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//! Error handling with the `Result` type.
//!
//! [`Result<T, E>`][`Result`] is the type used for returning and propagating
//! errors. It is an enum with the variants, [`Ok(T)`], representing
//! success and containing a value, and [`Err(E)`], representing error
//! and containing an error value.
//!
//! ```
//! # #[allow(dead_code)]
//! enum Result<T, E> {
//! Ok(T),
//! Err(E),
//! }
//! ```
//!
//! Functions return [`Result`] whenever errors are expected and
//! recoverable. In the `std` crate, [`Result`] is most prominently used
//! for [I/O](../../std/io/index.html).
//!
//! A simple function returning [`Result`] might be
//! defined and used like so:
//!
//! ```
//! #[derive(Debug)]
//! enum Version { Version1, Version2 }
//!
//! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
//! match header.get(0) {
//! None => Err("invalid header length"),
//! Some(&1) => Ok(Version::Version1),
//! Some(&2) => Ok(Version::Version2),
//! Some(_) => Err("invalid version"),
//! }
//! }
//!
//! let version = parse_version(&[1, 2, 3, 4]);
//! match version {
//! Ok(v) => println!("working with version: {v:?}"),
//! Err(e) => println!("error parsing header: {e:?}"),
//! }
//! ```
//!
//! Pattern matching on [`Result`]s is clear and straightforward for
//! simple cases, but [`Result`] comes with some convenience methods
//! that make working with it more succinct.
//!
//! ```
//! let good_result: Result<i32, i32> = Ok(10);
//! let bad_result: Result<i32, i32> = Err(10);
//!
//! // The `is_ok` and `is_err` methods do what they say.
//! assert!(good_result.is_ok() && !good_result.is_err());
//! assert!(bad_result.is_err() && !bad_result.is_ok());
//!
//! // `map` consumes the `Result` and produces another.
//! let good_result: Result<i32, i32> = good_result.map(|i| i + 1);
//! let bad_result: Result<i32, i32> = bad_result.map(|i| i - 1);
//!
//! // Use `and_then` to continue the computation.
//! let good_result: Result<bool, i32> = good_result.and_then(|i| Ok(i == 11));
//!
//! // Use `or_else` to handle the error.
//! let bad_result: Result<i32, i32> = bad_result.or_else(|i| Ok(i + 20));
//!
//! // Consume the result and return the contents with `unwrap`.
//! let final_awesome_result = good_result.unwrap();
//! ```
//!
//! # Results must be used
//!
//! A common problem with using return values to indicate errors is
//! that it is easy to ignore the return value, thus failing to handle
//! the error. [`Result`] is annotated with the `#[must_use]` attribute,
//! which will cause the compiler to issue a warning when a Result
//! value is ignored. This makes [`Result`] especially useful with
//! functions that may encounter errors but don't otherwise return a
//! useful value.
//!
//! Consider the [`write_all`] method defined for I/O types
//! by the [`Write`] trait:
//!
//! ```
//! use std::io;
//!
//! trait Write {
//! fn write_all(&mut self, bytes: &[u8]) -> Result<(), io::Error>;
//! }
//! ```
//!
//! *Note: The actual definition of [`Write`] uses [`io::Result`], which
//! is just a synonym for <code>[Result]<T, [io::Error]></code>.*
//!
//! This method doesn't produce a value, but the write may
//! fail. It's crucial to handle the error case, and *not* write
//! something like this:
//!
//! ```no_run
//! # #![allow(unused_must_use)] // \o/
//! use std::fs::File;
//! use std::io::prelude::*;
//!
//! let mut file = File::create("valuable_data.txt").unwrap();
//! // If `write_all` errors, then we'll never know, because the return
//! // value is ignored.
//! file.write_all(b"important message");
//! ```
//!
//! If you *do* write that in Rust, the compiler will give you a
//! warning (by default, controlled by the `unused_must_use` lint).
//!
//! You might instead, if you don't want to handle the error, simply
//! assert success with [`expect`]. This will panic if the
//! write fails, providing a marginally useful message indicating why:
//!
//! ```no_run
//! use std::fs::File;
//! use std::io::prelude::*;
//!
//! let mut file = File::create("valuable_data.txt").unwrap();
//! file.write_all(b"important message").expect("failed to write message");
//! ```
//!
//! You might also simply assert success:
//!
//! ```no_run
//! # use std::fs::File;
//! # use std::io::prelude::*;
//! # let mut file = File::create("valuable_data.txt").unwrap();
//! assert!(file.write_all(b"important message").is_ok());
//! ```
//!
//! Or propagate the error up the call stack with [`?`]:
//!
//! ```
//! # use std::fs::File;
//! # use std::io::prelude::*;
//! # use std::io;
//! # #[allow(dead_code)]
//! fn write_message() -> io::Result<()> {
//! let mut file = File::create("valuable_data.txt")?;
//! file.write_all(b"important message")?;
//! Ok(())
//! }
//! ```
//!
//! # The question mark operator, `?`
//!
//! When writing code that calls many functions that return the
//! [`Result`] type, the error handling can be tedious. The question mark
//! operator, [`?`], hides some of the boilerplate of propagating errors
//! up the call stack.
//!
//! It replaces this:
//!
//! ```
//! # #![allow(dead_code)]
//! use std::fs::File;
//! use std::io::prelude::*;
//! use std::io;
//!
//! struct Info {
//! name: String,
//! age: i32,
//! rating: i32,
//! }
//!
//! fn write_info(info: &Info) -> io::Result<()> {
//! // Early return on error
//! let mut file = match File::create("my_best_friends.txt") {
//! Err(e) => return Err(e),
//! Ok(f) => f,
//! };
//! if let Err(e) = file.write_all(format!("name: {}\n", info.name).as_bytes()) {
//! return Err(e)
//! }
//! if let Err(e) = file.write_all(format!("age: {}\n", info.age).as_bytes()) {
//! return Err(e)
//! }
//! if let Err(e) = file.write_all(format!("rating: {}\n", info.rating).as_bytes()) {
//! return Err(e)
//! }
//! Ok(())
//! }
//! ```
//!
//! With this:
//!
//! ```
//! # #![allow(dead_code)]
//! use std::fs::File;
//! use std::io::prelude::*;
//! use std::io;
//!
//! struct Info {
//! name: String,
//! age: i32,
//! rating: i32,
//! }
//!
//! fn write_info(info: &Info) -> io::Result<()> {
//! let mut file = File::create("my_best_friends.txt")?;
//! // Early return on error
//! file.write_all(format!("name: {}\n", info.name).as_bytes())?;
//! file.write_all(format!("age: {}\n", info.age).as_bytes())?;
//! file.write_all(format!("rating: {}\n", info.rating).as_bytes())?;
//! Ok(())
//! }
//! ```
//!
//! *It's much nicer!*
//!
//! Ending the expression with [`?`] will result in the [`Ok`]'s unwrapped value, unless the result
//! is [`Err`], in which case [`Err`] is returned early from the enclosing function.
//!
//! [`?`] can be used in functions that return [`Result`] because of the
//! early return of [`Err`] that it provides.
//!
//! [`expect`]: Result::expect
//! [`Write`]: ../../std/io/trait.Write.html "io::Write"
//! [`write_all`]: ../../std/io/trait.Write.html#method.write_all "io::Write::write_all"
//! [`io::Result`]: ../../std/io/type.Result.html "io::Result"
//! [`?`]: crate::ops::Try
//! [`Ok(T)`]: Ok
//! [`Err(E)`]: Err
//! [io::Error]: ../../std/io/struct.Error.html "io::Error"
//!
//! # Method overview
//!
//! In addition to working with pattern matching, [`Result`] provides a
//! wide variety of different methods.
//!
//! ## Querying the variant
//!
//! The [`is_ok`] and [`is_err`] methods return [`true`] if the [`Result`]
//! is [`Ok`] or [`Err`], respectively.
//!
//! [`is_err`]: Result::is_err
//! [`is_ok`]: Result::is_ok
//!
//! ## Adapters for working with references
//!
//! * [`as_ref`] converts from `&Result<T, E>` to `Result<&T, &E>`
//! * [`as_mut`] converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`
//! * [`as_deref`] converts from `&Result<T, E>` to `Result<&T::Target, &E>`
//! * [`as_deref_mut`] converts from `&mut Result<T, E>` to
//! `Result<&mut T::Target, &mut E>`
//!
//! [`as_deref`]: Result::as_deref
//! [`as_deref_mut`]: Result::as_deref_mut
//! [`as_mut`]: Result::as_mut
//! [`as_ref`]: Result::as_ref
//!
//! ## Extracting contained values
//!
//! These methods extract the contained value in a [`Result<T, E>`] when it
//! is the [`Ok`] variant. If the [`Result`] is [`Err`]:
//!
//! * [`expect`] panics with a provided custom message
//! * [`unwrap`] panics with a generic message
//! * [`unwrap_or`] returns the provided default value
//! * [`unwrap_or_default`] returns the default value of the type `T`
//! (which must implement the [`Default`] trait)
//! * [`unwrap_or_else`] returns the result of evaluating the provided
//! function
//!
//! The panicking methods [`expect`] and [`unwrap`] require `E` to
//! implement the [`Debug`] trait.
//!
//! [`Debug`]: crate::fmt::Debug
//! [`expect`]: Result::expect
//! [`unwrap`]: Result::unwrap
//! [`unwrap_or`]: Result::unwrap_or
//! [`unwrap_or_default`]: Result::unwrap_or_default
//! [`unwrap_or_else`]: Result::unwrap_or_else
//!
//! These methods extract the contained value in a [`Result<T, E>`] when it
//! is the [`Err`] variant. They require `T` to implement the [`Debug`]
//! trait. If the [`Result`] is [`Ok`]:
//!
//! * [`expect_err`] panics with a provided custom message
//! * [`unwrap_err`] panics with a generic message
//!
//! [`Debug`]: crate::fmt::Debug
//! [`expect_err`]: Result::expect_err
//! [`unwrap_err`]: Result::unwrap_err
//!
//! ## Transforming contained values
//!
//! These methods transform [`Result`] to [`Option`]:
//!
//! * [`err`][Result::err] transforms [`Result<T, E>`] into [`Option<E>`],
//! mapping [`Err(e)`] to [`Some(e)`] and [`Ok(v)`] to [`None`]
//! * [`ok`][Result::ok] transforms [`Result<T, E>`] into [`Option<T>`],
//! mapping [`Ok(v)`] to [`Some(v)`] and [`Err(e)`] to [`None`]
//! * [`transpose`] transposes a [`Result`] of an [`Option`] into an
//! [`Option`] of a [`Result`]
//!
// Do NOT add link reference definitions for `err` or `ok`, because they
// will generate numerous incorrect URLs for `Err` and `Ok` elsewhere, due
// to case folding.
//!
//! [`Err(e)`]: Err
//! [`Ok(v)`]: Ok
//! [`Some(e)`]: Option::Some
//! [`Some(v)`]: Option::Some
//! [`transpose`]: Result::transpose
//!
//! This method transforms the contained value of the [`Ok`] variant:
//!
//! * [`map`] transforms [`Result<T, E>`] into [`Result<U, E>`] by applying
//! the provided function to the contained value of [`Ok`] and leaving
//! [`Err`] values unchanged
//!
//! [`map`]: Result::map
//!
//! This method transforms the contained value of the [`Err`] variant:
//!
//! * [`map_err`] transforms [`Result<T, E>`] into [`Result<T, F>`] by
//! applying the provided function to the contained value of [`Err`] and
//! leaving [`Ok`] values unchanged
//!
//! [`map_err`]: Result::map_err
//!
//! These methods transform a [`Result<T, E>`] into a value of a possibly
//! different type `U`:
//!
//! * [`map_or`] applies the provided function to the contained value of
//! [`Ok`], or returns the provided default value if the [`Result`] is
//! [`Err`]
//! * [`map_or_else`] applies the provided function to the contained value
//! of [`Ok`], or applies the provided default fallback function to the
//! contained value of [`Err`]
//!
//! [`map_or`]: Result::map_or
//! [`map_or_else`]: Result::map_or_else
//!
//! ## Boolean operators
//!
//! These methods treat the [`Result`] as a boolean value, where [`Ok`]
//! acts like [`true`] and [`Err`] acts like [`false`]. There are two
//! categories of these methods: ones that take a [`Result`] as input, and
//! ones that take a function as input (to be lazily evaluated).
//!
//! The [`and`] and [`or`] methods take another [`Result`] as input, and
//! produce a [`Result`] as output. The [`and`] method can produce a
//! [`Result<U, E>`] value having a different inner type `U` than
//! [`Result<T, E>`]. The [`or`] method can produce a [`Result<T, F>`]
//! value having a different error type `F` than [`Result<T, E>`].
//!
//! | method | self | input | output |
//! |---------|----------|-----------|----------|
//! | [`and`] | `Err(e)` | (ignored) | `Err(e)` |
//! | [`and`] | `Ok(x)` | `Err(d)` | `Err(d)` |
//! | [`and`] | `Ok(x)` | `Ok(y)` | `Ok(y)` |
//! | [`or`] | `Err(e)` | `Err(d)` | `Err(d)` |
//! | [`or`] | `Err(e)` | `Ok(y)` | `Ok(y)` |
//! | [`or`] | `Ok(x)` | (ignored) | `Ok(x)` |
//!
//! [`and`]: Result::and
//! [`or`]: Result::or
//!
//! The [`and_then`] and [`or_else`] methods take a function as input, and
//! only evaluate the function when they need to produce a new value. The
//! [`and_then`] method can produce a [`Result<U, E>`] value having a
//! different inner type `U` than [`Result<T, E>`]. The [`or_else`] method
//! can produce a [`Result<T, F>`] value having a different error type `F`
//! than [`Result<T, E>`].
//!
//! | method | self | function input | function result | output |
//! |--------------|----------|----------------|-----------------|----------|
//! | [`and_then`] | `Err(e)` | (not provided) | (not evaluated) | `Err(e)` |
//! | [`and_then`] | `Ok(x)` | `x` | `Err(d)` | `Err(d)` |
//! | [`and_then`] | `Ok(x)` | `x` | `Ok(y)` | `Ok(y)` |
//! | [`or_else`] | `Err(e)` | `e` | `Err(d)` | `Err(d)` |
//! | [`or_else`] | `Err(e)` | `e` | `Ok(y)` | `Ok(y)` |
//! | [`or_else`] | `Ok(x)` | (not provided) | (not evaluated) | `Ok(x)` |
//!
//! [`and_then`]: Result::and_then
//! [`or_else`]: Result::or_else
//!
//! ## Comparison operators
//!
//! If `T` and `E` both implement [`PartialOrd`] then [`Result<T, E>`] will
//! derive its [`PartialOrd`] implementation. With this order, an [`Ok`]
//! compares as less than any [`Err`], while two [`Ok`] or two [`Err`]
//! compare as their contained values would in `T` or `E` respectively. If `T`
//! and `E` both also implement [`Ord`], then so does [`Result<T, E>`].
//!
//! ```
//! assert!(Ok(1) < Err(0));
//! let x: Result<i32, ()> = Ok(0);
//! let y = Ok(1);
//! assert!(x < y);
//! let x: Result<(), i32> = Err(0);
//! let y = Err(1);
//! assert!(x < y);
//! ```
//!
//! ## Iterating over `Result`
//!
//! A [`Result`] can be iterated over. This can be helpful if you need an
//! iterator that is conditionally empty. The iterator will either produce
//! a single value (when the [`Result`] is [`Ok`]), or produce no values
//! (when the [`Result`] is [`Err`]). For example, [`into_iter`] acts like
//! [`once(v)`] if the [`Result`] is [`Ok(v)`], and like [`empty()`] if the
//! [`Result`] is [`Err`].
//!
//! [`Ok(v)`]: Ok
//! [`empty()`]: crate::iter::empty
//! [`once(v)`]: crate::iter::once
//!
//! Iterators over [`Result<T, E>`] come in three types:
//!
//! * [`into_iter`] consumes the [`Result`] and produces the contained
//! value
//! * [`iter`] produces an immutable reference of type `&T` to the
//! contained value
//! * [`iter_mut`] produces a mutable reference of type `&mut T` to the
//! contained value
//!
//! See [Iterating over `Option`] for examples of how this can be useful.
//!
//! [Iterating over `Option`]: crate::option#iterating-over-option
//! [`into_iter`]: Result::into_iter
//! [`iter`]: Result::iter
//! [`iter_mut`]: Result::iter_mut
//!
//! You might want to use an iterator chain to do multiple instances of an
//! operation that can fail, but would like to ignore failures while
//! continuing to process the successful results. In this example, we take
//! advantage of the iterable nature of [`Result`] to select only the
//! [`Ok`] values using [`flatten`][Iterator::flatten].
//!
//! ```
//! # use std::str::FromStr;
//! let mut results = vec![];
//! let mut errs = vec![];
//! let nums: Vec<_> = ["17", "not a number", "99", "-27", "768"]
//! .into_iter()
//! .map(u8::from_str)
//! // Save clones of the raw `Result` values to inspect
//! .inspect(|x| results.push(x.clone()))
//! // Challenge: explain how this captures only the `Err` values
//! .inspect(|x| errs.extend(x.clone().err()))
//! .flatten()
//! .collect();
//! assert_eq!(errs.len(), 3);
//! assert_eq!(nums, [17, 99]);
//! println!("results {results:?}");
//! println!("errs {errs:?}");
//! println!("nums {nums:?}");
//! ```
//!
//! ## Collecting into `Result`
//!
//! [`Result`] implements the [`FromIterator`][impl-FromIterator] trait,
//! which allows an iterator over [`Result`] values to be collected into a
//! [`Result`] of a collection of each contained value of the original
//! [`Result`] values, or [`Err`] if any of the elements was [`Err`].
//!
//! [impl-FromIterator]: Result#impl-FromIterator%3CResult%3CA,+E%3E%3E-for-Result%3CV,+E%3E
//!
//! ```
//! let v = [Ok(2), Ok(4), Err("err!"), Ok(8)];
//! let res: Result<Vec<_>, &str> = v.into_iter().collect();
//! assert_eq!(res, Err("err!"));
//! let v = [Ok(2), Ok(4), Ok(8)];
//! let res: Result<Vec<_>, &str> = v.into_iter().collect();
//! assert_eq!(res, Ok(vec![2, 4, 8]));
//! ```
//!
//! [`Result`] also implements the [`Product`][impl-Product] and
//! [`Sum`][impl-Sum] traits, allowing an iterator over [`Result`] values
//! to provide the [`product`][Iterator::product] and
//! [`sum`][Iterator::sum] methods.
//!
//! [impl-Product]: Result#impl-Product%3CResult%3CU,+E%3E%3E-for-Result%3CT,+E%3E
//! [impl-Sum]: Result#impl-Sum%3CResult%3CU,+E%3E%3E-for-Result%3CT,+E%3E
//!
//! ```
//! let v = [Err("error!"), Ok(1), Ok(2), Ok(3), Err("foo")];
//! let res: Result<i32, &str> = v.into_iter().sum();
//! assert_eq!(res, Err("error!"));
//! let v = [Ok(1), Ok(2), Ok(21)];
//! let res: Result<i32, &str> = v.into_iter().product();
//! assert_eq!(res, Ok(42));
//! ```
#![stable(feature = "rust1", since = "1.0.0")]
use crate::iter::{self, FromIterator, FusedIterator, TrustedLen};
use crate::ops::{self, ControlFlow, Deref, DerefMut};
use crate::{convert, fmt, hint};
/// `Result` is a type that represents either success ([`Ok`]) or failure ([`Err`]).
///
/// See the [module documentation](self) for details.
#[derive(Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
#[must_use = "this `Result` may be an `Err` variant, which should be handled"]
#[rustc_diagnostic_item = "Result"]
#[stable(feature = "rust1", since = "1.0.0")]
pub enum Result<T, E> {
/// Contains the success value
#[lang = "Ok"]
#[stable(feature = "rust1", since = "1.0.0")]
Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
/// Contains the error value
#[lang = "Err"]
#[stable(feature = "rust1", since = "1.0.0")]
Err(#[stable(feature = "rust1", since = "1.0.0")] E),
}
/////////////////////////////////////////////////////////////////////////////
// Type implementation
/////////////////////////////////////////////////////////////////////////////
impl<T, E> Result<T, E> {
/////////////////////////////////////////////////////////////////////////
// Querying the contained values
/////////////////////////////////////////////////////////////////////////
/// Returns `true` if the result is [`Ok`].
///
/// # Examples
///
/// ```
/// let x: Result<i32, &str> = Ok(-3);
/// assert_eq!(x.is_ok(), true);
///
/// let x: Result<i32, &str> = Err("Some error message");
/// assert_eq!(x.is_ok(), false);
/// ```
#[must_use = "if you intended to assert that this is ok, consider `.unwrap()` instead"]
#[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub const fn is_ok(&self) -> bool {
matches!(*self, Ok(_))
}
/// Returns `true` if the result is [`Ok`] and the value inside of it matches a predicate.
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// assert_eq!(x.is_ok_and(|x| x > 1), true);
///
/// let x: Result<u32, &str> = Ok(0);
/// assert_eq!(x.is_ok_and(|x| x > 1), false);
///
/// let x: Result<u32, &str> = Err("hey");
/// assert_eq!(x.is_ok_and(|x| x > 1), false);
/// ```
#[must_use]
#[inline]
#[stable(feature = "is_some_and", since = "1.70.0")]
pub fn is_ok_and(self, f: impl FnOnce(T) -> bool) -> bool {
match self {
Err(_) => false,
Ok(x) => f(x),
}
}
/// Returns `true` if the result is [`Err`].
///
/// # Examples
///
/// ```
/// let x: Result<i32, &str> = Ok(-3);
/// assert_eq!(x.is_err(), false);
///
/// let x: Result<i32, &str> = Err("Some error message");
/// assert_eq!(x.is_err(), true);
/// ```
#[must_use = "if you intended to assert that this is err, consider `.unwrap_err()` instead"]
#[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub const fn is_err(&self) -> bool {
!self.is_ok()
}
/// Returns `true` if the result is [`Err`] and the value inside of it matches a predicate.
///
/// # Examples
///
/// ```
/// use std::io::{Error, ErrorKind};
///
/// let x: Result<u32, Error> = Err(Error::new(ErrorKind::NotFound, "!"));
/// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), true);
///
/// let x: Result<u32, Error> = Err(Error::new(ErrorKind::PermissionDenied, "!"));
/// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
///
/// let x: Result<u32, Error> = Ok(123);
/// assert_eq!(x.is_err_and(|x| x.kind() == ErrorKind::NotFound), false);
/// ```
#[must_use]
#[inline]
#[stable(feature = "is_some_and", since = "1.70.0")]
pub fn is_err_and(self, f: impl FnOnce(E) -> bool) -> bool {
match self {
Ok(_) => false,
Err(e) => f(e),
}
}
/////////////////////////////////////////////////////////////////////////
// Adapter for each variant
/////////////////////////////////////////////////////////////////////////
/// Converts from `Result<T, E>` to [`Option<T>`].
///
/// Converts `self` into an [`Option<T>`], consuming `self`,
/// and discarding the error, if any.
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// assert_eq!(x.ok(), Some(2));
///
/// let x: Result<u32, &str> = Err("Nothing here");
/// assert_eq!(x.ok(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn ok(self) -> Option<T> {
match self {
Ok(x) => Some(x),
Err(_) => None,
}
}
/// Converts from `Result<T, E>` to [`Option<E>`].
///
/// Converts `self` into an [`Option<E>`], consuming `self`,
/// and discarding the success value, if any.
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// assert_eq!(x.err(), None);
///
/// let x: Result<u32, &str> = Err("Nothing here");
/// assert_eq!(x.err(), Some("Nothing here"));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn err(self) -> Option<E> {
match self {
Ok(_) => None,
Err(x) => Some(x),
}
}
/////////////////////////////////////////////////////////////////////////
// Adapter for working with references
/////////////////////////////////////////////////////////////////////////
/// Converts from `&Result<T, E>` to `Result<&T, &E>`.
///
/// Produces a new `Result`, containing a reference
/// into the original, leaving the original in place.
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// assert_eq!(x.as_ref(), Ok(&2));
///
/// let x: Result<u32, &str> = Err("Error");
/// assert_eq!(x.as_ref(), Err(&"Error"));
/// ```
#[inline]
#[rustc_const_stable(feature = "const_result_basics", since = "1.48.0")]
#[stable(feature = "rust1", since = "1.0.0")]
pub const fn as_ref(&self) -> Result<&T, &E> {
match *self {
Ok(ref x) => Ok(x),
Err(ref x) => Err(x),
}
}
/// Converts from `&mut Result<T, E>` to `Result<&mut T, &mut E>`.
///
/// # Examples
///
/// ```
/// fn mutate(r: &mut Result<i32, i32>) {
/// match r.as_mut() {
/// Ok(v) => *v = 42,
/// Err(e) => *e = 0,
/// }
/// }
///
/// let mut x: Result<i32, i32> = Ok(2);
/// mutate(&mut x);
/// assert_eq!(x.unwrap(), 42);
///
/// let mut x: Result<i32, i32> = Err(13);
/// mutate(&mut x);
/// assert_eq!(x.unwrap_err(), 0);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
#[rustc_const_unstable(feature = "const_result", issue = "82814")]
pub const fn as_mut(&mut self) -> Result<&mut T, &mut E> {
match *self {
Ok(ref mut x) => Ok(x),
Err(ref mut x) => Err(x),
}
}
/////////////////////////////////////////////////////////////////////////
// Transforming contained values
/////////////////////////////////////////////////////////////////////////
/// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to a
/// contained [`Ok`] value, leaving an [`Err`] value untouched.
///
/// This function can be used to compose the results of two functions.
///
/// # Examples
///
/// Print the numbers on each line of a string multiplied by two.
///
/// ```
/// let line = "1\n2\n3\n4\n";
///
/// for num in line.lines() {
/// match num.parse::<i32>().map(|i| i * 2) {
/// Ok(n) => println!("{n}"),
/// Err(..) => {}
/// }
/// }
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U, E> {
match self {
Ok(t) => Ok(op(t)),
Err(e) => Err(e),
}
}
/// Returns the provided default (if [`Err`]), or
/// applies a function to the contained value (if [`Ok`]).
///
/// Arguments passed to `map_or` are eagerly evaluated; if you are passing
/// the result of a function call, it is recommended to use [`map_or_else`],
/// which is lazily evaluated.
///
/// [`map_or_else`]: Result::map_or_else
///
/// # Examples
///
/// ```
/// let x: Result<_, &str> = Ok("foo");
/// assert_eq!(x.map_or(42, |v| v.len()), 3);
///
/// let x: Result<&str, _> = Err("bar");
/// assert_eq!(x.map_or(42, |v| v.len()), 42);
/// ```
#[inline]
#[stable(feature = "result_map_or", since = "1.41.0")]
#[must_use = "if you don't need the returned value, use `if let` instead"]
pub fn map_or<U, F: FnOnce(T) -> U>(self, default: U, f: F) -> U {
match self {
Ok(t) => f(t),
Err(_) => default,
}
}
/// Maps a `Result<T, E>` to `U` by applying fallback function `default` to
/// a contained [`Err`] value, or function `f` to a contained [`Ok`] value.
///
/// This function can be used to unpack a successful result
/// while handling an error.
///
///
/// # Examples
///
/// ```
/// let k = 21;
///
/// let x : Result<_, &str> = Ok("foo");
/// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 3);
///
/// let x : Result<&str, _> = Err("bar");
/// assert_eq!(x.map_or_else(|e| k * 2, |v| v.len()), 42);
/// ```
#[inline]
#[stable(feature = "result_map_or_else", since = "1.41.0")]
pub fn map_or_else<U, D: FnOnce(E) -> U, F: FnOnce(T) -> U>(self, default: D, f: F) -> U {
match self {
Ok(t) => f(t),
Err(e) => default(e),
}
}
/// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to a
/// contained [`Err`] value, leaving an [`Ok`] value untouched.
///
/// This function can be used to pass through a successful result while handling
/// an error.
///
///
/// # Examples
///
/// ```
/// fn stringify(x: u32) -> String { format!("error code: {x}") }
///
/// let x: Result<u32, u32> = Ok(2);
/// assert_eq!(x.map_err(stringify), Ok(2));
///
/// let x: Result<u32, u32> = Err(13);
/// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T, F> {
match self {
Ok(t) => Ok(t),
Err(e) => Err(op(e)),
}
}
/// Calls the provided closure with a reference to the contained value (if [`Ok`]).
///
/// # Examples
///
/// ```
/// let x: u8 = "4"
/// .parse::<u8>()
/// .inspect(|x| println!("original: {x}"))
/// .map(|x| x.pow(3))
/// .expect("failed to parse number");
/// ```
#[inline]
#[stable(feature = "result_option_inspect", since = "1.76.0")]
pub fn inspect<F: FnOnce(&T)>(self, f: F) -> Self {
if let Ok(ref t) = self {
f(t);
}
self
}
/// Calls the provided closure with a reference to the contained error (if [`Err`]).
///
/// # Examples
///
/// ```
/// use std::{fs, io};
///
/// fn read() -> io::Result<String> {
/// fs::read_to_string("address.txt")
/// .inspect_err(|e| eprintln!("failed to read file: {e}"))
/// }
/// ```
#[inline]
#[stable(feature = "result_option_inspect", since = "1.76.0")]
pub fn inspect_err<F: FnOnce(&E)>(self, f: F) -> Self {
if let Err(ref e) = self {
f(e);
}
self
}
/// Converts from `Result<T, E>` (or `&Result<T, E>`) to `Result<&<T as Deref>::Target, &E>`.
///
/// Coerces the [`Ok`] variant of the original [`Result`] via [`Deref`](crate::ops::Deref)
/// and returns the new [`Result`].
///
/// # Examples
///
/// ```
/// let x: Result<String, u32> = Ok("hello".to_string());
/// let y: Result<&str, &u32> = Ok("hello");
/// assert_eq!(x.as_deref(), y);
///
/// let x: Result<String, u32> = Err(42);
/// let y: Result<&str, &u32> = Err(&42);
/// assert_eq!(x.as_deref(), y);
/// ```
#[inline]
#[stable(feature = "inner_deref", since = "1.47.0")]
pub fn as_deref(&self) -> Result<&T::Target, &E>
where
T: Deref,
{
self.as_ref().map(|t| t.deref())
}
/// Converts from `Result<T, E>` (or `&mut Result<T, E>`) to `Result<&mut <T as DerefMut>::Target, &mut E>`.
///
/// Coerces the [`Ok`] variant of the original [`Result`] via [`DerefMut`](crate::ops::DerefMut)
/// and returns the new [`Result`].
///
/// # Examples
///
/// ```
/// let mut s = "HELLO".to_string();
/// let mut x: Result<String, u32> = Ok("hello".to_string());
/// let y: Result<&mut str, &mut u32> = Ok(&mut s);
/// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
///
/// let mut i = 42;
/// let mut x: Result<String, u32> = Err(42);
/// let y: Result<&mut str, &mut u32> = Err(&mut i);
/// assert_eq!(x.as_deref_mut().map(|x| { x.make_ascii_uppercase(); x }), y);
/// ```
#[inline]
#[stable(feature = "inner_deref", since = "1.47.0")]
pub fn as_deref_mut(&mut self) -> Result<&mut T::Target, &mut E>
where
T: DerefMut,
{
self.as_mut().map(|t| t.deref_mut())
}
/////////////////////////////////////////////////////////////////////////
// Iterator constructors
/////////////////////////////////////////////////////////////////////////
/// Returns an iterator over the possibly contained value.
///
/// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(7);
/// assert_eq!(x.iter().next(), Some(&7));
///
/// let x: Result<u32, &str> = Err("nothing!");
/// assert_eq!(x.iter().next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn iter(&self) -> Iter<'_, T> {
Iter { inner: self.as_ref().ok() }
}
/// Returns a mutable iterator over the possibly contained value.
///
/// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
///
/// # Examples
///
/// ```
/// let mut x: Result<u32, &str> = Ok(7);
/// match x.iter_mut().next() {
/// Some(v) => *v = 40,
/// None => {},
/// }
/// assert_eq!(x, Ok(40));
///
/// let mut x: Result<u32, &str> = Err("nothing!");
/// assert_eq!(x.iter_mut().next(), None);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn iter_mut(&mut self) -> IterMut<'_, T> {
IterMut { inner: self.as_mut().ok() }
}
/////////////////////////////////////////////////////////////////////////
// Extract a value
/////////////////////////////////////////////////////////////////////////
/// Returns the contained [`Ok`] value, consuming the `self` value.
///
/// Because this function may panic, its use is generally discouraged.
/// Instead, prefer to use pattern matching and handle the [`Err`]
/// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
/// [`unwrap_or_default`].
///
/// [`unwrap_or`]: Result::unwrap_or
/// [`unwrap_or_else`]: Result::unwrap_or_else
/// [`unwrap_or_default`]: Result::unwrap_or_default
///
/// # Panics
///
/// Panics if the value is an [`Err`], with a panic message including the
/// passed message, and the content of the [`Err`].
///
///
/// # Examples
///
/// ```should_panic
/// let x: Result<u32, &str> = Err("emergency failure");
/// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
/// ```
///
/// # Recommended Message Style
///
/// We recommend that `expect` messages are used to describe the reason you
/// _expect_ the `Result` should be `Ok`.
///
/// ```should_panic
/// let path = std::env::var("IMPORTANT_PATH")
/// .expect("env variable `IMPORTANT_PATH` should be set by `wrapper_script.sh`");
/// ```
///
/// **Hint**: If you're having trouble remembering how to phrase expect
/// error messages remember to focus on the word "should" as in "env
/// variable should be set by blah" or "the given binary should be available
/// and executable by the current user".
///
/// For more detail on expect message styles and the reasoning behind our recommendation please
/// refer to the section on ["Common Message
/// Styles"](../../std/error/index.html#common-message-styles) in the
/// [`std::error`](../../std/error/index.html) module docs.
#[inline]
#[track_caller]
#[stable(feature = "result_expect", since = "1.4.0")]
pub fn expect(self, msg: &str) -> T
where
E: fmt::Debug,
{
match self {
Ok(t) => t,
Err(e) => unwrap_failed(msg, &e),
}
}
/// Returns the contained [`Ok`] value, consuming the `self` value.
///
/// Because this function may panic, its use is generally discouraged.
/// Instead, prefer to use pattern matching and handle the [`Err`]
/// case explicitly, or call [`unwrap_or`], [`unwrap_or_else`], or
/// [`unwrap_or_default`].
///
/// [`unwrap_or`]: Result::unwrap_or
/// [`unwrap_or_else`]: Result::unwrap_or_else
/// [`unwrap_or_default`]: Result::unwrap_or_default
///
/// # Panics
///
/// Panics if the value is an [`Err`], with a panic message provided by the
/// [`Err`]'s value.
///
///
/// # Examples
///
/// Basic usage:
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// assert_eq!(x.unwrap(), 2);
/// ```
///
/// ```should_panic
/// let x: Result<u32, &str> = Err("emergency failure");
/// x.unwrap(); // panics with `emergency failure`
/// ```
#[inline(always)]
#[track_caller]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn unwrap(self) -> T
where
E: fmt::Debug,
{
match self {
Ok(t) => t,
Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", &e),
}
}
/// Returns the contained [`Ok`] value or a default
///
/// Consumes the `self` argument then, if [`Ok`], returns the contained
/// value, otherwise if [`Err`], returns the default value for that
/// type.
///
/// # Examples
///
/// Converts a string to an integer, turning poorly-formed strings
/// into 0 (the default value for integers). [`parse`] converts
/// a string to any other type that implements [`FromStr`], returning an
/// [`Err`] on error.
///
/// ```
/// let good_year_from_input = "1909";
/// let bad_year_from_input = "190blarg";
/// let good_year = good_year_from_input.parse().unwrap_or_default();
/// let bad_year = bad_year_from_input.parse().unwrap_or_default();
///
/// assert_eq!(1909, good_year);
/// assert_eq!(0, bad_year);
/// ```
///
/// [`parse`]: str::parse
/// [`FromStr`]: crate::str::FromStr
#[inline]
#[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
pub fn unwrap_or_default(self) -> T
where
T: Default,
{
match self {
Ok(x) => x,
Err(_) => Default::default(),
}
}
/// Returns the contained [`Err`] value, consuming the `self` value.
///
/// # Panics
///
/// Panics if the value is an [`Ok`], with a panic message including the
/// passed message, and the content of the [`Ok`].
///
///
/// # Examples
///
/// ```should_panic
/// let x: Result<u32, &str> = Ok(10);
/// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
/// ```
#[inline]
#[track_caller]
#[stable(feature = "result_expect_err", since = "1.17.0")]
pub fn expect_err(self, msg: &str) -> E
where
T: fmt::Debug,
{
match self {
Ok(t) => unwrap_failed(msg, &t),
Err(e) => e,
}
}
/// Returns the contained [`Err`] value, consuming the `self` value.
///
/// # Panics
///
/// Panics if the value is an [`Ok`], with a custom panic message provided
/// by the [`Ok`]'s value.
///
/// # Examples
///
/// ```should_panic
/// let x: Result<u32, &str> = Ok(2);
/// x.unwrap_err(); // panics with `2`
/// ```
///
/// ```
/// let x: Result<u32, &str> = Err("emergency failure");
/// assert_eq!(x.unwrap_err(), "emergency failure");
/// ```
#[inline]
#[track_caller]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn unwrap_err(self) -> E
where
T: fmt::Debug,
{
match self {
Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", &t),
Err(e) => e,
}
}
/// Returns the contained [`Ok`] value, but never panics.
///
/// Unlike [`unwrap`], this method is known to never panic on the
/// result types it is implemented for. Therefore, it can be used
/// instead of `unwrap` as a maintainability safeguard that will fail
/// to compile if the error type of the `Result` is later changed
/// to an error that can actually occur.
///
/// [`unwrap`]: Result::unwrap
///
/// # Examples
///
/// ```
/// # #![feature(never_type)]
/// # #![feature(unwrap_infallible)]
///
/// fn only_good_news() -> Result<String, !> {
/// Ok("this is fine".into())
/// }
///
/// let s: String = only_good_news().into_ok();
/// println!("{s}");
/// ```
#[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
#[inline]
pub fn into_ok(self) -> T
where
E: Into<!>,
{
match self {
Ok(x) => x,
Err(e) => e.into(),
}
}
/// Returns the contained [`Err`] value, but never panics.
///
/// Unlike [`unwrap_err`], this method is known to never panic on the
/// result types it is implemented for. Therefore, it can be used
/// instead of `unwrap_err` as a maintainability safeguard that will fail
/// to compile if the ok type of the `Result` is later changed
/// to a type that can actually occur.
///
/// [`unwrap_err`]: Result::unwrap_err
///
/// # Examples
///
/// ```
/// # #![feature(never_type)]
/// # #![feature(unwrap_infallible)]
///
/// fn only_bad_news() -> Result<!, String> {
/// Err("Oops, it failed".into())
/// }
///
/// let error: String = only_bad_news().into_err();
/// println!("{error}");
/// ```
#[unstable(feature = "unwrap_infallible", reason = "newly added", issue = "61695")]
#[inline]
pub fn into_err(self) -> E
where
T: Into<!>,
{
match self {
Ok(x) => x.into(),
Err(e) => e,
}
}
////////////////////////////////////////////////////////////////////////
// Boolean operations on the values, eager and lazy
/////////////////////////////////////////////////////////////////////////
/// Returns `res` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
///
/// Arguments passed to `and` are eagerly evaluated; if you are passing the
/// result of a function call, it is recommended to use [`and_then`], which is
/// lazily evaluated.
///
/// [`and_then`]: Result::and_then
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// let y: Result<&str, &str> = Err("late error");
/// assert_eq!(x.and(y), Err("late error"));
///
/// let x: Result<u32, &str> = Err("early error");
/// let y: Result<&str, &str> = Ok("foo");
/// assert_eq!(x.and(y), Err("early error"));
///
/// let x: Result<u32, &str> = Err("not a 2");
/// let y: Result<&str, &str> = Err("late error");
/// assert_eq!(x.and(y), Err("not a 2"));
///
/// let x: Result<u32, &str> = Ok(2);
/// let y: Result<&str, &str> = Ok("different result type");
/// assert_eq!(x.and(y), Ok("different result type"));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
match self {
Ok(_) => res,
Err(e) => Err(e),
}
}
/// Calls `op` if the result is [`Ok`], otherwise returns the [`Err`] value of `self`.
///
///
/// This function can be used for control flow based on `Result` values.
///
/// # Examples
///
/// ```
/// fn sq_then_to_string(x: u32) -> Result<String, &'static str> {
/// x.checked_mul(x).map(|sq| sq.to_string()).ok_or("overflowed")
/// }
///
/// assert_eq!(Ok(2).and_then(sq_then_to_string), Ok(4.to_string()));
/// assert_eq!(Ok(1_000_000).and_then(sq_then_to_string), Err("overflowed"));
/// assert_eq!(Err("not a number").and_then(sq_then_to_string), Err("not a number"));
/// ```
///
/// Often used to chain fallible operations that may return [`Err`].
///
/// ```
/// use std::{io::ErrorKind, path::Path};
///
/// // Note: on Windows "/" maps to "C:\"
/// let root_modified_time = Path::new("/").metadata().and_then(|md| md.modified());
/// assert!(root_modified_time.is_ok());
///
/// let should_fail = Path::new("/bad/path").metadata().and_then(|md| md.modified());
/// assert!(should_fail.is_err());
/// assert_eq!(should_fail.unwrap_err().kind(), ErrorKind::NotFound);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
match self {
Ok(t) => op(t),
Err(e) => Err(e),
}
}
/// Returns `res` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
///
/// Arguments passed to `or` are eagerly evaluated; if you are passing the
/// result of a function call, it is recommended to use [`or_else`], which is
/// lazily evaluated.
///
/// [`or_else`]: Result::or_else
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// let y: Result<u32, &str> = Err("late error");
/// assert_eq!(x.or(y), Ok(2));
///
/// let x: Result<u32, &str> = Err("early error");
/// let y: Result<u32, &str> = Ok(2);
/// assert_eq!(x.or(y), Ok(2));
///
/// let x: Result<u32, &str> = Err("not a 2");
/// let y: Result<u32, &str> = Err("late error");
/// assert_eq!(x.or(y), Err("late error"));
///
/// let x: Result<u32, &str> = Ok(2);
/// let y: Result<u32, &str> = Ok(100);
/// assert_eq!(x.or(y), Ok(2));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn or<F>(self, res: Result<T, F>) -> Result<T, F> {
match self {
Ok(v) => Ok(v),
Err(_) => res,
}
}
/// Calls `op` if the result is [`Err`], otherwise returns the [`Ok`] value of `self`.
///
/// This function can be used for control flow based on result values.
///
///
/// # Examples
///
/// ```
/// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
/// fn err(x: u32) -> Result<u32, u32> { Err(x) }
///
/// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
/// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
/// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
/// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
match self {
Ok(t) => Ok(t),
Err(e) => op(e),
}
}
/// Returns the contained [`Ok`] value or a provided default.
///
/// Arguments passed to `unwrap_or` are eagerly evaluated; if you are passing
/// the result of a function call, it is recommended to use [`unwrap_or_else`],
/// which is lazily evaluated.
///
/// [`unwrap_or_else`]: Result::unwrap_or_else
///
/// # Examples
///
/// ```
/// let default = 2;
/// let x: Result<u32, &str> = Ok(9);
/// assert_eq!(x.unwrap_or(default), 9);
///
/// let x: Result<u32, &str> = Err("error");
/// assert_eq!(x.unwrap_or(default), default);
/// ```
#[inline]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn unwrap_or(self, default: T) -> T {
match self {
Ok(t) => t,
Err(_) => default,
}
}
/// Returns the contained [`Ok`] value or computes it from a closure.
///
///
/// # Examples
///
/// ```
/// fn count(x: &str) -> usize { x.len() }
///
/// assert_eq!(Ok(2).unwrap_or_else(count), 2);
/// assert_eq!(Err("foo").unwrap_or_else(count), 3);
/// ```
#[inline]
#[track_caller]
#[stable(feature = "rust1", since = "1.0.0")]
pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
match self {
Ok(t) => t,
Err(e) => op(e),
}
}
/// Returns the contained [`Ok`] value, consuming the `self` value,
/// without checking that the value is not an [`Err`].
///
/// # Safety
///
/// Calling this method on an [`Err`] is *[undefined behavior]*.
///
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(2);
/// assert_eq!(unsafe { x.unwrap_unchecked() }, 2);
/// ```
///
/// ```no_run
/// let x: Result<u32, &str> = Err("emergency failure");
/// unsafe { x.unwrap_unchecked(); } // Undefined behavior!
/// ```
#[inline]
#[track_caller]
#[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
pub unsafe fn unwrap_unchecked(self) -> T {
debug_assert!(self.is_ok());
match self {
Ok(t) => t,
// SAFETY: the safety contract must be upheld by the caller.
Err(_) => unsafe { hint::unreachable_unchecked() },
}
}
/// Returns the contained [`Err`] value, consuming the `self` value,
/// without checking that the value is not an [`Ok`].
///
/// # Safety
///
/// Calling this method on an [`Ok`] is *[undefined behavior]*.
///
/// [undefined behavior]: https://doc.rust-lang.org/reference/behavior-considered-undefined.html
///
/// # Examples
///
/// ```no_run
/// let x: Result<u32, &str> = Ok(2);
/// unsafe { x.unwrap_err_unchecked() }; // Undefined behavior!
/// ```
///
/// ```
/// let x: Result<u32, &str> = Err("emergency failure");
/// assert_eq!(unsafe { x.unwrap_err_unchecked() }, "emergency failure");
/// ```
#[inline]
#[track_caller]
#[stable(feature = "option_result_unwrap_unchecked", since = "1.58.0")]
pub unsafe fn unwrap_err_unchecked(self) -> E {
debug_assert!(self.is_err());
match self {
// SAFETY: the safety contract must be upheld by the caller.
Ok(_) => unsafe { hint::unreachable_unchecked() },
Err(e) => e,
}
}
}
impl<T, E> Result<&T, E> {
/// Maps a `Result<&T, E>` to a `Result<T, E>` by copying the contents of the
/// `Ok` part.
///
/// # Examples
///
/// ```
/// let val = 12;
/// let x: Result<&i32, i32> = Ok(&val);
/// assert_eq!(x, Ok(&12));
/// let copied = x.copied();
/// assert_eq!(copied, Ok(12));
/// ```
#[inline]
#[stable(feature = "result_copied", since = "1.59.0")]
pub fn copied(self) -> Result<T, E>
where
T: Copy,
{
self.map(|&t| t)
}
/// Maps a `Result<&T, E>` to a `Result<T, E>` by cloning the contents of the
/// `Ok` part.
///
/// # Examples
///
/// ```
/// let val = 12;
/// let x: Result<&i32, i32> = Ok(&val);
/// assert_eq!(x, Ok(&12));
/// let cloned = x.cloned();
/// assert_eq!(cloned, Ok(12));
/// ```
#[inline]
#[stable(feature = "result_cloned", since = "1.59.0")]
pub fn cloned(self) -> Result<T, E>
where
T: Clone,
{
self.map(|t| t.clone())
}
}
impl<T, E> Result<&mut T, E> {
/// Maps a `Result<&mut T, E>` to a `Result<T, E>` by copying the contents of the
/// `Ok` part.
///
/// # Examples
///
/// ```
/// let mut val = 12;
/// let x: Result<&mut i32, i32> = Ok(&mut val);
/// assert_eq!(x, Ok(&mut 12));
/// let copied = x.copied();
/// assert_eq!(copied, Ok(12));
/// ```
#[inline]
#[stable(feature = "result_copied", since = "1.59.0")]
pub fn copied(self) -> Result<T, E>
where
T: Copy,
{
self.map(|&mut t| t)
}
/// Maps a `Result<&mut T, E>` to a `Result<T, E>` by cloning the contents of the
/// `Ok` part.
///
/// # Examples
///
/// ```
/// let mut val = 12;
/// let x: Result<&mut i32, i32> = Ok(&mut val);
/// assert_eq!(x, Ok(&mut 12));
/// let cloned = x.cloned();
/// assert_eq!(cloned, Ok(12));
/// ```
#[inline]
#[stable(feature = "result_cloned", since = "1.59.0")]
pub fn cloned(self) -> Result<T, E>
where
T: Clone,
{
self.map(|t| t.clone())
}
}
impl<T, E> Result<Option<T>, E> {
/// Transposes a `Result` of an `Option` into an `Option` of a `Result`.
///
/// `Ok(None)` will be mapped to `None`.
/// `Ok(Some(_))` and `Err(_)` will be mapped to `Some(Ok(_))` and `Some(Err(_))`.
///
/// # Examples
///
/// ```
/// #[derive(Debug, Eq, PartialEq)]
/// struct SomeErr;
///
/// let x: Result<Option<i32>, SomeErr> = Ok(Some(5));
/// let y: Option<Result<i32, SomeErr>> = Some(Ok(5));
/// assert_eq!(x.transpose(), y);
/// ```
#[inline]
#[stable(feature = "transpose_result", since = "1.33.0")]
#[rustc_const_unstable(feature = "const_result", issue = "82814")]
pub const fn transpose(self) -> Option<Result<T, E>> {
match self {
Ok(Some(x)) => Some(Ok(x)),
Ok(None) => None,
Err(e) => Some(Err(e)),
}
}
}
impl<T, E> Result<Result<T, E>, E> {
/// Converts from `Result<Result<T, E>, E>` to `Result<T, E>`
///
/// # Examples
///
/// ```
/// #![feature(result_flattening)]
/// let x: Result<Result<&'static str, u32>, u32> = Ok(Ok("hello"));
/// assert_eq!(Ok("hello"), x.flatten());
///
/// let x: Result<Result<&'static str, u32>, u32> = Ok(Err(6));
/// assert_eq!(Err(6), x.flatten());
///
/// let x: Result<Result<&'static str, u32>, u32> = Err(6);
/// assert_eq!(Err(6), x.flatten());
/// ```
///
/// Flattening only removes one level of nesting at a time:
///
/// ```
/// #![feature(result_flattening)]
/// let x: Result<Result<Result<&'static str, u32>, u32>, u32> = Ok(Ok(Ok("hello")));
/// assert_eq!(Ok(Ok("hello")), x.flatten());
/// assert_eq!(Ok("hello"), x.flatten().flatten());
/// ```
#[inline]
#[unstable(feature = "result_flattening", issue = "70142")]
pub fn flatten(self) -> Result<T, E> {
self.and_then(convert::identity)
}
}
// This is a separate function to reduce the code size of the methods
#[cfg(not(feature = "panic_immediate_abort"))]
#[inline(never)]
#[cold]
#[track_caller]
fn unwrap_failed(msg: &str, error: &dyn fmt::Debug) -> ! {
panic!("{msg}: {error:?}")
}
// This is a separate function to avoid constructing a `dyn Debug`
// that gets immediately thrown away, since vtables don't get cleaned up
// by dead code elimination if a trait object is constructed even if it goes
// unused
#[cfg(feature = "panic_immediate_abort")]
#[inline]
#[cold]
#[track_caller]
fn unwrap_failed<T>(_msg: &str, _error: &T) -> ! {
panic!()
}
/////////////////////////////////////////////////////////////////////////////
// Trait implementations
/////////////////////////////////////////////////////////////////////////////
#[stable(feature = "rust1", since = "1.0.0")]
impl<T, E> Clone for Result<T, E>
where
T: Clone,
E: Clone,
{
#[inline]
fn clone(&self) -> Self {
match self {
Ok(x) => Ok(x.clone()),
Err(x) => Err(x.clone()),
}
}
#[inline]
fn clone_from(&mut self, source: &Self) {
match (self, source) {
(Ok(to), Ok(from)) => to.clone_from(from),
(Err(to), Err(from)) => to.clone_from(from),
(to, from) => *to = from.clone(),
}
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T, E> IntoIterator for Result<T, E> {
type Item = T;
type IntoIter = IntoIter<T>;
/// Returns a consuming iterator over the possibly contained value.
///
/// The iterator yields one value if the result is [`Result::Ok`], otherwise none.
///
/// # Examples
///
/// ```
/// let x: Result<u32, &str> = Ok(5);
/// let v: Vec<u32> = x.into_iter().collect();
/// assert_eq!(v, [5]);
///
/// let x: Result<u32, &str> = Err("nothing!");
/// let v: Vec<u32> = x.into_iter().collect();
/// assert_eq!(v, []);
/// ```
#[inline]
fn into_iter(self) -> IntoIter<T> {
IntoIter { inner: self.ok() }
}
}
#[stable(since = "1.4.0", feature = "result_iter")]
impl<'a, T, E> IntoIterator for &'a Result<T, E> {
type Item = &'a T;
type IntoIter = Iter<'a, T>;
fn into_iter(self) -> Iter<'a, T> {
self.iter()
}
}
#[stable(since = "1.4.0", feature = "result_iter")]
impl<'a, T, E> IntoIterator for &'a mut Result<T, E> {
type Item = &'a mut T;
type IntoIter = IterMut<'a, T>;
fn into_iter(self) -> IterMut<'a, T> {
self.iter_mut()
}
}
/////////////////////////////////////////////////////////////////////////////
// The Result Iterators
/////////////////////////////////////////////////////////////////////////////
/// An iterator over a reference to the [`Ok`] variant of a [`Result`].
///
/// The iterator yields one value if the result is [`Ok`], otherwise none.
///
/// Created by [`Result::iter`].
#[derive(Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct Iter<'a, T: 'a> {
inner: Option<&'a T>,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for Iter<'a, T> {
type Item = &'a T;
#[inline]
fn next(&mut self) -> Option<&'a T> {
self.inner.take()
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let n = if self.inner.is_some() { 1 } else { 0 };
(n, Some(n))
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
#[inline]
fn next_back(&mut self) -> Option<&'a T> {
self.inner.take()
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for Iter<'_, T> {}
#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for Iter<'_, T> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A> TrustedLen for Iter<'_, A> {}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Clone for Iter<'_, T> {
#[inline]
fn clone(&self) -> Self {
Iter { inner: self.inner }
}
}
/// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
///
/// Created by [`Result::iter_mut`].
#[derive(Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IterMut<'a, T: 'a> {
inner: Option<&'a mut T>,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> Iterator for IterMut<'a, T> {
type Item = &'a mut T;
#[inline]
fn next(&mut self) -> Option<&'a mut T> {
self.inner.take()
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let n = if self.inner.is_some() { 1 } else { 0 };
(n, Some(n))
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
#[inline]
fn next_back(&mut self) -> Option<&'a mut T> {
self.inner.take()
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for IterMut<'_, T> {}
#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for IterMut<'_, T> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A> TrustedLen for IterMut<'_, A> {}
/// An iterator over the value in a [`Ok`] variant of a [`Result`].
///
/// The iterator yields one value if the result is [`Ok`], otherwise none.
///
/// This struct is created by the [`into_iter`] method on
/// [`Result`] (provided by the [`IntoIterator`] trait).
///
/// [`into_iter`]: IntoIterator::into_iter
#[derive(Clone, Debug)]
#[stable(feature = "rust1", since = "1.0.0")]
pub struct IntoIter<T> {
inner: Option<T>,
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> Iterator for IntoIter<T> {
type Item = T;
#[inline]
fn next(&mut self) -> Option<T> {
self.inner.take()
}
#[inline]
fn size_hint(&self) -> (usize, Option<usize>) {
let n = if self.inner.is_some() { 1 } else { 0 };
(n, Some(n))
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> DoubleEndedIterator for IntoIter<T> {
#[inline]
fn next_back(&mut self) -> Option<T> {
self.inner.take()
}
}
#[stable(feature = "rust1", since = "1.0.0")]
impl<T> ExactSizeIterator for IntoIter<T> {}
#[stable(feature = "fused", since = "1.26.0")]
impl<T> FusedIterator for IntoIter<T> {}
#[unstable(feature = "trusted_len", issue = "37572")]
unsafe impl<A> TrustedLen for IntoIter<A> {}
/////////////////////////////////////////////////////////////////////////////
// FromIterator
/////////////////////////////////////////////////////////////////////////////
#[stable(feature = "rust1", since = "1.0.0")]
impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
/// Takes each element in the `Iterator`: if it is an `Err`, no further
/// elements are taken, and the `Err` is returned. Should no `Err` occur, a
/// container with the values of each `Result` is returned.
///
/// Here is an example which increments every integer in a vector,
/// checking for overflow:
///
/// ```
/// let v = vec![1, 2];
/// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
/// x.checked_add(1).ok_or("Overflow!")
/// ).collect();
/// assert_eq!(res, Ok(vec![2, 3]));
/// ```
///
/// Here is another example that tries to subtract one from another list
/// of integers, this time checking for underflow:
///
/// ```
/// let v = vec![1, 2, 0];
/// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32|
/// x.checked_sub(1).ok_or("Underflow!")
/// ).collect();
/// assert_eq!(res, Err("Underflow!"));
/// ```
///
/// Here is a variation on the previous example, showing that no
/// further elements are taken from `iter` after the first `Err`.
///
/// ```
/// let v = vec![3, 2, 1, 10];
/// let mut shared = 0;
/// let res: Result<Vec<u32>, &'static str> = v.iter().map(|x: &u32| {
/// shared += x;
/// x.checked_sub(2).ok_or("Underflow!")
/// }).collect();
/// assert_eq!(res, Err("Underflow!"));
/// assert_eq!(shared, 6);
/// ```
///
/// Since the third element caused an underflow, no further elements were taken,
/// so the final value of `shared` is 6 (= `3 + 2 + 1`), not 16.
#[inline]
fn from_iter<I: IntoIterator<Item = Result<A, E>>>(iter: I) -> Result<V, E> {
iter::try_process(iter.into_iter(), |i| i.collect())
}
}
#[unstable(feature = "try_trait_v2", issue = "84277")]
impl<T, E> ops::Try for Result<T, E> {
type Output = T;
type Residual = Result<convert::Infallible, E>;
#[inline]
fn from_output(output: Self::Output) -> Self {
Ok(output)
}
#[inline]
fn branch(self) -> ControlFlow<Self::Residual, Self::Output> {
match self {
Ok(v) => ControlFlow::Continue(v),
Err(e) => ControlFlow::Break(Err(e)),
}
}
}
#[unstable(feature = "try_trait_v2", issue = "84277")]
impl<T, E, F: From<E>> ops::FromResidual<Result<convert::Infallible, E>> for Result<T, F> {
#[inline]
#[track_caller]
fn from_residual(residual: Result<convert::Infallible, E>) -> Self {
match residual {
Err(e) => Err(From::from(e)),
}
}
}
#[unstable(feature = "try_trait_v2_yeet", issue = "96374")]
impl<T, E, F: From<E>> ops::FromResidual<ops::Yeet<E>> for Result<T, F> {
#[inline]
fn from_residual(ops::Yeet(e): ops::Yeet<E>) -> Self {
Err(From::from(e))
}
}
#[unstable(feature = "try_trait_v2_residual", issue = "91285")]
impl<T, E> ops::Residual<T> for Result<convert::Infallible, E> {
type TryType = Result<T, E>;
}