1 // Copyright 2012-2014 The Rust Project Developers. See the COPYRIGHT
2 // file at the top-level directory of this distribution and at
3 // http://rust-lang.org/COPYRIGHT.
5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
8 // option. This file may not be copied, modified, or distributed
9 // except according to those terms.
11 //! Error handling with the `Result` type
13 //! `Result<T, E>` is the type used for returning and propagating
14 //! errors. It is an enum with the variants, `Ok(T)`, representing
15 //! success and containing a value, and `Err(E)`, representing error
16 //! and containing an error value.
19 //! enum Result<T, E> {
25 //! Functions return `Result` whenever errors are expected and
26 //! recoverable. In the `std` crate `Result` is most prominently used
27 //! for [I/O](../../std/io/index.html).
29 //! A simple function returning `Result` might be
30 //! defined and used like so:
34 //! enum Version { Version1, Version2 }
36 //! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
37 //! if header.len() < 1 {
38 //! return Err("invalid header length");
41 //! 1 => Ok(Version::Version1),
42 //! 2 => Ok(Version::Version2),
43 //! _ => Err("invalid version")
47 //! let version = parse_version(&[1, 2, 3, 4]);
50 //! println!("working with version: {:?}", v);
53 //! println!("error parsing header: {:?}", e);
58 //! Pattern matching on `Result`s is clear and straightforward for
59 //! simple cases, but `Result` comes with some convenience methods
60 //! that make working with it more succinct.
63 //! let good_result: Result<int, int> = Ok(10);
64 //! let bad_result: Result<int, int> = Err(10);
66 //! // The `is_ok` and `is_err` methods do what they say.
67 //! assert!(good_result.is_ok() && !good_result.is_err());
68 //! assert!(bad_result.is_err() && !bad_result.is_ok());
70 //! // `map` consumes the `Result` and produces another.
71 //! let good_result: Result<int, int> = good_result.map(|i| i + 1);
72 //! let bad_result: Result<int, int> = bad_result.map(|i| i - 1);
74 //! // Use `and_then` to continue the computation.
75 //! let good_result: Result<bool, int> = good_result.and_then(|i| Ok(i == 11));
77 //! // Use `or_else` to handle the error.
78 //! let bad_result: Result<int, int> = bad_result.or_else(|i| Ok(11));
80 //! // Consume the result and return the contents with `unwrap`.
81 //! let final_awesome_result = good_result.ok().unwrap();
84 //! # Results must be used
86 //! A common problem with using return values to indicate errors is
87 //! that it is easy to ignore the return value, thus failing to handle
88 //! the error. Result is annotated with the #[must_use] attribute,
89 //! which will cause the compiler to issue a warning when a Result
90 //! value is ignored. This makes `Result` especially useful with
91 //! functions that may encounter errors but don't otherwise return a
94 //! Consider the `write_line` method defined for I/O types
95 //! by the [`Writer`](../io/trait.Writer.html) trait:
98 //! use std::io::IoError;
101 //! fn write_line(&mut self, s: &str) -> Result<(), IoError>;
105 //! *Note: The actual definition of `Writer` uses `IoResult`, which
106 //! is just a synonym for `Result<T, IoError>`.*
108 //! This method doesn't produce a value, but the write may
109 //! fail. It's crucial to handle the error case, and *not* write
110 //! something like this:
113 //! use std::io::{File, Open, Write};
115 //! let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
116 //! // If `write_line` errors, then we'll never know, because the return
117 //! // value is ignored.
118 //! file.write_line("important message");
122 //! If you *do* write that in Rust, the compiler will give you a
123 //! warning (by default, controlled by the `unused_must_use` lint).
125 //! You might instead, if you don't want to handle the error, simply
126 //! panic, by converting to an `Option` with `ok`, then asserting
127 //! success with `expect`. This will panic if the write fails, proving
128 //! a marginally useful message indicating why:
131 //! use std::io::{File, Open, Write};
133 //! let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
134 //! file.write_line("important message").ok().expect("failed to write message");
138 //! You might also simply assert success:
141 //! # use std::io::{File, Open, Write};
143 //! # let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
144 //! assert!(file.write_line("important message").is_ok());
148 //! Or propagate the error up the call stack with `try!`:
151 //! # use std::io::{File, Open, Write, IoError};
152 //! fn write_message() -> Result<(), IoError> {
153 //! let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
154 //! try!(file.write_line("important message"));
160 //! # The `try!` macro
162 //! When writing code that calls many functions that return the
163 //! `Result` type, the error handling can be tedious. The `try!`
164 //! macro hides some of the boilerplate of propagating errors up the
167 //! It replaces this:
170 //! use std::io::{File, Open, Write, IoError};
178 //! fn write_info(info: &Info) -> Result<(), IoError> {
179 //! let mut file = File::open_mode(&Path::new("my_best_friends.txt"), Open, Write);
180 //! // Early return on error
181 //! if let Err(e) = file.write_line(format!("name: {}", info.name).as_slice()) {
184 //! if let Err(e) = file.write_line(format!("age: {}", info.age).as_slice()) {
187 //! return file.write_line(format!("rating: {}", info.rating).as_slice());
194 //! use std::io::{File, Open, Write, IoError};
202 //! fn write_info(info: &Info) -> Result<(), IoError> {
203 //! let mut file = File::open_mode(&Path::new("my_best_friends.txt"), Open, Write);
204 //! // Early return on error
205 //! try!(file.write_line(format!("name: {}", info.name).as_slice()));
206 //! try!(file.write_line(format!("age: {}", info.age).as_slice()));
207 //! try!(file.write_line(format!("rating: {}", info.rating).as_slice()));
212 //! *It's much nicer!*
214 //! Wrapping an expression in `try!` will result in the unwrapped
215 //! success (`Ok`) value, unless the result is `Err`, in which case
216 //! `Err` is returned early from the enclosing function. Its simple definition
220 //! macro_rules! try {
221 //! ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
225 //! `try!` is imported by the prelude, and is available everywhere.
229 use self::Result::{Ok, Err};
233 use iter::{Iterator, IteratorExt, DoubleEndedIterator, FromIterator, ExactSizeIterator};
234 use ops::{FnMut, FnOnce};
235 use option::Option::{self, None, Some};
239 /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
241 /// See the [`std::result`](index.html) module documentation for details.
242 #[derive(Clone, Copy, PartialEq, PartialOrd, Eq, Ord, Show, Hash)]
245 pub enum Result<T, E> {
246 /// Contains the success value
250 /// Contains the error value
255 /////////////////////////////////////////////////////////////////////////////
256 // Type implementation
257 /////////////////////////////////////////////////////////////////////////////
260 impl<T, E> Result<T, E> {
261 /////////////////////////////////////////////////////////////////////////
262 // Querying the contained values
263 /////////////////////////////////////////////////////////////////////////
265 /// Returns true if the result is `Ok`
270 /// let x: Result<int, &str> = Ok(-3);
271 /// assert_eq!(x.is_ok(), true);
273 /// let x: Result<int, &str> = Err("Some error message");
274 /// assert_eq!(x.is_ok(), false);
278 pub fn is_ok(&self) -> bool {
285 /// Returns true if the result is `Err`
290 /// let x: Result<int, &str> = Ok(-3);
291 /// assert_eq!(x.is_err(), false);
293 /// let x: Result<int, &str> = Err("Some error message");
294 /// assert_eq!(x.is_err(), true);
298 pub fn is_err(&self) -> bool {
302 /////////////////////////////////////////////////////////////////////////
303 // Adapter for each variant
304 /////////////////////////////////////////////////////////////////////////
306 /// Convert from `Result<T, E>` to `Option<T>`
308 /// Converts `self` into an `Option<T>`, consuming `self`,
309 /// and discarding the error, if any.
314 /// let x: Result<uint, &str> = Ok(2);
315 /// assert_eq!(x.ok(), Some(2));
317 /// let x: Result<uint, &str> = Err("Nothing here");
318 /// assert_eq!(x.ok(), None);
322 pub fn ok(self) -> Option<T> {
329 /// Convert from `Result<T, E>` to `Option<E>`
331 /// Converts `self` into an `Option<E>`, consuming `self`,
332 /// and discarding the value, if any.
337 /// let x: Result<uint, &str> = Ok(2);
338 /// assert_eq!(x.err(), None);
340 /// let x: Result<uint, &str> = Err("Nothing here");
341 /// assert_eq!(x.err(), Some("Nothing here"));
345 pub fn err(self) -> Option<E> {
352 /////////////////////////////////////////////////////////////////////////
353 // Adapter for working with references
354 /////////////////////////////////////////////////////////////////////////
356 /// Convert from `Result<T, E>` to `Result<&T, &E>`
358 /// Produces a new `Result`, containing a reference
359 /// into the original, leaving the original in place.
362 /// let x: Result<uint, &str> = Ok(2);
363 /// assert_eq!(x.as_ref(), Ok(&2));
365 /// let x: Result<uint, &str> = Err("Error");
366 /// assert_eq!(x.as_ref(), Err(&"Error"));
370 pub fn as_ref(&self) -> Result<&T, &E> {
373 Err(ref x) => Err(x),
377 /// Convert from `Result<T, E>` to `Result<&mut T, &mut E>`
380 /// fn mutate(r: &mut Result<int, int>) {
381 /// match r.as_mut() {
382 /// Ok(&mut ref mut v) => *v = 42,
383 /// Err(&mut ref mut e) => *e = 0,
387 /// let mut x: Result<int, int> = Ok(2);
389 /// assert_eq!(x.unwrap(), 42);
391 /// let mut x: Result<int, int> = Err(13);
393 /// assert_eq!(x.unwrap_err(), 0);
397 pub fn as_mut(&mut self) -> Result<&mut T, &mut E> {
399 Ok(ref mut x) => Ok(x),
400 Err(ref mut x) => Err(x),
404 /// Convert from `Result<T, E>` to `&mut [T]` (without copying)
407 /// let mut x: Result<&str, uint> = Ok("Gold");
409 /// let v = x.as_mut_slice();
410 /// assert!(v == ["Gold"]);
412 /// assert!(v == ["Silver"]);
414 /// assert_eq!(x, Ok("Silver"));
416 /// let mut x: Result<&str, uint> = Err(45);
417 /// assert!(x.as_mut_slice().is_empty());
420 #[unstable = "waiting for mut conventions"]
421 pub fn as_mut_slice(&mut self) -> &mut [T] {
423 Ok(ref mut x) => slice::mut_ref_slice(x),
425 // work around lack of implicit coercion from fixed-size array to slice
426 let emp: &mut [_] = &mut [];
432 /////////////////////////////////////////////////////////////////////////
433 // Transforming contained values
434 /////////////////////////////////////////////////////////////////////////
436 /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to an
437 /// contained `Ok` value, leaving an `Err` value untouched.
439 /// This function can be used to compose the results of two functions.
443 /// Sum the lines of a buffer by mapping strings to numbers,
444 /// ignoring I/O and parse errors:
447 /// use std::io::IoResult;
449 /// let mut buffer = &mut b"1\n2\n3\n4\n";
453 /// while !buffer.is_empty() {
454 /// let line: IoResult<String> = buffer.read_line();
455 /// // Convert the string line to a number using `map` and `from_str`
456 /// let val: IoResult<int> = line.map(|line| {
457 /// line.as_slice().trim_right().parse::<int>().unwrap_or(0)
459 /// // Add the value if there were no errors, otherwise add 0
460 /// sum += val.ok().unwrap_or(0);
463 /// assert!(sum == 10);
467 pub fn map<U, F: FnOnce(T) -> U>(self, op: F) -> Result<U,E> {
474 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to an
475 /// contained `Err` value, leaving an `Ok` value untouched.
477 /// This function can be used to pass through a successful result while handling
483 /// fn stringify(x: uint) -> String { format!("error code: {}", x) }
485 /// let x: Result<uint, uint> = Ok(2u);
486 /// assert_eq!(x.map_err(stringify), Ok(2u));
488 /// let x: Result<uint, uint> = Err(13);
489 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
493 pub fn map_err<F, O: FnOnce(E) -> F>(self, op: O) -> Result<T,F> {
500 /////////////////////////////////////////////////////////////////////////
501 // Iterator constructors
502 /////////////////////////////////////////////////////////////////////////
504 /// Returns an iterator over the possibly contained value.
509 /// let x: Result<uint, &str> = Ok(7);
510 /// assert_eq!(x.iter().next(), Some(&7));
512 /// let x: Result<uint, &str> = Err("nothing!");
513 /// assert_eq!(x.iter().next(), None);
517 pub fn iter(&self) -> Iter<T> {
518 Iter { inner: self.as_ref().ok() }
521 /// Returns a mutable iterator over the possibly contained value.
526 /// let mut x: Result<uint, &str> = Ok(7);
527 /// match x.iter_mut().next() {
528 /// Some(&mut ref mut x) => *x = 40,
531 /// assert_eq!(x, Ok(40));
533 /// let mut x: Result<uint, &str> = Err("nothing!");
534 /// assert_eq!(x.iter_mut().next(), None);
538 pub fn iter_mut(&mut self) -> IterMut<T> {
539 IterMut { inner: self.as_mut().ok() }
542 /// Returns a consuming iterator over the possibly contained value.
547 /// let x: Result<uint, &str> = Ok(5);
548 /// let v: Vec<uint> = x.into_iter().collect();
549 /// assert_eq!(v, vec![5u]);
551 /// let x: Result<uint, &str> = Err("nothing!");
552 /// let v: Vec<uint> = x.into_iter().collect();
553 /// assert_eq!(v, vec![]);
557 pub fn into_iter(self) -> IntoIter<T> {
558 IntoIter { inner: self.ok() }
561 ////////////////////////////////////////////////////////////////////////
562 // Boolean operations on the values, eager and lazy
563 /////////////////////////////////////////////////////////////////////////
565 /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
570 /// let x: Result<uint, &str> = Ok(2);
571 /// let y: Result<&str, &str> = Err("late error");
572 /// assert_eq!(x.and(y), Err("late error"));
574 /// let x: Result<uint, &str> = Err("early error");
575 /// let y: Result<&str, &str> = Ok("foo");
576 /// assert_eq!(x.and(y), Err("early error"));
578 /// let x: Result<uint, &str> = Err("not a 2");
579 /// let y: Result<&str, &str> = Err("late error");
580 /// assert_eq!(x.and(y), Err("not a 2"));
582 /// let x: Result<uint, &str> = Ok(2);
583 /// let y: Result<&str, &str> = Ok("different result type");
584 /// assert_eq!(x.and(y), Ok("different result type"));
588 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
595 /// Calls `op` if the result is `Ok`, otherwise returns the `Err` value of `self`.
597 /// This function can be used for control flow based on result values.
602 /// fn sq(x: uint) -> Result<uint, uint> { Ok(x * x) }
603 /// fn err(x: uint) -> Result<uint, uint> { Err(x) }
605 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
606 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
607 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
608 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
612 pub fn and_then<U, F: FnOnce(T) -> Result<U, E>>(self, op: F) -> Result<U, E> {
619 /// Returns `res` if the result is `Err`, otherwise returns the `Ok` value of `self`.
624 /// let x: Result<uint, &str> = Ok(2);
625 /// let y: Result<uint, &str> = Err("late error");
626 /// assert_eq!(x.or(y), Ok(2));
628 /// let x: Result<uint, &str> = Err("early error");
629 /// let y: Result<uint, &str> = Ok(2);
630 /// assert_eq!(x.or(y), Ok(2));
632 /// let x: Result<uint, &str> = Err("not a 2");
633 /// let y: Result<uint, &str> = Err("late error");
634 /// assert_eq!(x.or(y), Err("late error"));
636 /// let x: Result<uint, &str> = Ok(2);
637 /// let y: Result<uint, &str> = Ok(100);
638 /// assert_eq!(x.or(y), Ok(2));
642 pub fn or(self, res: Result<T, E>) -> Result<T, E> {
649 /// Calls `op` if the result is `Err`, otherwise returns the `Ok` value of `self`.
651 /// This function can be used for control flow based on result values.
656 /// fn sq(x: uint) -> Result<uint, uint> { Ok(x * x) }
657 /// fn err(x: uint) -> Result<uint, uint> { Err(x) }
659 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
660 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
661 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
662 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
666 pub fn or_else<F, O: FnOnce(E) -> Result<T, F>>(self, op: O) -> Result<T, F> {
673 /// Unwraps a result, yielding the content of an `Ok`.
674 /// Else it returns `optb`.
680 /// let x: Result<uint, &str> = Ok(9u);
681 /// assert_eq!(x.unwrap_or(optb), 9u);
683 /// let x: Result<uint, &str> = Err("error");
684 /// assert_eq!(x.unwrap_or(optb), optb);
688 pub fn unwrap_or(self, optb: T) -> T {
695 /// Unwraps a result, yielding the content of an `Ok`.
696 /// If the value is an `Err` then it calls `op` with its value.
701 /// fn count(x: &str) -> uint { x.len() }
703 /// assert_eq!(Ok(2u).unwrap_or_else(count), 2u);
704 /// assert_eq!(Err("foo").unwrap_or_else(count), 3u);
708 pub fn unwrap_or_else<F: FnOnce(E) -> T>(self, op: F) -> T {
717 impl<T, E: Show> Result<T, E> {
718 /// Unwraps a result, yielding the content of an `Ok`.
722 /// Panics if the value is an `Err`, with a custom panic message provided
723 /// by the `Err`'s value.
728 /// let x: Result<uint, &str> = Ok(2u);
729 /// assert_eq!(x.unwrap(), 2u);
732 /// ```{.should_fail}
733 /// let x: Result<uint, &str> = Err("emergency failure");
734 /// x.unwrap(); // panics with `emergency failure`
738 pub fn unwrap(self) -> T {
742 panic!("called `Result::unwrap()` on an `Err` value: {:?}", e)
748 impl<T: Show, E> Result<T, E> {
749 /// Unwraps a result, yielding the content of an `Err`.
753 /// Panics if the value is an `Ok`, with a custom panic message provided
754 /// by the `Ok`'s value.
758 /// ```{.should_fail}
759 /// let x: Result<uint, &str> = Ok(2u);
760 /// x.unwrap_err(); // panics with `2`
764 /// let x: Result<uint, &str> = Err("emergency failure");
765 /// assert_eq!(x.unwrap_err(), "emergency failure");
769 pub fn unwrap_err(self) -> E {
772 panic!("called `Result::unwrap_err()` on an `Ok` value: {:?}", t),
778 /////////////////////////////////////////////////////////////////////////////
779 // Trait implementations
780 /////////////////////////////////////////////////////////////////////////////
782 impl<T, E> AsSlice<T> for Result<T, E> {
783 /// Convert from `Result<T, E>` to `&[T]` (without copying)
786 fn as_slice<'a>(&'a self) -> &'a [T] {
788 Ok(ref x) => slice::ref_slice(x),
790 // work around lack of implicit coercion from fixed-size array to slice
798 /////////////////////////////////////////////////////////////////////////////
799 // The Result Iterators
800 /////////////////////////////////////////////////////////////////////////////
802 /// An iterator over a reference to the `Ok` variant of a `Result`.
804 pub struct Iter<'a, T: 'a> { inner: Option<&'a T> }
807 impl<'a, T> Iterator for Iter<'a, T> {
811 fn next(&mut self) -> Option<&'a T> { self.inner.take() }
813 fn size_hint(&self) -> (uint, Option<uint>) {
814 let n = if self.inner.is_some() {1} else {0};
820 impl<'a, T> DoubleEndedIterator for Iter<'a, T> {
822 fn next_back(&mut self) -> Option<&'a T> { self.inner.take() }
826 impl<'a, T> ExactSizeIterator for Iter<'a, T> {}
828 impl<'a, T> Clone for Iter<'a, T> {
829 fn clone(&self) -> Iter<'a, T> { Iter { inner: self.inner } }
832 /// An iterator over a mutable reference to the `Ok` variant of a `Result`.
834 pub struct IterMut<'a, T: 'a> { inner: Option<&'a mut T> }
837 impl<'a, T> Iterator for IterMut<'a, T> {
838 type Item = &'a mut T;
841 fn next(&mut self) -> Option<&'a mut T> { self.inner.take() }
843 fn size_hint(&self) -> (uint, Option<uint>) {
844 let n = if self.inner.is_some() {1} else {0};
850 impl<'a, T> DoubleEndedIterator for IterMut<'a, T> {
852 fn next_back(&mut self) -> Option<&'a mut T> { self.inner.take() }
856 impl<'a, T> ExactSizeIterator for IterMut<'a, T> {}
858 /// An iterator over the value in a `Ok` variant of a `Result`.
860 pub struct IntoIter<T> { inner: Option<T> }
863 impl<T> Iterator for IntoIter<T> {
867 fn next(&mut self) -> Option<T> { self.inner.take() }
869 fn size_hint(&self) -> (uint, Option<uint>) {
870 let n = if self.inner.is_some() {1} else {0};
876 impl<T> DoubleEndedIterator for IntoIter<T> {
878 fn next_back(&mut self) -> Option<T> { self.inner.take() }
882 impl<T> ExactSizeIterator for IntoIter<T> {}
884 /////////////////////////////////////////////////////////////////////////////
886 /////////////////////////////////////////////////////////////////////////////
889 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
890 /// Takes each element in the `Iterator`: if it is an `Err`, no further
891 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
892 /// container with the values of each `Result` is returned.
894 /// Here is an example which increments every integer in a vector,
895 /// checking for overflow:
900 /// let v = vec!(1u, 2u);
901 /// let res: Result<Vec<uint>, &'static str> = v.iter().map(|&x: &uint|
902 /// if x == uint::MAX { Err("Overflow!") }
903 /// else { Ok(x + 1) }
905 /// assert!(res == Ok(vec!(2u, 3u)));
908 fn from_iter<I: Iterator<Item=Result<A, E>>>(iter: I) -> Result<V, E> {
909 // FIXME(#11084): This could be replaced with Iterator::scan when this
910 // performance bug is closed.
912 struct Adapter<Iter, E> {
917 impl<T, E, Iter: Iterator<Item=Result<T, E>>> Iterator for Adapter<Iter, E> {
921 fn next(&mut self) -> Option<T> {
922 match self.iter.next() {
923 Some(Ok(value)) => Some(value),
925 self.err = Some(err);
933 let mut adapter = Adapter { iter: iter, err: None };
934 let v: V = FromIterator::from_iter(adapter.by_ref());
937 Some(err) => Err(err),
943 /////////////////////////////////////////////////////////////////////////////
945 /////////////////////////////////////////////////////////////////////////////
947 /// Perform a fold operation over the result values from an iterator.
949 /// If an `Err` is encountered, it is immediately returned.
950 /// Otherwise, the folded value is returned.
957 Iter: Iterator<Item=Result<T, E>>>(
964 Ok(v) => init = f(init, v),
965 Err(u) => return Err(u)