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 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 by 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 //! match file.write_line(format!("name: {}", info.name).as_slice()) {
183 //! Err(e) => return Err(e)
185 //! match file.write_line(format!("age: {}", info.age).as_slice()) {
187 //! Err(e) => return Err(e)
189 //! return file.write_line(format!("rating: {}", info.rating).as_slice());
196 //! use std::io::{File, Open, Write, IoError};
204 //! fn write_info(info: &Info) -> Result<(), IoError> {
205 //! let mut file = File::open_mode(&Path::new("my_best_friends.txt"), Open, Write);
206 //! // Early return on error
207 //! try!(file.write_line(format!("name: {}", info.name).as_slice()));
208 //! try!(file.write_line(format!("age: {}", info.age).as_slice()));
209 //! try!(file.write_line(format!("rating: {}", info.rating).as_slice()));
214 //! *It's much nicer!*
216 //! Wrapping an expression in `try!` will result in the unwrapped
217 //! success (`Ok`) value, unless the result is `Err`, in which case
218 //! `Err` is returned early from the enclosing function. Its simple definition
222 //! # #![feature(macro_rules)]
223 //! macro_rules! try(
224 //! ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
229 //! `try!` is imported by the prelude, and is available everywhere.
233 pub use self::Result::*;
238 use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSize};
239 use option::{None, Option, Some};
241 /// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
243 /// See the [`std::result`](index.html) module documentation for details.
244 #[deriving(Clone, PartialEq, PartialOrd, Eq, Ord, Show)]
247 pub enum Result<T, E> {
248 /// Contains the success value
251 /// Contains the error value
255 /////////////////////////////////////////////////////////////////////////////
256 // Type implementation
257 /////////////////////////////////////////////////////////////////////////////
259 impl<T, E> Result<T, E> {
260 /////////////////////////////////////////////////////////////////////////
261 // Querying the contained values
262 /////////////////////////////////////////////////////////////////////////
264 /// Returns true if the result is `Ok`
269 /// let x: Result<int, &str> = Ok(-3);
270 /// assert_eq!(x.is_ok(), true);
272 /// let x: Result<int, &str> = Err("Some error message");
273 /// assert_eq!(x.is_ok(), false);
277 pub fn is_ok(&self) -> bool {
284 /// Returns true if the result is `Err`
289 /// let x: Result<int, &str> = Ok(-3);
290 /// assert_eq!(x.is_err(), false);
292 /// let x: Result<int, &str> = Err("Some error message");
293 /// assert_eq!(x.is_err(), true);
297 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<T>`, 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<'r>(&'r self) -> Result<&'r T, &'r 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(&ref mut v) => *v = 42,
383 /// Err(&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);
396 #[unstable = "waiting for mut conventions"]
397 pub fn as_mut<'r>(&'r mut self) -> Result<&'r mut T, &'r 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 == &mut ["Gold"]);
412 /// assert!(v == &mut ["Silver"]);
414 /// assert_eq!(x, Ok("Silver"));
416 /// let mut x: Result<&str, uint> = Err(45);
417 /// assert!(x.as_mut_slice() == &mut []);
420 #[unstable = "waiting for mut conventions"]
421 pub fn as_mut_slice<'r>(&'r mut self) -> &'r 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::{BufReader, IoResult};
449 /// let buffer = "1\n2\n3\n4\n";
450 /// let mut reader = BufReader::new(buffer.as_bytes());
454 /// while !reader.eof() {
455 /// let line: IoResult<String> = reader.read_line();
456 /// // Convert the string line to a number using `map` and `from_str`
457 /// let val: IoResult<int> = line.map(|line| {
458 /// from_str::<int>(line.as_slice().trim_right()).unwrap_or(0)
460 /// // Add the value if there were no errors, otherwise add 0
461 /// sum += val.ok().unwrap_or(0);
464 /// assert!(sum == 10);
467 #[unstable = "waiting for unboxed closures"]
468 pub fn map<U>(self, op: |T| -> U) -> Result<U,E> {
475 /// Maps a `Result<T, E>` to `Result<T, F>` by applying a function to an
476 /// contained `Err` value, leaving an `Ok` value untouched.
478 /// This function can be used to pass through a successful result while handling
484 /// fn stringify(x: uint) -> String { format!("error code: {}", x) }
486 /// let x: Result<uint, uint> = Ok(2u);
487 /// assert_eq!(x.map_err(stringify), Ok(2u));
489 /// let x: Result<uint, uint> = Err(13);
490 /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
493 #[unstable = "waiting for unboxed closures"]
494 pub fn map_err<F>(self, op: |E| -> F) -> Result<T,F> {
502 /////////////////////////////////////////////////////////////////////////
503 // Iterator constructors
504 /////////////////////////////////////////////////////////////////////////
506 /// Returns an iterator over the possibly contained value.
511 /// let x: Result<uint, &str> = Ok(7);
512 /// assert_eq!(x.iter().next(), Some(&7));
514 /// let x: Result<uint, &str> = Err("nothing!");
515 /// assert_eq!(x.iter().next(), None);
518 #[unstable = "waiting for iterator conventions"]
519 pub fn iter<'r>(&'r self) -> Item<&'r T> {
520 Item{opt: self.as_ref().ok()}
523 /// Returns a mutable iterator over the possibly contained value.
528 /// let mut x: Result<uint, &str> = Ok(7);
529 /// match x.iter_mut().next() {
530 /// Some(&ref mut x) => *x = 40,
533 /// assert_eq!(x, Ok(40));
535 /// let mut x: Result<uint, &str> = Err("nothing!");
536 /// assert_eq!(x.iter_mut().next(), None);
539 #[unstable = "waiting for iterator conventions"]
540 pub fn iter_mut<'r>(&'r mut self) -> Item<&'r mut T> {
541 Item{opt: self.as_mut().ok()}
544 /// Returns a consuming iterator over the possibly contained value.
549 /// let x: Result<uint, &str> = Ok(5);
550 /// let v: Vec<uint> = x.into_iter().collect();
551 /// assert_eq!(v, vec![5u]);
553 /// let x: Result<uint, &str> = Err("nothing!");
554 /// let v: Vec<uint> = x.into_iter().collect();
555 /// assert_eq!(v, vec![]);
558 #[unstable = "waiting for iterator conventions"]
559 pub fn into_iter(self) -> Item<T> {
563 ////////////////////////////////////////////////////////////////////////
564 // Boolean operations on the values, eager and lazy
565 /////////////////////////////////////////////////////////////////////////
567 /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
572 /// let x: Result<uint, &str> = Ok(2);
573 /// let y: Result<&str, &str> = Err("late error");
574 /// assert_eq!(x.and(y), Err("late error"));
576 /// let x: Result<uint, &str> = Err("early error");
577 /// let y: Result<&str, &str> = Ok("foo");
578 /// assert_eq!(x.and(y), Err("early error"));
580 /// let x: Result<uint, &str> = Err("not a 2");
581 /// let y: Result<&str, &str> = Err("late error");
582 /// assert_eq!(x.and(y), Err("not a 2"));
584 /// let x: Result<uint, &str> = Ok(2);
585 /// let y: Result<&str, &str> = Ok("different result type");
586 /// assert_eq!(x.and(y), Ok("different result type"));
590 pub fn and<U>(self, res: Result<U, E>) -> Result<U, E> {
597 /// Calls `op` if the result is `Ok`, otherwise returns the `Err` value of `self`.
599 /// This function can be used for control flow based on result values.
604 /// fn sq(x: uint) -> Result<uint, uint> { Ok(x * x) }
605 /// fn err(x: uint) -> Result<uint, uint> { Err(x) }
607 /// assert_eq!(Ok(2).and_then(sq).and_then(sq), Ok(16));
608 /// assert_eq!(Ok(2).and_then(sq).and_then(err), Err(4));
609 /// assert_eq!(Ok(2).and_then(err).and_then(sq), Err(2));
610 /// assert_eq!(Err(3).and_then(sq).and_then(sq), Err(3));
613 #[unstable = "waiting for unboxed closures"]
614 pub fn and_then<U>(self, op: |T| -> Result<U, E>) -> Result<U, E> {
621 /// Returns `res` if the result is `Err`, otherwise returns the `Ok` value of `self`.
626 /// let x: Result<uint, &str> = Ok(2);
627 /// let y: Result<uint, &str> = Err("late error");
628 /// assert_eq!(x.or(y), Ok(2));
630 /// let x: Result<uint, &str> = Err("early error");
631 /// let y: Result<uint, &str> = Ok(2);
632 /// assert_eq!(x.or(y), Ok(2));
634 /// let x: Result<uint, &str> = Err("not a 2");
635 /// let y: Result<uint, &str> = Err("late error");
636 /// assert_eq!(x.or(y), Err("late error"));
638 /// let x: Result<uint, &str> = Ok(2);
639 /// let y: Result<uint, &str> = Ok(100);
640 /// assert_eq!(x.or(y), Ok(2));
644 pub fn or(self, res: Result<T, E>) -> Result<T, E> {
651 /// Calls `op` if the result is `Err`, otherwise returns the `Ok` value of `self`.
653 /// This function can be used for control flow based on result values.
658 /// fn sq(x: uint) -> Result<uint, uint> { Ok(x * x) }
659 /// fn err(x: uint) -> Result<uint, uint> { Err(x) }
661 /// assert_eq!(Ok(2).or_else(sq).or_else(sq), Ok(2));
662 /// assert_eq!(Ok(2).or_else(err).or_else(sq), Ok(2));
663 /// assert_eq!(Err(3).or_else(sq).or_else(err), Ok(9));
664 /// assert_eq!(Err(3).or_else(err).or_else(err), Err(3));
667 #[unstable = "waiting for unboxed closures"]
668 pub fn or_else<F>(self, op: |E| -> Result<T, F>) -> Result<T, F> {
675 /// Unwraps a result, yielding the content of an `Ok`.
676 /// Else it returns `optb`.
682 /// let x: Result<uint, &str> = Ok(9u);
683 /// assert_eq!(x.unwrap_or(optb), 9u);
685 /// let x: Result<uint, &str> = Err("error");
686 /// assert_eq!(x.unwrap_or(optb), optb);
689 #[unstable = "waiting for conventions"]
690 pub fn unwrap_or(self, optb: T) -> T {
697 /// Unwraps a result, yielding the content of an `Ok`.
698 /// If the value is an `Err` then it calls `op` with its value.
703 /// fn count(x: &str) -> uint { x.len() }
705 /// assert_eq!(Ok(2u).unwrap_or_else(count), 2u);
706 /// assert_eq!(Err("foo").unwrap_or_else(count), 3u);
709 #[unstable = "waiting for conventions"]
710 pub fn unwrap_or_else(self, op: |E| -> T) -> T {
718 impl<T, E: Show> Result<T, E> {
719 /// Unwraps a result, yielding the content of an `Ok`.
723 /// Panics if the value is an `Err`, with a custom panic message provided
724 /// by the `Err`'s value.
729 /// let x: Result<uint, &str> = Ok(2u);
730 /// assert_eq!(x.unwrap(), 2u);
733 /// ```{.should_fail}
734 /// let x: Result<uint, &str> = Err("emergency failure");
735 /// x.unwrap(); // panics with `emergency failure`
738 #[unstable = "waiting for conventions"]
739 pub fn unwrap(self) -> T {
743 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");
768 #[unstable = "waiting for conventions"]
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 Iterator
800 /////////////////////////////////////////////////////////////////////////////
802 /// A `Result` iterator that yields either one or zero elements
804 /// The `Item` iterator is returned by the `iter`, `iter_mut` and `into_iter`
805 /// methods on `Result`.
807 #[unstable = "waiting for iterator conventions"]
812 impl<T> Iterator<T> for Item<T> {
814 fn next(&mut self) -> Option<T> {
819 fn size_hint(&self) -> (uint, Option<uint>) {
821 Some(_) => (1, Some(1)),
822 None => (0, Some(0)),
827 impl<A> DoubleEndedIterator<A> for Item<A> {
829 fn next_back(&mut self) -> Option<A> {
834 impl<A> ExactSize<A> for Item<A> {}
836 /////////////////////////////////////////////////////////////////////////////
838 /////////////////////////////////////////////////////////////////////////////
841 impl<A, E, V: FromIterator<A>> FromIterator<Result<A, E>> for Result<V, E> {
842 /// Takes each element in the `Iterator`: if it is an `Err`, no further
843 /// elements are taken, and the `Err` is returned. Should no `Err` occur, a
844 /// container with the values of each `Result` is returned.
846 /// Here is an example which increments every integer in a vector,
847 /// checking for overflow:
852 /// let v = vec!(1u, 2u);
853 /// let res: Result<Vec<uint>, &'static str> = v.iter().map(|&x: &uint|
854 /// if x == uint::MAX { Err("Overflow!") }
855 /// else { Ok(x + 1) }
857 /// assert!(res == Ok(vec!(2u, 3u)));
860 fn from_iter<I: Iterator<Result<A, E>>>(iter: I) -> Result<V, E> {
861 // FIXME(#11084): This could be replaced with Iterator::scan when this
862 // performance bug is closed.
864 struct Adapter<Iter, E> {
869 impl<T, E, Iter: Iterator<Result<T, E>>> Iterator<T> for Adapter<Iter, E> {
871 fn next(&mut self) -> Option<T> {
872 match self.iter.next() {
873 Some(Ok(value)) => Some(value),
875 self.err = Some(err);
883 let mut adapter = Adapter { iter: iter, err: None };
884 let v: V = FromIterator::from_iter(adapter.by_ref());
887 Some(err) => Err(err),
893 /////////////////////////////////////////////////////////////////////////////
895 /////////////////////////////////////////////////////////////////////////////
897 /// Perform a fold operation over the result values from an iterator.
899 /// If an `Err` is encountered, it is immediately returned.
900 /// Otherwise, the folded value is returned.
906 Iter: Iterator<Result<T, E>>>(
913 Ok(v) => init = f(init, v),
914 Err(u) => return Err(u)