1 //! Error handling with the `Result` type.
3 //! [`Result<T, E>`][`Result`] is the type used for returning and propagating
4 //! errors. It is an enum with the variants, [`Ok(T)`], representing
5 //! success and containing a value, and [`Err(E)`], representing error
6 //! and containing an error value.
9 //! # #[allow(dead_code)]
10 //! enum Result<T, E> {
16 //! Functions return [`Result`] whenever errors are expected and
17 //! recoverable. In the `std` crate, [`Result`] is most prominently used
18 //! for [I/O](../../std/io/index.html).
20 //! A simple function returning [`Result`] might be
21 //! defined and used like so:
25 //! enum Version { Version1, Version2 }
27 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
28 //! match header.get(0) {
29 //! None => Err("invalid header length"),
30 //! Some(&1) => Ok(Version::Version1),
31 //! Some(&2) => Ok(Version::Version2),
32 //! Some(_) => Err("invalid version"),
36 //! let version = parse_version(&[1, 2, 3, 4]);
38 //! Ok(v) => println!("working with version: {:?}", v),
39 //! Err(e) => println!("error parsing header: {:?}", e),
43 //! Pattern matching on [`Result`]s is clear and straightforward for
44 //! simple cases, but [`Result`] comes with some convenience methods
45 //! that make working with it more succinct.
48 //! let good_result: Result<i32, i32> = Ok(10);
49 //! let bad_result: Result<i32, i32> = Err(10);
51 //! // The `is_ok` and `is_err` methods do what they say.
52 //! assert!(good_result.is_ok() && !good_result.is_err());
53 //! assert!(bad_result.is_err() && !bad_result.is_ok());
55 //! // `map` consumes the `Result` and produces another.
56 //! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
57 //! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
59 //! // Use `and_then` to continue the computation.
60 //! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
62 //! // Use `or_else` to handle the error.
63 //! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
65 //! // Consume the result and return the contents with `unwrap`.
66 //! let final_awesome_result = good_result.unwrap();
69 //! # Results must be used
71 //! A common problem with using return values to indicate errors is
72 //! that it is easy to ignore the return value, thus failing to handle
73 //! the error. [`Result`] is annotated with the `#[must_use]` attribute,
74 //! which will cause the compiler to issue a warning when a Result
75 //! value is ignored. This makes [`Result`] especially useful with
76 //! functions that may encounter errors but don't otherwise return a
79 //! Consider the [`write_all`] method defined for I/O types
80 //! by the [`Write`] trait:
86 //! fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
90 //! *Note: The actual definition of [`Write`] uses [`io::Result`], which
91 //! is just a synonym for [`Result`]`<T, `[`io::Error`]`>`.*
93 //! This method doesn't produce a value, but the write may
94 //! fail. It's crucial to handle the error case, and *not* write
95 //! something like this:
98 //! # #![allow(unused_must_use)] // \o/
99 //! use std::fs::File;
100 //! use std::io::prelude::*;
102 //! let mut file = File::create("valuable_data.txt").unwrap();
103 //! // If `write_all` errors, then we'll never know, because the return
104 //! // value is ignored.
105 //! file.write_all(b"important message");
108 //! If you *do* write that in Rust, the compiler will give you a
109 //! warning (by default, controlled by the `unused_must_use` lint).
111 //! You might instead, if you don't want to handle the error, simply
112 //! assert success with [`expect`]. This will panic if the
113 //! write fails, providing a marginally useful message indicating why:
116 //! use std::fs::File;
117 //! use std::io::prelude::*;
119 //! let mut file = File::create("valuable_data.txt").unwrap();
120 //! file.write_all(b"important message").expect("failed to write message");
123 //! You might also simply assert success:
126 //! # use std::fs::File;
127 //! # use std::io::prelude::*;
128 //! # let mut file = File::create("valuable_data.txt").unwrap();
129 //! assert!(file.write_all(b"important message").is_ok());
132 //! Or propagate the error up the call stack with [`?`]:
135 //! # use std::fs::File;
136 //! # use std::io::prelude::*;
138 //! # #[allow(dead_code)]
139 //! fn write_message() -> io::Result<()> {
140 //! let mut file = File::create("valuable_data.txt")?;
141 //! file.write_all(b"important message")?;
146 //! # The question mark operator, `?`
148 //! When writing code that calls many functions that return the
149 //! [`Result`] type, the error handling can be tedious. The question mark
150 //! operator, [`?`], hides some of the boilerplate of propagating errors
151 //! up the call stack.
153 //! It replaces this:
156 //! # #![allow(dead_code)]
157 //! use std::fs::File;
158 //! use std::io::prelude::*;
167 //! fn write_info(info: &Info) -> io::Result<()> {
168 //! // Early return on error
169 //! let mut file = match File::create("my_best_friends.txt") {
170 //! Err(e) => return Err(e),
173 //! if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
176 //! if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
179 //! if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
189 //! # #![allow(dead_code)]
190 //! use std::fs::File;
191 //! use std::io::prelude::*;
200 //! fn write_info(info: &Info) -> io::Result<()> {
201 //! let mut file = File::create("my_best_friends.txt")?;
202 //! // Early return on error
203 //! file.write_all(format!("name: {}\n", info.name).as_bytes())?;
204 //! file.write_all(format!("age: {}\n", info.age).as_bytes())?;
205 //! file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
210 //! *It's much nicer!*
212 //! Ending the expression with [`?`] will result in the unwrapped
213 //! success ([`Ok`]) value, unless the result is [`Err`], in which case
214 //! [`Err`] is returned early from the enclosing function.
216 //! [`?`] can only be used in functions that return [`Result`] because of the
217 //! early return of [`Err`] that it provides.
219 //! [`expect`]: enum.Result.html#method.expect
220 //! [`Write`]: ../../std/io/trait.Write.html
221 //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
222 //! [`io::Result`]: ../../std/io/type.Result.html
223 //! [`?`]: ../../std/macro.try.html
224 //! [`Result`]: enum.Result.html
225 //! [`Ok(T)`]: enum.Result.html#variant.Ok
226 //! [`Err(E)`]: enum.Result.html#variant.Err
227 //! [`io::Error`]: ../../std/io/struct.Error.html
228 //! [`Ok`]: enum.Result.html#variant.Ok
229 //! [`Err`]: enum.Result.html#variant.Err
231 #![stable(feature = "rust1", since = "1.0.0")]
234 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
235 use crate::ops::{self, Deref};
237 /// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
239 /// See the [`std::result`](index.html) module documentation for details.
241 /// [`Ok`]: enum.Result.html#variant.Ok
242 /// [`Err`]: enum.Result.html#variant.Err
243 #[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
244 #[must_use = "this `Result` may be an `Err` variant, which should be handled"]
245 #[stable(feature = "rust1", since = "1.0.0")]
246 pub enum Result<T, E> {
247 /// Contains the success value
248 #[stable(feature = "rust1", since = "1.0.0")]
249 Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
251 /// Contains the error value
252 #[stable(feature = "rust1", since = "1.0.0")]
253 Err(#[stable(feature = "rust1", since = "1.0.0")] E),
256 /////////////////////////////////////////////////////////////////////////////
257 // Type implementation
258 /////////////////////////////////////////////////////////////////////////////
260 impl<T, E> Result<T, E> {
261 /////////////////////////////////////////////////////////////////////////
262 // Querying the contained values
263 /////////////////////////////////////////////////////////////////////////
265 /// Returns `true` if the result is [`Ok`].
267 /// [`Ok`]: enum.Result.html#variant.Ok
274 /// let x: Result<i32, &str> = Ok(-3);
275 /// assert_eq!(x.is_ok(), true);
277 /// let x: Result<i32, &str> = Err("Some error message");
278 /// assert_eq!(x.is_ok(), false);
282 #[stable(feature = "rust1", since = "1.0.0")]
283 pub fn is_ok(&self) -> bool {
290 /// Returns `true` if the result is [`Err`].
292 /// [`Err`]: enum.Result.html#variant.Err
299 /// let x: Result<i32, &str> = Ok(-3);
300 /// assert_eq!(x.is_err(), false);
302 /// let x: Result<i32, &str> = Err("Some error message");
303 /// assert_eq!(x.is_err(), true);
307 #[stable(feature = "rust1", since = "1.0.0")]
308 pub fn is_err(&self) -> bool {
312 /// Returns `true` if the result is an [`Ok`] value containing the given value.
317 /// #![feature(option_result_contains)]
319 /// let x: Result<u32, &str> = Ok(2);
320 /// assert_eq!(x.contains(&2), true);
322 /// let x: Result<u32, &str> = Ok(3);
323 /// assert_eq!(x.contains(&2), false);
325 /// let x: Result<u32, &str> = Err("Some error message");
326 /// assert_eq!(x.contains(&2), false);
330 #[unstable(feature = "option_result_contains", issue = "62358")]
331 pub fn contains<U>(&self, x: &U) -> bool where U: PartialEq<T> {
338 /// Returns `true` if the result is an [`Err`] value containing the given value.
343 /// #![feature(result_contains_err)]
345 /// let x: Result<u32, &str> = Ok(2);
346 /// assert_eq!(x.contains_err(&"Some error message"), false);
348 /// let x: Result<u32, &str> = Err("Some error message");
349 /// assert_eq!(x.contains_err(&"Some error message"), true);
351 /// let x: Result<u32, &str> = Err("Some other error message");
352 /// assert_eq!(x.contains_err(&"Some error message"), false);
356 #[unstable(feature = "result_contains_err", issue = "62358")]
357 pub fn contains_err<F>(&self, f: &F) -> bool where F: PartialEq<E> {
364 /////////////////////////////////////////////////////////////////////////
365 // Adapter for each variant
366 /////////////////////////////////////////////////////////////////////////
368 /// Converts from `Result<T, E>` to [`Option<T>`].
370 /// Converts `self` into an [`Option<T>`], consuming `self`,
371 /// and discarding the error, if any.
373 /// [`Option<T>`]: ../../std/option/enum.Option.html
380 /// let x: Result<u32, &str> = Ok(2);
381 /// assert_eq!(x.ok(), Some(2));
383 /// let x: Result<u32, &str> = Err("Nothing here");
384 /// assert_eq!(x.ok(), None);
387 #[stable(feature = "rust1", since = "1.0.0")]
388 pub fn ok(self) -> Option<T> {
395 /// Converts from `Result<T, E>` to [`Option<E>`].
397 /// Converts `self` into an [`Option<E>`], consuming `self`,
398 /// and discarding the success value, if any.
400 /// [`Option<E>`]: ../../std/option/enum.Option.html
407 /// let x: Result<u32, &str> = Ok(2);
408 /// assert_eq!(x.err(), None);
410 /// let x: Result<u32, &str> = Err("Nothing here");
411 /// assert_eq!(x.err(), Some("Nothing here"));
414 #[stable(feature = "rust1", since = "1.0.0")]
415 pub fn err(self) -> Option<E> {
422 /////////////////////////////////////////////////////////////////////////
423 // Adapter for working with references
424 /////////////////////////////////////////////////////////////////////////
426 /// Converts from `&Result<T, E>` to `Result<&T, &E>`.
428 /// Produces a new `Result`, containing a reference
429 /// into the original, leaving the original in place.
436 /// let x: Result<u32, &str> = Ok(2);
437 /// assert_eq!(x.as_ref(), Ok(&2));
439 /// let x: Result<u32, &str> = Err("Error");
440 /// assert_eq!(x.as_ref(), Err(&"Error"));
443 #[stable(feature = "rust1", since = "1.0.0")]
444 pub fn as_ref(&self) -> Result<&T, &E> {
447 Err(ref x) => Err(x),
451 /// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
458 /// fn mutate(r: &mut Result<i32, i32>) {
459 /// match r.as_mut() {
460 /// Ok(v) => *v = 42,
461 /// Err(e) => *e = 0,
465 /// let mut x: Result<i32, i32> = Ok(2);
467 /// assert_eq!(x.unwrap(), 42);
469 /// let mut x: Result<i32, i32> = Err(13);
471 /// assert_eq!(x.unwrap_err(), 0);
474 #[stable(feature = "rust1", since = "1.0.0")]
475 pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
477 Ok(ref mut x) => Ok(x),
478 Err(ref mut x) => Err(x),
482 /////////////////////////////////////////////////////////////////////////
483 // Transforming contained values
484 /////////////////////////////////////////////////////////////////////////
486 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
487 /// contained [`Ok`] value, leaving an [`Err`] value untouched.
489 /// This function can be used to compose the results of two functions.
491 /// [`Ok`]: enum.Result.html#variant.Ok
492 /// [`Err`]: enum.Result.html#variant.Err
496 /// Print the numbers on each line of a string multiplied by two.
499 /// let line = "1\n2\n3\n4\n";
501 /// for num in line.lines() {
502 /// match num.parse::<i32>().map(|i| i * 2) {
503 /// Ok(n) => println!("{}", n),
509 #[stable(feature = "rust1", since = "1.0.0")]
510 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
517 /// Maps a `Result<T, E>` to `U` by applying a function to a
518 /// contained [`Ok`] value, or a fallback function to a
519 /// contained [`Err`] value.
521 /// This function can be used to unpack a successful result
522 /// while handling an error.
524 /// [`Ok`]: enum.Result.html#variant.Ok
525 /// [`Err`]: enum.Result.html#variant.Err
532 /// #![feature(result_map_or_else)]
535 /// let x : Result<_, &str> = Ok("foo");
536 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
538 /// let x : Result<&str, _> = Err("bar");
539 /// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
542 #[unstable(feature = "result_map_or_else", issue = "53268")]
543 pub fn map_or_else<U, M: FnOnce(T) -> U, F: FnOnce(E) -> U>(self, fallback: F, map: M) -> U {
544 self.map(map).unwrap_or_else(fallback)
547 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
548 /// contained [`Err`] value, leaving an [`Ok`] value untouched.
550 /// This function can be used to pass through a successful result while handling
553 /// [`Ok`]: enum.Result.html#variant.Ok
554 /// [`Err`]: enum.Result.html#variant.Err
561 /// fn stringify(x: u32) -> String { format!("error code: {}", x) }
563 /// let x: Result<u32, u32> = Ok(2);
564 /// assert_eq!(x.map_err(stringify), Ok(2));
566 /// let x: Result<u32, u32> = Err(13);
567 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
570 #[stable(feature = "rust1", since = "1.0.0")]
571 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
578 /////////////////////////////////////////////////////////////////////////
579 // Iterator constructors
580 /////////////////////////////////////////////////////////////////////////
582 /// Returns an iterator over the possibly contained value.
584 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
591 /// let x: Result<u32, &str> = Ok(7);
592 /// assert_eq!(x.iter().next(), Some(&7));
594 /// let x: Result<u32, &str> = Err("nothing!");
595 /// assert_eq!(x.iter().next(), None);
598 #[stable(feature = "rust1", since = "1.0.0")]
599 pub fn iter(&self) -> Iter<'_, T> {
600 Iter { inner: self.as_ref().ok() }
603 /// Returns a mutable iterator over the possibly contained value.
605 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
612 /// let mut x: Result<u32, &str> = Ok(7);
613 /// match x.iter_mut().next() {
614 /// Some(v) => *v = 40,
617 /// assert_eq!(x, Ok(40));
619 /// let mut x: Result<u32, &str> = Err("nothing!");
620 /// assert_eq!(x.iter_mut().next(), None);
623 #[stable(feature = "rust1", since = "1.0.0")]
624 pub fn iter_mut(&mut self) -> IterMut<'_, T> {
625 IterMut { inner: self.as_mut().ok() }
628 ////////////////////////////////////////////////////////////////////////
629 // Boolean operations on the values, eager and lazy
630 /////////////////////////////////////////////////////////////////////////
632 /// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
634 /// [`Ok`]: enum.Result.html#variant.Ok
635 /// [`Err`]: enum.Result.html#variant.Err
642 /// let x: Result<u32, &str> = Ok(2);
643 /// let y: Result<&str, &str> = Err("late error");
644 /// assert_eq!(x.and(y), Err("late error"));
646 /// let x: Result<u32, &str> = Err("early error");
647 /// let y: Result<&str, &str> = Ok("foo");
648 /// assert_eq!(x.and(y), Err("early error"));
650 /// let x: Result<u32, &str> = Err("not a 2");
651 /// let y: Result<&str, &str> = Err("late error");
652 /// assert_eq!(x.and(y), Err("not a 2"));
654 /// let x: Result<u32, &str> = Ok(2);
655 /// let y: Result<&str, &str> = Ok("different result type");
656 /// assert_eq!(x.and(y), Ok("different result type"));
659 #[stable(feature = "rust1", since = "1.0.0")]
660 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
667 /// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
669 /// [`Ok`]: enum.Result.html#variant.Ok
670 /// [`Err`]: enum.Result.html#variant.Err
672 /// This function can be used for control flow based on `Result` values.
679 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
680 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
682 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
683 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
684 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
685 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
688 #[stable(feature = "rust1", since = "1.0.0")]
689 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
696 /// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
698 /// Arguments passed to `or` are eagerly evaluated; if you are passing the
699 /// result of a function call, it is recommended to use [`or_else`], which is
700 /// lazily evaluated.
702 /// [`Ok`]: enum.Result.html#variant.Ok
703 /// [`Err`]: enum.Result.html#variant.Err
704 /// [`or_else`]: #method.or_else
711 /// let x: Result<u32, &str> = Ok(2);
712 /// let y: Result<u32, &str> = Err("late error");
713 /// assert_eq!(x.or(y), Ok(2));
715 /// let x: Result<u32, &str> = Err("early error");
716 /// let y: Result<u32, &str> = Ok(2);
717 /// assert_eq!(x.or(y), Ok(2));
719 /// let x: Result<u32, &str> = Err("not a 2");
720 /// let y: Result<u32, &str> = Err("late error");
721 /// assert_eq!(x.or(y), Err("late error"));
723 /// let x: Result<u32, &str> = Ok(2);
724 /// let y: Result<u32, &str> = Ok(100);
725 /// assert_eq!(x.or(y), Ok(2));
728 #[stable(feature = "rust1", since = "1.0.0")]
729 pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
736 /// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
738 /// This function can be used for control flow based on result values.
740 /// [`Ok`]: enum.Result.html#variant.Ok
741 /// [`Err`]: enum.Result.html#variant.Err
748 /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
749 /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
751 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
752 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
753 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
754 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
757 #[stable(feature = "rust1", since = "1.0.0")]
758 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
765 /// Unwraps a result, yielding the content of an [`Ok`].
766 /// Else, it returns `optb`.
768 /// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
769 /// the result of a function call, it is recommended to use [`unwrap_or_else`],
770 /// which is lazily evaluated.
772 /// [`Ok`]: enum.Result.html#variant.Ok
773 /// [`Err`]: enum.Result.html#variant.Err
774 /// [`unwrap_or_else`]: #method.unwrap_or_else
782 /// let x: Result<u32, &str> = Ok(9);
783 /// assert_eq!(x.unwrap_or(optb), 9);
785 /// let x: Result<u32, &str> = Err("error");
786 /// assert_eq!(x.unwrap_or(optb), optb);
789 #[stable(feature = "rust1", since = "1.0.0")]
790 pub fn unwrap_or(self, optb: T) -> T {
797 /// Unwraps a result, yielding the content of an [`Ok`].
798 /// If the value is an [`Err`] then it calls `op` with its value.
800 /// [`Ok`]: enum.Result.html#variant.Ok
801 /// [`Err`]: enum.Result.html#variant.Err
808 /// fn count(x: &str) -> usize { x.len() }
810 /// assert_eq!(Ok(2).unwrap_or_else(count), 2);
811 /// assert_eq!(Err("foo").unwrap_or_else(count), 3);
814 #[stable(feature = "rust1", since = "1.0.0")]
815 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
823 impl<T, E: fmt::Debug> Result<T, E> {
824 /// Unwraps a result, yielding the content of an [`Ok`].
828 /// Panics if the value is an [`Err`], with a panic message provided by the
831 /// [`Ok`]: enum.Result.html#variant.Ok
832 /// [`Err`]: enum.Result.html#variant.Err
839 /// let x: Result<u32, &str> = Ok(2);
840 /// assert_eq!(x.unwrap(), 2);
843 /// ```{.should_panic}
844 /// let x: Result<u32, &str> = Err("emergency failure");
845 /// x.unwrap(); // panics with `emergency failure`
848 #[stable(feature = "rust1", since = "1.0.0")]
849 pub fn unwrap(self) -> T {
852 Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
856 /// Unwraps a result, yielding the content of an [`Ok`].
860 /// Panics if the value is an [`Err`], with a panic message including the
861 /// passed message, and the content of the [`Err`].
863 /// [`Ok`]: enum.Result.html#variant.Ok
864 /// [`Err`]: enum.Result.html#variant.Err
870 /// ```{.should_panic}
871 /// let x: Result<u32, &str> = Err("emergency failure");
872 /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
875 #[stable(feature = "result_expect", since = "1.4.0")]
876 pub fn expect(self, msg: &str) -> T {
879 Err(e) => unwrap_failed(msg, &e),
884 impl<T: fmt::Debug, E> Result<T, E> {
885 /// Unwraps a result, yielding the content of an [`Err`].
889 /// Panics if the value is an [`Ok`], with a custom panic message provided
890 /// by the [`Ok`]'s value.
892 /// [`Ok`]: enum.Result.html#variant.Ok
893 /// [`Err`]: enum.Result.html#variant.Err
898 /// ```{.should_panic}
899 /// let x: Result<u32, &str> = Ok(2);
900 /// x.unwrap_err(); // panics with `2`
904 /// let x: Result<u32, &str> = Err("emergency failure");
905 /// assert_eq!(x.unwrap_err(), "emergency failure");
908 #[stable(feature = "rust1", since = "1.0.0")]
909 pub fn unwrap_err(self) -> E {
911 Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
916 /// Unwraps a result, yielding the content of an [`Err`].
920 /// Panics if the value is an [`Ok`], with a panic message including the
921 /// passed message, and the content of the [`Ok`].
923 /// [`Ok`]: enum.Result.html#variant.Ok
924 /// [`Err`]: enum.Result.html#variant.Err
930 /// ```{.should_panic}
931 /// let x: Result<u32, &str> = Ok(10);
932 /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
935 #[stable(feature = "result_expect_err", since = "1.17.0")]
936 pub fn expect_err(self, msg: &str) -> E {
938 Ok(t) => unwrap_failed(msg, &t),
944 impl<T: Default, E> Result<T, E> {
945 /// Returns the contained value or a default
947 /// Consumes the `self` argument then, if [`Ok`], returns the contained
948 /// value, otherwise if [`Err`], returns the default value for that
953 /// Converts a string to an integer, turning poorly-formed strings
954 /// into 0 (the default value for integers). [`parse`] converts
955 /// a string to any other type that implements [`FromStr`], returning an
956 /// [`Err`] on error.
959 /// let good_year_from_input = "1909";
960 /// let bad_year_from_input = "190blarg";
961 /// let good_year = good_year_from_input.parse().unwrap_or_default();
962 /// let bad_year = bad_year_from_input.parse().unwrap_or_default();
964 /// assert_eq!(1909, good_year);
965 /// assert_eq!(0, bad_year);
968 /// [`parse`]: ../../std/primitive.str.html#method.parse
969 /// [`FromStr`]: ../../std/str/trait.FromStr.html
970 /// [`Ok`]: enum.Result.html#variant.Ok
971 /// [`Err`]: enum.Result.html#variant.Err
973 #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
974 pub fn unwrap_or_default(self) -> T {
977 Err(_) => Default::default(),
982 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
983 impl<T: Deref, E> Result<T, E> {
984 /// Converts from `&Result<T, E>` to `Result<&T::Target, &E>`.
986 /// Leaves the original Result in-place, creating a new one with a reference
987 /// to the original one, additionally coercing the `Ok` arm of the Result via
989 pub fn deref_ok(&self) -> Result<&T::Target, &E> {
990 self.as_ref().map(|t| t.deref())
994 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
995 impl<T, E: Deref> Result<T, E> {
996 /// Converts from `&Result<T, E>` to `Result<&T, &E::Target>`.
998 /// Leaves the original Result in-place, creating a new one with a reference
999 /// to the original one, additionally coercing the `Err` arm of the Result via
1001 pub fn deref_err(&self) -> Result<&T, &E::Target>
1003 self.as_ref().map_err(|e| e.deref())
1007 #[unstable(feature = "inner_deref", reason = "newly added", issue = "50264")]
1008 impl<T: Deref, E: Deref> Result<T, E> {
1009 /// Converts from `&Result<T, E>` to `Result<&T::Target, &E::Target>`.
1011 /// Leaves the original Result in-place, creating a new one with a reference
1012 /// to the original one, additionally coercing both the `Ok` and `Err` arms
1013 /// of the Result via `Deref`.
1014 pub fn deref(&self) -> Result<&T::Target, &E::Target>
1016 self.as_ref().map(|t| t.deref()).map_err(|e| e.deref())
1020 impl<T, E> Result<Option<T>, E> {
1021 /// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
1023 /// `Ok(None)` will be mapped to `None`.
1024 /// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
1029 /// #[derive(Debug, Eq, PartialEq)]
1032 /// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
1033 /// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
1034 /// assert_eq!(x.transpose(), y);
1037 #[stable(feature = "transpose_result", since = "1.33.0")]
1038 pub fn transpose(self) -> Option<Result<T, E>> {
1040 Ok(Some(x)) => Some(Ok(x)),
1042 Err(e) => Some(Err(e)),
1047 // This is a separate function to reduce the code size of the methods
1050 fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
1051 panic!("{}: {:?}", msg, error)
1054 /////////////////////////////////////////////////////////////////////////////
1055 // Trait implementations
1056 /////////////////////////////////////////////////////////////////////////////
1058 #[stable(feature = "rust1", since = "1.0.0")]
1059 impl<T: Clone, E: Clone> Clone for Result<T, E> {
1061 fn clone(&self) -> Self {
1063 Ok(x) => Ok(x.clone()),
1064 Err(x) => Err(x.clone()),
1069 fn clone_from(&mut self, source: &Self) {
1070 match (self, source) {
1071 (Ok(to), Ok(from)) => to.clone_from(from),
1072 (Err(to), Err(from)) => to.clone_from(from),
1073 (to, from) => *to = from.clone(),
1079 #[stable(feature = "rust1", since = "1.0.0")]
1080 impl<T, E> IntoIterator for Result<T, E> {
1082 type IntoIter = IntoIter<T>;
1084 /// Returns a consuming iterator over the possibly contained value.
1086 /// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
1093 /// let x: Result<u32, &str> = Ok(5);
1094 /// let v: Vec<u32> = x.into_iter().collect();
1095 /// assert_eq!(v, [5]);
1097 /// let x: Result<u32, &str> = Err("nothing!");
1098 /// let v: Vec<u32> = x.into_iter().collect();
1099 /// assert_eq!(v, []);
1102 fn into_iter(self) -> IntoIter<T> {
1103 IntoIter { inner: self.ok() }
1107 #[stable(since = "1.4.0", feature = "result_iter")]
1108 impl<'a, T, E> IntoIterator for &'a Result<T, E> {
1110 type IntoIter = Iter<'a, T>;
1112 fn into_iter(self) -> Iter<'a, T> {
1117 #[stable(since = "1.4.0", feature = "result_iter")]
1118 impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
1119 type Item = &'a mut T;
1120 type IntoIter = IterMut<'a, T>;
1122 fn into_iter(self) -> IterMut<'a, T> {
1127 /////////////////////////////////////////////////////////////////////////////
1128 // The Result Iterators
1129 /////////////////////////////////////////////////////////////////////////////
1131 /// An iterator over a reference to the [`Ok`] variant of a [`Result`].
1133 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1135 /// Created by [`Result::iter`].
1137 /// [`Ok`]: enum.Result.html#variant.Ok
1138 /// [`Result`]: enum.Result.html
1139 /// [`Result::iter`]: enum.Result.html#method.iter
1141 #[stable(feature = "rust1", since = "1.0.0")]
1142 pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }
1144 #[stable(feature = "rust1", since = "1.0.0")]
1145 impl<'a, T> Iterator for Iter<'a, T> {
1149 fn next(&mut self) -> Option<&'a T> { self.inner.take() }
1151 fn size_hint(&self) -> (usize, Option<usize>) {
1152 let n = if self.inner.is_some() {1} else {0};
1157 #[stable(feature = "rust1", since = "1.0.0")]
1158 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
1160 fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
1163 #[stable(feature = "rust1", since = "1.0.0")]
1164 impl<T> ExactSizeIterator for Iter<'_, T> {}
1166 #[stable(feature = "fused", since = "1.26.0")]
1167 impl<T> FusedIterator for Iter<'_, T> {}
1169 #[unstable(feature = "trusted_len", issue = "37572")]
1170 unsafe impl<A> TrustedLen for Iter<'_, A> {}
1172 #[stable(feature = "rust1", since = "1.0.0")]
1173 impl<T> Clone for Iter<'_, T> {
1175 fn clone(&self) -> Self { Iter { inner: self.inner } }
1178 /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
1180 /// Created by [`Result::iter_mut`].
1182 /// [`Ok`]: enum.Result.html#variant.Ok
1183 /// [`Result`]: enum.Result.html
1184 /// [`Result::iter_mut`]: enum.Result.html#method.iter_mut
1186 #[stable(feature = "rust1", since = "1.0.0")]
1187 pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }
1189 #[stable(feature = "rust1", since = "1.0.0")]
1190 impl<'a, T> Iterator for IterMut<'a, T> {
1191 type Item = &'a mut T;
1194 fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
1196 fn size_hint(&self) -> (usize, Option<usize>) {
1197 let n = if self.inner.is_some() {1} else {0};
1202 #[stable(feature = "rust1", since = "1.0.0")]
1203 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
1205 fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
1208 #[stable(feature = "rust1", since = "1.0.0")]
1209 impl<T> ExactSizeIterator for IterMut<'_, T> {}
1211 #[stable(feature = "fused", since = "1.26.0")]
1212 impl<T> FusedIterator for IterMut<'_, T> {}
1214 #[unstable(feature = "trusted_len", issue = "37572")]
1215 unsafe impl<A> TrustedLen for IterMut<'_, A> {}
1217 /// An iterator over the value in a [`Ok`] variant of a [`Result`].
1219 /// The iterator yields one value if the result is [`Ok`], otherwise none.
1221 /// This struct is created by the [`into_iter`] method on
1222 /// [`Result`][`Result`] (provided by the [`IntoIterator`] trait).
1224 /// [`Ok`]: enum.Result.html#variant.Ok
1225 /// [`Result`]: enum.Result.html
1226 /// [`into_iter`]: ../iter/trait.IntoIterator.html#tymethod.into_iter
1227 /// [`IntoIterator`]: ../iter/trait.IntoIterator.html
1228 #[derive(Clone, Debug)]
1229 #[stable(feature = "rust1", since = "1.0.0")]
1230 pub struct IntoIter<T> { inner: Option<T> }
1232 #[stable(feature = "rust1", since = "1.0.0")]
1233 impl<T> Iterator for IntoIter<T> {
1237 fn next(&mut self) -> Option<T> { self.inner.take() }
1239 fn size_hint(&self) -> (usize, Option<usize>) {
1240 let n = if self.inner.is_some() {1} else {0};
1245 #[stable(feature = "rust1", since = "1.0.0")]
1246 impl<T> DoubleEndedIterator for IntoIter<T> {
1248 fn next_back(&mut self) -> Option<T> { self.inner.take() }
1251 #[stable(feature = "rust1", since = "1.0.0")]
1252 impl<T> ExactSizeIterator for IntoIter<T> {}
1254 #[stable(feature = "fused", since = "1.26.0")]
1255 impl<T> FusedIterator for IntoIter<T> {}
1257 #[unstable(feature = "trusted_len", issue = "37572")]
1258 unsafe impl<A> TrustedLen for IntoIter<A> {}
1260 /////////////////////////////////////////////////////////////////////////////
1262 /////////////////////////////////////////////////////////////////////////////
1264 #[stable(feature = "rust1", since = "1.0.0")]
1265 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
1266 /// Takes each element in the `Iterator`: if it is an `Err`, no further
1267 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
1268 /// container with the values of each `Result` is returned.
1270 /// Here is an example which increments every integer in a vector,
1271 /// checking for overflow:
1274 /// let v = vec![1, 2];
1275 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1276 /// x.checked_add(1).ok_or("Overflow!")
1278 /// assert_eq!(res, Ok(vec![2, 3]));
1281 /// Here is another example that tries to subtract one from another list
1282 /// of integers, this time checking for underflow:
1285 /// let v = vec![1, 2, 0];
1286 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
1287 /// x.checked_sub(1).ok_or("Underflow!")
1289 /// assert_eq!(res, Err("Underflow!"));
1292 /// Here is a variation on the previous example, showing that no
1293 /// further elements are taken from `iter` after the first `Err`.
1296 /// let v = vec![3, 2, 1, 10];
1297 /// let mut shared = 0;
1298 /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
1300 /// x.checked_sub(2).ok_or("Underflow!")
1302 /// assert_eq!(res, Err("Underflow!"));
1303 /// assert_eq!(shared, 6);
1306 /// Since the third element caused an underflow, no further elements were taken,
1307 /// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
1309 fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
1310 // FIXME(#11084): This could be replaced with Iterator::scan when this
1311 // performance bug is closed.
1313 struct Adapter<Iter, E> {
1318 impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
1322 fn next(&mut self) -> Option<T> {
1323 match self.iter.next() {
1324 Some(Ok(value)) => Some(value),
1326 self.err = Some(err);
1333 fn size_hint(&self) -> (usize, Option<usize>) {
1334 let (_min, max) = self.iter.size_hint();
1339 let mut adapter = Adapter { iter: iter.into_iter(), err: None };
1340 let v: V = FromIterator::from_iter(adapter.by_ref());
1343 Some(err) => Err(err),
1349 #[unstable(feature = "try_trait", issue = "42327")]
1350 impl<T,E> ops::Try for Result<T, E> {
1355 fn into_result(self) -> Self {
1360 fn from_ok(v: T) -> Self {
1365 fn from_error(v: E) -> Self {