infallible)]
1268 /// fn only_bad_news() -> Result<!, String> {
1269 /// Err("Oops, it failed".into())
1272 /// let error: String = only_bad_news().into_err();
1273 /// println!("{error}");
1275 #[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
1277 pub fn into_err(self) -> E
1287 ////////////////////////////////////////////////////////////////////////
1288 // Boolean operations on the values, eager and lazy
1289 /////////////////////////////////////////////////////////////////////////
1291 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1293 /// Arguments passed to `and` are eagerly evaluated; if you are passing the
1294 /// result of a function call, it is recommended to use [`and_then`], which is
1295 /// lazily evaluated.
1297 /// [`and_then`]: Result::and_then
1304 /// let x: Result<u32, &str> = Ok(2);
1305 /// let y: Result<&str, &str> = Err("late error");
1306 /// assert_eq!(x.and(y), Err("late error"));
1308 /// let x: Result<u32, &str> = Err("early error");
1309 /// let y: Result<&str, &str> = Ok("foo");
1310 /// assert_eq!(x.and(y), Err("early error"));
1312 /// let x: Result<u32, &str> = Err("not a 2");
1313 /// let y: Result<&str, &str> = Err("late error");
1314 /// assert_eq!(x.and(y), Err("not a 2"));
1316 /// let x: Result<u32, &str> = Ok(2);
1317 /// let y: Result<&str, &str> = Ok("different result type");
1318 /// assert_eq!(x.and(y), Ok("different result type"));
1321 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
1322 #[stable(feature = "rust1", since = "1.0.0")]
1323 pub const fn and<U>(self, res: Result<U, E>) -> Result<U, E>
1330 // FIXME: ~const Drop doesn't quite work right yet
1331 #[allow(unused_variables)]
1337 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
1340 /// This function can be used for control flow based on `Result` values.
1345 /// fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
1346 /// x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
1349 /// assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
1350 /// assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
1351 /// assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));
1354 /// Often used to chain fallible operations that may return [`Err`].
1357 /// use std::{io::ErrorKind, path::Path};
1359 /// // Note: on Windows "/" maps to "C:\"
1360 /// let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
1361 /// assert!(root_modified_time.is_ok());
1363 /// let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
1364 /// assert!(should_fail.is_err());
1365 /// assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);
1368 #[stable(feature = "rust1", since = "1.0.0")]
1369 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
1376 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1378 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
1379 /// result of a function call, it is recommended to use [`or_else`], which is
1380 /// lazily evaluated.
1382 /// [`or_else`]: Result::or_else
1389 /// let x: Result<u32, &str> = Ok(2);
1390 /// let y: Result<u32, &str> = Err("late error");
1391 /// assert_eq!(x.or(y), Ok(2));
1393 /// let x: Result<u32, &str> = Err("early error");
1394 /// let y: Result<u32, &str> = Ok(2);
1395 /// assert_eq!(x.or(y), Ok(2));
1397 /// let x: Result<u32, &str> = Err("not a 2");
1398 /// let y: Result<u32, &str> = Err("late error");
1399 /// assert_eq!(x.or(y), Err("late error"));
1401 /// let x: Result<u32, &str> = Ok(2);
1402 /// let y: Result<u32, &str> = Ok(100);
1403 /// assert_eq!(x.or(y), Ok(2));
1406 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
1407 #[stable(feature = "rust1", since = "1.0.0")]
1408 pub const fn or<F>(self, res: Result<T, F>) -> Result<T, F>
1416 // FIXME: ~const Drop doesn't quite work right yet
1417 #[allow(unused_variables)]
1422 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
1424 /// This function can be used for control flow based on result values.
1432 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
1433 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
1435 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
1436 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
1437 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
1438 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
1441 #[stable(feature = "rust1", since = "1.0.0")]
1442 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
1449 /// Returns the contained [`Ok`] value or a provided default.
1451 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
1452 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
1453 /// which is lazily evaluated.
1455 /// [`unwrap_or_else`]: Result::unwrap_or_else
1462 /// let default = 2;
1463 /// let x: Result<u32, &str> = Ok(9);
1464 /// assert_eq!(x.unwrap_or(default), 9);
1466 /// let x: Result<u32, &str> = Err("error");
1467 /// assert_eq!(x.unwrap_or(default), default);
1470 #[rustc_const_unstable(feature = "const_result_drop", issue = "92384")]
1471 #[stable(feature = "rust1", since = "1.0.0")]
1472 pub const fn unwrap_or(self, default: T) -> T
1479 // FIXME: ~const Drop doesn't quite work right yet
1480 #[allow(unused_variables)]
1485 /// Returns the contained [`Ok`] value or computes it from a closure.
1493 /// fn count(x: &str) -> usize { x.len() }
1495 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
1496 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
1499 #[stable(feature = "rust1", since = "1.0.0")]
1500 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
1507 /// Returns the contained [`Ok`] value, consuming the `self` value,
1508 /// without checking that the value is not an [`Err`].
1512 /// Calling this method on an [`Err`] is *[undefined behavior]*.
1514 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1519 /// let x: Result<u32, &str> = Ok(2);
1520 /// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
1524 /// let x: Result<u32, &str> = Err("emergency failure");
1525 /// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
1529 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1530 pub unsafe fn unwrap_unchecked(self) -> T {
1531 debug_assert!(self.is_ok());
1534 // SAFETY: the safety contract must be upheld by the caller.
1535 Err(_) => unsafe { hint::unreachable_unchecked() },
1539 /// Returns the contained [`Err`] value, consuming the `self` value,
1540 /// without checking that the value is not an [`Ok`].
1544 /// Calling this method on an [`Ok`] is *[undefined behavior]*.
1546 /// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
1551 /// let x: Result<u32, &str> = Ok(2);
1552 /// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
1556 /// let x: Result<u32, &str> = Err("emergency failure");
1557 /// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
1561 #[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
1562 pub unsafe fn unwrap_err_unchecked(self) -> E {
1563 debug_assert!(self.is_err());
1565 // SAFETY: the safety contract must be upheld by the caller.
1566 Ok(_) => unsafe { hint::unreachable_unchecked() },
1571 /////////////////////////////////////////////////////////////////////////
1573 /////////////////////////////////////////////////////////////////////////
1575 /// Returns `true` if the result is an [`Ok`] value containing the given value.
1580 /// #![feature(option_result_contains)]
1582 /// let x: Result<u32, &str> = Ok(2);
1583 /// assert_eq!(x.contains(&2), true);
1585 /// let x: Result<u32, &str> = Ok(3);
1586 /// assert_eq!(x.contains(&2), false);
1588 /// let x: Result<u32, &str> = Err("Some error message");
1589 /// assert_eq!(x.contains(&2), false);
1593 #[unstable(feature = "option_result_contains", issue = "62358")]
1594 pub fn contains<U>(&self, x: &U) -> bool
1604 /// Returns `true` if the result is an [`Err`] value containing the given value.
1609 /// #![feature(result_contains_err)]
1611 /// let x: Result<u32, &str> = Ok(2);
1612 /// assert_eq!(x.contains_err(&"Some error message"), false);
1614 /// let x: Result<u32, &str> = Err("Some error message");
1615 /// assert_eq!(x.contains_err(&"Some error message"), true);
1617 /// let x: Result<u32, &str> = Err("Some other error message");
1618 /// assert_eq!(x.contains_err(&"Some error message"), false);
1622 #[unstable(feature = "result_contains_err", issue = "62358")]
1623 pub fn contains_err<F>(&self, f: &F) -> bool
1634 impl<T, E> Result<&T, E> {
1635 /// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
1642 /// let x: Result<&i32, i32> = Ok(&val);
1643 /// assert_eq!(x, Ok(&12));
1644 /// let copied = x.copied();
1645 /// assert_eq!(copied, Ok(12));
1648 #[stable(feature = "result_copied", since = "1.59.0")]
1649 pub fn copied(self) -> Result<T, E>
1656 /// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
1663 /// let x: Result<&i32, i32> = Ok(&val);
1664 /// assert_eq!(x, Ok(&12));
1665 /// let cloned = x.cloned();
1666 /// assert_eq!(cloned, Ok(12));
1669 #[stable(feature = "result_cloned", since = "1.59.0")]
1670 pub fn cloned(self) -> Result<T, E>
1674 self.map(|t| t.clone())
1678 impl<T, E> Result<&mut T, E> {
1679 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
1685 /// let mut val = 12;
1686 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1687 /// assert_eq!(x, Ok(&mut 12));
1688 /// let copied = x.copied();
1689 /// assert_eq!(copied, Ok(12));
1692 #[stable(feature = "result_copied", since = "1.59.0")]
1693 pub fn copied(self) -> Result<T, E>
1697 self.map(|&mut t| t)
1700 /// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
1706 /// let mut val = 12;
1707 /// let x: Result<&mut i32, i32> = Ok(&mut val);
1708 /// assert_eq!(x, Ok(&mut 12));
1709 /// let cloned = x.cloned();
1710 /// assert_eq!(cloned, Ok(12));
1713 #[stable(feature = "result_cloned", since = "1.59.0")]
1714 pub fn cloned(self) -> Result<T, E>
1718 self.map(|t| t.clone())
1722 impl<T, E> Result<Option<T>, E> {
1723 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1725 /// `Ok(None)` will be mapped to `None`.
1726 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1731 /// #[derive(Debug, Eq, PartialEq)]
1734 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1735 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1736 /// assert_eq!(x.transpose(), y);
1739 #[stable(feature = "transpose_result", since = "1.33.0")]
1740 #[rustc_const_unstable(feature = "const_result", issue = "82814")]
1741 pub const fn transpose(self) -> Option<Result<T, E>> {
1743 Ok(Some(x)) => Some(Ok(x)),
1745 Err(e) => Some(Err(e)),
1750 impl<T, E> Result<Result<T, E>, E> {
1751 /// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
1758 /// #![feature(result_flattening)]
1759 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
1760 /// assert_eq!(Ok("hello"), x.flatten());
1762 /// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
1763 /// assert_eq!(Err(6), x.flatten());
1765 /// let x: Result<Result<&'static str, u32>, u32> = Err(6);
1766 /// assert_eq!(Err(6), x.flatten());
1769 /// Flattening only removes one level of nesting at a time:
1772 /// #![feature(result_flattening)]
1773 /// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
1774 /// assert_eq!(Ok(Ok("hello")), x.flatten());
1775 /// assert_eq!(Ok("hello"), x.flatten().flatten());
1778 #[unstable(feature = "result_flattening", issue = "70142")]
1779 pub fn flatten(self) -> Result<T, E> {
1780 self.and_then(convert::identity)
1784 // This is a separate function to reduce the code size of the methods
1785 #[cfg(not(feature = "panic_immediate_abort"))]
1789 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1790 panic!("{msg}: {error:?}")
1793 // This is a separate function to avoid constructing a `dyn Debug`
1794 // that gets immediately thrown away, since vtables don't get cleaned up
1795 // by dead code elimination if a trait object is constructed even if it goes
1797 #[cfg(feature = "panic_immediate_abort")]
1801 fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
1805 /////////////////////////////////////////////////////////////////////////////
1806 // Trait implementations
1807 /////////////////////////////////////////////////////////////////////////////
1809 #[stable(feature = "rust1", since = "1.0.0")]
1810 #[rustc_const_unstable(feature = "const_clone", issue = "91805")]
1811 impl<T, E> const Clone for Result<T, E>
1813 T: ~const Clone + ~const Destruct,
1814 E: ~const Clone + ~const Destruct,
1817 fn clone(&self) -> Self {
1819 Ok(x) => Ok(x.clone()),
1820 Err(x) => Err(x.clone()),
1825 fn clone_from(&mut self, source: &Self) {
1826 match (self, source) {
1827 (Ok(to), Ok(from)) => to.clone_from(from),
1828 (Err(to), Err(from)) => to.clone_from(from),
1829 (to, from) => *to = from.clone(),
1834 #[stable(feature = "rust1", since = "1.0.0")]
1835 impl<T, E> IntoIterator for Result<T, E> {
1837 type IntoIter = IntoIter<T>;
1839 /// Returns a consuming iterator over the possibly contained value.
1841 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1848 /// let x: Result<u32, &str> = Ok(5);
1849 /// let v: Vec<u32> = x.into_iter().collect();
1850 /// assert_eq!(v, [5]);
1852 /// let x: Result<u32, &str> = Err("nothing!");
1853 /// let v: Vec<u32> = x.into_iter().collect();
1854 /// assert_eq!(v, []);
1857 fn into_iter(self) -> IntoIter<T> {
1858 IntoIter { inner: self.ok() }
1862 #[stable(since = "1.4.0", feature = "result_iter")]
1863 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1865 type IntoIter = Iter<'a, T>;
1867 fn into_iter(self) -> Iter<'a, T> {
1872 #[stable(since = "1.4.0", feature = "result_iter")]
1873 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1874 type Item = &'a mut T;
1875 type IntoIter = IterMut<'a, T>;
1877 fn into_iter(self) -> IterMut<'a, T> {
1882 /////////////////////////////////////////////////////////////////////////////
1883 // The Result Iterators
1884 /////////////////////////////////////////////////////////////////////////////
1886 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1888 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1890 /// Created by [`Result::iter`].
1892 #[stable(feature = "rust1", since = "1.0.0")]
1893 pub struct Iter<'a, T: 'a> {
1894 inner: Option<&'a T>,
1897 #[stable(feature = "rust1", since = "1.0.0")]
1898 impl<'a, T> Iterator for Iter<'a, T> {
1902 fn next(&mut self) -> Option<&'a T> {
1906 fn size_hint(&self) -> (usize, Option<usize>) {
1907 let n = if self.inner.is_some() { 1 } else { 0 };
1912 #[stable(feature = "rust1", since = "1.0.0")]
1913 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1915 fn next_back(&mut self) -> Option<&'a T> {
1920 #[stable(feature = "rust1", since = "1.0.0")]
1921 impl<T> ExactSizeIterator for Iter<'_, T> {}
1923 #[stable(feature = "fused", since = "1.26.0")]
1924 impl<T> FusedIterator for Iter<'_, T> {}
1926 #[unstable(feature = "trusted_len", issue = "37572")]
1927 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1929 #[stable(feature = "rust1", since = "1.0.0")]
1930 impl<T> Clone for Iter<'_, T> {
1932 fn clone(&self) -> Self {
1933 Iter { inner: self.inner }
1937 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1939 /// Created by [`Result::iter_mut`].
1941 #[stable(feature = "rust1", since = "1.0.0")]
1942 pub struct IterMut<'a, T: 'a> {
1943 inner: Option<&'a mut T>,
1946 #[stable(feature = "rust1", since = "1.0.0")]
1947 impl<'a, T> Iterator for IterMut<'a, T> {
1948 type Item = &'a mut T;
1951 fn next(&mut self) -> Option<&'a mut T> {
1955 fn size_hint(&self) -> (usize, Option<usize>) {
1956 let n = if self.inner.is_some() { 1 } else { 0 };
1961 #[stable(feature = "rust1", since = "1.0.0")]
1962 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1964 fn next_back(&mut self) -> Option<&'a mut T> {
1969 #[stable(feature = "rust1", since = "1.0.0")]
1970 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1972 #[stable(feature = "fused", since = "1.26.0")]
1973 impl<T> FusedIterator for IterMut<'_, T> {}
1975 #[unstable(feature = "trusted_len", issue = "37572")]
1976 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1978 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1980 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1982 /// This struct is created by the [`into_iter`] method on
1983 /// [`Result`] (provided by the [`IntoIterator`] trait).
1985 /// [`into_iter`]: IntoIterator::into_iter
1986 #[derive(Clone, Debug)]
1987 #[stable(feature = "rust1", since = "1.0.0")]
1988 pub struct IntoIter<T> {
1992 #[stable(feature = "rust1", since = "1.0.0")]
1993 impl<T> Iterator for IntoIter<T> {
1997 fn next(&mut self) -> Option<T> {
2001 fn size_hint(&self) -> (usize, Option<usize>) {
2002 let n = if self.inner.is_some() { 1 } else { 0 };
2007 #[stable(feature = "rust1", since = "1.0.0")]
2008 impl<T> DoubleEndedIterator for IntoIter<T> {
2010 fn next_back(&mut self) -> Option<T> {
2015 #[stable(feature = "rust1", since = "1.0.0")]
2016 impl<T> ExactSizeIterator for IntoIter<T> {}
2018 #[stable(feature = "fused", since = "1.26.0")]
2019 impl<T> FusedIterator for IntoIter<T> {}
2021 #[unstable(feature = "trusted_len", issue = "37572")]
2022 unsafe impl<A> TrustedLen for IntoIter<A> {}
2024 /////////////////////////////////////////////////////////////////////////////
2026 /////////////////////////////////////////////////////////////////////////////
2028 #[stable(feature = "rust1", since = "1.0.0")]
2029 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
2030 /// Takes each element in the `Iterator`: if it is an `Err`, no further
2031 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
2032 /// container with the values of each `Result` is returned.
2034 /// Here is an example which increments every integer in a vector,
2035 /// checking for overflow:
2038 /// let v = vec![1, 2];
2039 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
2040 /// x.checked_add(1).ok_or("Overflow!")
2042 /// assert_eq!(res, Ok(vec![2, 3]));
2045 /// Here is another example that tries to subtract one from another list
2046 /// of integers, this time checking for underflow:
2049 /// let v = vec![1, 2, 0];
2050 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
2051 /// x.checked_sub(1).ok_or("Underflow!")
2053 /// assert_eq!(res, Err("Underflow!"));
2056 /// Here is a variation on the previous example, showing that no
2057 /// further elements are taken from `iter` after the first `Err`.
2060 /// let v = vec![3, 2, 1, 10];
2061 /// let mut shared = 0;
2062 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
2064 /// x.checked_sub(2).ok_or("Underflow!")
2066 /// assert_eq!(res, Err("Underflow!"));
2067 /// assert_eq!(shared, 6);
2070 /// Since the third element caused an underflow, no further elements were taken,
2071 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
2073 fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
2074 iter::try_process(iter.into_iter(), |i| i.collect())
2078 #[unstable(feature = "try_trait_v2", issue = "84277")]
2079 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2080 impl<T, E> const ops::Try for Result<T, E> {
2082 type Residual = Result<convert::Infallible, E>;
2085 fn from_output(output: Self::Output) -> Self {
2090 fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
2092 Ok(v) => ControlFlow::Continue(v),
2093 Err(e) => ControlFlow::Break(Err(e)),
2098 #[unstable(feature = "try_trait_v2", issue = "84277")]
2099 #[rustc_const_unstable(feature = "const_convert", issue = "88674")]
2100 impl<T, E, F: ~const From<E>> const ops::FromResidual<Result<convert::Infallible, E>>
2105 fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
2107 Err(e) => Err(From::from(e)),
2112 #[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
2113 impl<T, E, F: From<E>> ops::FromResidual<ops::Yeet<E>> for Result<T, F> {
2115 fn from_residual(ops::Yeet(e): ops::Yeet<E>) -> Self {
2120 #[unstable(feature = "try_trait_v2_residual", issue = "91285")]
2121 #[rustc_const_unstable(feature = "const_try", issue = "74935")]
2122 impl<T, E> const ops::Residual<T> for Result<convert::Infallible, E> {
2123 type TryType = Result<T, E>;