/src/libcore/result.rs
Rust | 1110 lines | 322 code | 72 blank | 716 comment | 27 complexity | ade915818cc02cdddccbf9c97784863d MD5 | raw file
Possible License(s): AGPL-1.0, BSD-2-Clause, 0BSD, Apache-2.0, MIT, LGPL-2.0
- // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
- // file at the top-level directory of this distribution and at
- // http://rust-lang.org/COPYRIGHT.
- //
- // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
- // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
- // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
- // option. This file may not be copied, modified, or distributed
- // except according to those terms.
- //! 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 [`Result`]`<T, `[`io::Error`]`>`.*
- //!
- //! 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 `?` syntax
- //!
- //! When writing code that calls many functions that return the
- //! [`Result`] type, the error handling can be tedious. The [`?`]
- //! syntax 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 unwrapped
- //! success ([`Ok`]) value, unless the result is [`Err`], in which case
- //! [`Err`] is returned early from the enclosing function.
- //!
- //! [`?`] can only be used in functions that return [`Result`] because of the
- //! early return of [`Err`] that it provides.
- //!
- //! [`expect`]: enum.Result.html#method.expect
- //! [`Write`]: ../../std/io/trait.Write.html
- //! [`write_all`]: ../../std/io/trait.Write.html#method.write_all
- //! [`io::Result`]: ../../std/io/type.Result.html
- //! [`?`]: ../../std/macro.try.html
- //! [`Result`]: enum.Result.html
- //! [`Ok(T)`]: enum.Result.html#variant.Ok
- //! [`Err(E)`]: enum.Result.html#variant.Err
- //! [`io::Error`]: ../../std/io/struct.Error.html
- //! [`Ok`]: enum.Result.html#variant.Ok
- //! [`Err`]: enum.Result.html#variant.Err
- #![stable(feature = "rust1", since = "1.0.0")]
- use fmt;
- use iter::{FromIterator, FusedIterator, TrustedLen};
- /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
- ///
- /// See the [`std::result`](index.html) module documentation for details.
- #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Debug, Hash)]
- #[must_use]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub enum Result<T, E> {
- /// Contains the success value
- #[stable(feature = "rust1", since = "1.0.0")]
- Ok(#[stable(feature = "rust1", since = "1.0.0")] T),
- /// Contains the error value
- #[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
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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);
- /// ```
- #[inline]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn is_ok(&self) -> bool {
- match *self {
- Ok(_) => true,
- Err(_) => false
- }
- }
- /// Returns `true` if the result is `Err`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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);
- /// ```
- #[inline]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn is_err(&self) -> bool {
- !self.is_ok()
- }
- /////////////////////////////////////////////////////////////////////////
- // 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.
- ///
- /// [`Option<T>`]: ../../std/option/enum.Option.html
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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.
- ///
- /// [`Option<E>`]: ../../std/option/enum.Option.html
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn as_ref(&self) -> Result<&T, &E> {
- match *self {
- Ok(ref x) => Ok(x),
- Err(ref x) => Err(x),
- }
- }
- /// Converts from `Result<T, E>` to `Result<&mut T, &mut E>`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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")]
- pub 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)
- }
- }
- /// 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
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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))
- }
- }
- /////////////////////////////////////////////////////////////////////////
- // Iterator constructors
- /////////////////////////////////////////////////////////////////////////
- /// Returns an iterator over the possibly contained value.
- ///
- /// The iterator yields one value if the result is [`Ok`], otherwise none.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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);
- /// ```
- ///
- /// [`Ok`]: enum.Result.html#variant.Ok
- #[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 [`Ok`], otherwise none.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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);
- /// ```
- ///
- /// [`Ok`]: enum.Result.html#variant.Ok
- #[inline]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn iter_mut(&mut self) -> IterMut<T> {
- IterMut { inner: self.as_mut().ok() }
- }
- ////////////////////////////////////////////////////////////////////////
- // Boolean operations on the values, eager and lazy
- /////////////////////////////////////////////////////////////////////////
- /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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
- ///
- /// Basic usage:
- ///
- /// ```
- /// fn sq(x: u32) -> Result<u32, u32> { Ok(x * x) }
- /// fn err(x: u32) -> Result<u32, u32> { Err(x) }
- ///
- /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
- /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
- /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
- /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
- /// ```
- #[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`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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),
- }
- }
- /// Unwraps a result, yielding the content of an `Ok`.
- /// Else, it returns `optb`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// let optb = 2;
- /// let x: Result<u32, &str> = Ok(9);
- /// assert_eq!(x.unwrap_or(optb), 9);
- ///
- /// let x: Result<u32, &str> = Err("error");
- /// assert_eq!(x.unwrap_or(optb), optb);
- /// ```
- #[inline]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn unwrap_or(self, optb: T) -> T {
- match self {
- Ok(t) => t,
- Err(_) => optb
- }
- }
- /// Unwraps a result, yielding the content of an `Ok`.
- /// If the value is an `Err` then it calls `op` with its value.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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]
- #[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)
- }
- }
- }
- impl<T, E: fmt::Debug> Result<T, E> {
- /// Unwraps a result, yielding the content of an `Ok`.
- ///
- /// # 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]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn unwrap(self) -> T {
- match self {
- Ok(t) => t,
- Err(e) => unwrap_failed("called `Result::unwrap()` on an `Err` value", e),
- }
- }
- /// Unwraps a result, yielding the content of an `Ok`.
- ///
- /// # Panics
- ///
- /// Panics if the value is an `Err`, with a panic message including the
- /// passed message, and the content of the `Err`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```{.should_panic}
- /// let x: Result<u32, &str> = Err("emergency failure");
- /// x.expect("Testing expect"); // panics with `Testing expect: emergency failure`
- /// ```
- #[inline]
- #[stable(feature = "result_expect", since = "1.4.0")]
- pub fn expect(self, msg: &str) -> T {
- match self {
- Ok(t) => t,
- Err(e) => unwrap_failed(msg, e),
- }
- }
- }
- impl<T: fmt::Debug, E> Result<T, E> {
- /// Unwraps a result, yielding the content of an `Err`.
- ///
- /// # 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]
- #[stable(feature = "rust1", since = "1.0.0")]
- pub fn unwrap_err(self) -> E {
- match self {
- Ok(t) => unwrap_failed("called `Result::unwrap_err()` on an `Ok` value", t),
- Err(e) => e,
- }
- }
- /// Unwraps a result, yielding the content of an `Err`.
- ///
- /// # Panics
- ///
- /// Panics if the value is an `Ok`, with a panic message including the
- /// passed message, and the content of the `Ok`.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```{.should_panic}
- /// let x: Result<u32, &str> = Ok(10);
- /// x.expect_err("Testing expect_err"); // panics with `Testing expect_err: 10`
- /// ```
- #[inline]
- #[stable(feature = "result_expect_err", since = "1.17.0")]
- pub fn expect_err(self, msg: &str) -> E {
- match self {
- Ok(t) => unwrap_failed(msg, t),
- Err(e) => e,
- }
- }
- }
- impl<T: Default, E> Result<T, E> {
- /// Returns the contained 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
- ///
- /// Convert 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`]: ../../std/primitive.str.html#method.parse
- /// [`FromStr`]: ../../std/str/trait.FromStr.html
- #[inline]
- #[stable(feature = "result_unwrap_or_default", since = "1.16.0")]
- pub fn unwrap_or_default(self) -> T {
- match self {
- Ok(x) => x,
- Err(_) => Default::default(),
- }
- }
- }
- // This is a separate function to reduce the code size of the methods
- #[inline(never)]
- #[cold]
- fn unwrap_failed<E: fmt::Debug>(msg: &str, error: E) -> ! {
- panic!("{}: {:?}", msg, error)
- }
- /////////////////////////////////////////////////////////////////////////////
- // Trait implementations
- /////////////////////////////////////////////////////////////////////////////
- #[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 [`Ok`], otherwise none.
- ///
- /// # Examples
- ///
- /// Basic usage:
- ///
- /// ```
- /// 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, []);
- /// ```
- ///
- /// [`Ok`]: enum.Result.html#variant.Ok
- #[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(mut 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`].
- ///
- /// [`Ok`]: enum.Result.html#variant.Ok
- /// [`Result`]: enum.Result.html
- /// [`Result::iter`]: enum.Result.html#method.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<'a, T> ExactSizeIterator for Iter<'a, T> {}
- #[unstable(feature = "fused", issue = "35602")]
- impl<'a, T> FusedIterator for Iter<'a, T> {}
- #[unstable(feature = "trusted_len", issue = "37572")]
- unsafe impl<'a, A> TrustedLen for Iter<'a, A> {}
- #[stable(feature = "rust1", since = "1.0.0")]
- impl<'a, T> Clone for Iter<'a, T> {
- fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
- }
- /// An iterator over a mutable reference to the [`Ok`] variant of a [`Result`].
- ///
- /// Created by [`Result::iter_mut`].
- ///
- /// [`Ok`]: enum.Result.html#variant.Ok
- /// [`Result`]: enum.Result.html
- /// [`Result::iter_mut`]: enum.Result.html#method.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<'a, T> ExactSizeIterator for IterMut<'a, T> {}
- #[unstable(feature = "fused", issue = "35602")]
- impl<'a, T> FusedIterator for IterMut<'a, T> {}
- #[unstable(feature = "trusted_len", issue = "37572")]
- unsafe impl<'a, A> TrustedLen for IterMut<'a, 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`][`Result`] (provided by the [`IntoIterator`] trait).
- ///
- /// [`Ok`]: enum.Result.html#variant.Ok
- /// [`Result`]: enum.Result.html
- /// [`into_iter`]: ../iter/trait.IntoIterator.html#tymethod.into_iter
- /// [`IntoIterator`]: ../iter/trait.IntoIterator.html
- #[derive(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> {}
- #[unstable(feature = "fused", issue = "35602")]
- 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:
- ///
- /// ```
- /// use std::u32;
- ///
- /// let v = vec![1, 2];
- /// let res: Result<Vec<u32>, &'static str> = v.iter().map(|&x: &u32|
- /// if x == u32::MAX { Err("Overflow!") }
- /// else { Ok(x + 1) }
- /// ).collect();
- /// assert!(res == Ok(vec![2, 3]));
- /// ```
- #[inline]
- fn from_iter<I: IntoIterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
- // FIXME(#11084): This could be replaced with Iterator::scan when this
- // performance bug is closed.
- struct Adapter<Iter, E> {
- iter: Iter,
- err: Option<E>,
- }
- impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
- type Item = T;
- #[inline]
- fn next(&mut self) -> Option<T> {
- match self.iter.next() {
- Some(Ok(value)) => Some(value),
- Some(Err(err)) => {
- self.err = Some(err);
- None
- }
- None => None,
- }
- }
- fn size_hint(&self) -> (usize, Option<usize>) {
- let (_min, max) = self.iter.size_hint();
- (0, max)
- }
- }
- let mut adapter = Adapter { iter: iter.into_iter(), err: None };
- let v: V = FromIterator::from_iter(adapter.by_ref());
- match adapter.err {
- Some(err) => Err(err),
- None => Ok(v),
- }
- }
- }