rust/src/libcore/result.rs

// 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>` 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.
//!
//! ```
//! 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:
//!
//! ```
//! #[deriving(Show)]
//! enum Version { Version1, Version2 }
//!
//! fn parse_version(header: &[u8]) -> Result<Version, &'static str> {
//!     if header.len() < 1 {
//!         return Err("invalid header length");
//!     }
//!     match header[0] {
//!         1 => Ok(Version::Version1),
//!         2 => Ok(Version::Version2),
//!         _ => 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<int, int> = Ok(10);
//! let bad_result: Result<int, int> = 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<int, int> = good_result.map(|i| i + 1);
//! let bad_result: Result<int, int> = bad_result.map(|i| i - 1);
//!
//! // Use `and_then` to continue the computation.
//! let good_result: Result<bool, int> = good_result.and_then(|i| Ok(i == 11));
//!
//! // Use `or_else` to handle the error.
//! let bad_result: Result<int, int> = bad_result.or_else(|i| Ok(11));
//!
//! // Consume the result and return the contents with `unwrap`.
//! let final_awesome_result = good_result.ok().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_line` method defined for I/O types
//! by the [`Writer`](../io/trait.Writer.html) trait:
//!
//! ```
//! use std::io::IoError;
//!
//! trait Writer {
//!     fn write_line(&mut self, s: &str) -> Result<(), IoError>;
//! }
//! ```
//!
//! *Note: The actual definition of `Writer` uses `IoResult`, which
//! is just a synonym for `Result<T, IoError>`.*
//!
//! 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:
//!
//! ```{.ignore}
//! use std::io::{File, Open, Write};
//!
//! let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
//! // If `write_line` errors, then we'll never know, because the return
//! // value is ignored.
//! file.write_line("important message");
//! drop(file);
//! ```
//!
//! If you *do* write that in Rust, the compiler will by 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
//! panic, by converting to an `Option` with `ok`, then asserting
//! success with `expect`. This will panic if the write fails, proving
//! a marginally useful message indicating why:
//!
//! ```{.no_run}
//! use std::io::{File, Open, Write};
//!
//! let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
//! file.write_line("important message").ok().expect("failed to write message");
//! drop(file);
//! ```
//!
//! You might also simply assert success:
//!
//! ```{.no_run}
//! # use std::io::{File, Open, Write};
//!
//! # let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
//! assert!(file.write_line("important message").is_ok());
//! # drop(file);
//! ```
//!
//! Or propagate the error up the call stack with `try!`:
//!
//! ```
//! # use std::io::{File, Open, Write, IoError};
//! fn write_message() -> Result<(), IoError> {
//!     let mut file = File::open_mode(&Path::new("valuable_data.txt"), Open, Write);
//!     try!(file.write_line("important message"));
//!     drop(file);
//!     return Ok(());
//! }
//! ```
//!
//! # The `try!` macro
//!
//! When writing code that calls many functions that return the
//! `Result` type, the error handling can be tedious.  The `try!`
//! macro hides some of the boilerplate of propagating errors up the
//! call stack.
//!
//! It replaces this:
//!
//! ```
//! use std::io::{File, Open, Write, IoError};
//!
//! struct Info {
//!     name: String,
//!     age: int,
//!     rating: int
//! }
//!
//! fn write_info(info: &Info) -> Result<(), IoError> {
//!     let mut file = File::open_mode(&Path::new("my_best_friends.txt"), Open, Write);
//!     // Early return on error
//!     match file.write_line(format!("name: {}", info.name).as_slice()) {
//!         Ok(_) => (),
//!         Err(e) => return Err(e)
//!     }
//!     match file.write_line(format!("age: {}", info.age).as_slice()) {
//!         Ok(_) => (),
//!         Err(e) => return Err(e)
//!     }
//!     return file.write_line(format!("rating: {}", info.rating).as_slice());
//! }
//! ```
//!
//! With this:
//!
//! ```
//! use std::io::{File, Open, Write, IoError};
//!
//! struct Info {
//!     name: String,
//!     age: int,
//!     rating: int
//! }
//!
//! fn write_info(info: &Info) -> Result<(), IoError> {
//!     let mut file = File::open_mode(&Path::new("my_best_friends.txt"), Open, Write);
//!     // Early return on error
//!     try!(file.write_line(format!("name: {}", info.name).as_slice()));
//!     try!(file.write_line(format!("age: {}", info.age).as_slice()));
//!     try!(file.write_line(format!("rating: {}", info.rating).as_slice()));
//!     return Ok(());
//! }
//! ```
//!
//! *It's much nicer!*
//!
//! Wrapping an expression in `try!` will result in the unwrapped
//! success (`Ok`) value, unless the result is `Err`, in which case
//! `Err` is returned early from the enclosing function. Its simple definition
//! makes it clear:
//!
//! ```
//! # #![feature(macro_rules)]
//! macro_rules! try(
//!     ($e:expr) => (match $e { Ok(e) => e, Err(e) => return Err(e) })
//! );
//! # fn main() { }
//! ```
//!
//! `try!` is imported by the prelude, and is available everywhere.

#![stable]

use self::Result::*;

use kinds::Copy;
use std::fmt::Show;
use slice;
use slice::AsSlice;
use iter::{Iterator, IteratorExt, DoubleEndedIterator, FromIterator, ExactSizeIterator};
use option::Option;
use option::Option::{None, Some};
use ops::{FnMut, FnOnce};

/// `Result` is a type that represents either success (`Ok`) or failure (`Err`).
///
/// See the [`std::result`](index.html) module documentation for details.
#[deriving(Clone, PartialEq, PartialOrd, Eq, Ord, Show, Hash)]
#[must_use]
#[stable]
pub enum Result<T, E> {
    /// Contains the success value
    Ok(T),

    /// Contains the error value
    Err(E)
}

/////////////////////////////////////////////////////////////////////////////
// Type implementation
/////////////////////////////////////////////////////////////////////////////

impl<T, E> Result<T, E> {
    /////////////////////////////////////////////////////////////////////////
    // Querying the contained values
    /////////////////////////////////////////////////////////////////////////

    /// Returns true if the result is `Ok`
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<int, &str> = Ok(-3);
    /// assert_eq!(x.is_ok(), true);
    ///
    /// let x: Result<int, &str> = Err("Some error message");
    /// assert_eq!(x.is_ok(), false);
    /// ```
    #[inline]
    #[stable]
    pub fn is_ok(&self) -> bool {
        match *self {
            Ok(_) => true,
            Err(_) => false
        }
    }

    /// Returns true if the result is `Err`
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<int, &str> = Ok(-3);
    /// assert_eq!(x.is_err(), false);
    ///
    /// let x: Result<int, &str> = Err("Some error message");
    /// assert_eq!(x.is_err(), true);
    /// ```
    #[inline]
    #[stable]
    pub fn is_err(&self) -> bool {
        !self.is_ok()
    }


    /////////////////////////////////////////////////////////////////////////
    // Adapter for each variant
    /////////////////////////////////////////////////////////////////////////

    /// Convert from `Result<T, E>` to `Option<T>`
    ///
    /// Converts `self` into an `Option<T>`, consuming `self`,
    /// and discarding the error, if any.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(2);
    /// assert_eq!(x.ok(), Some(2));
    ///
    /// let x: Result<uint, &str> = Err("Nothing here");
    /// assert_eq!(x.ok(), None);
    /// ```
    #[inline]
    #[stable]
    pub fn ok(self) -> Option<T> {
        match self {
            Ok(x)  => Some(x),
            Err(_) => None,
        }
    }

    /// Convert from `Result<T, E>` to `Option<E>`
    ///
    /// Converts `self` into an `Option<T>`, consuming `self`,
    /// and discarding the value, if any.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(2);
    /// assert_eq!(x.err(), None);
    ///
    /// let x: Result<uint, &str> = Err("Nothing here");
    /// assert_eq!(x.err(), Some("Nothing here"));
    /// ```
    #[inline]
    #[stable]
    pub fn err(self) -> Option<E> {
        match self {
            Ok(_)  => None,
            Err(x) => Some(x),
        }
    }

    /////////////////////////////////////////////////////////////////////////
    // Adapter for working with references
    /////////////////////////////////////////////////////////////////////////

    /// Convert from `Result<T, E>` to `Result<&T, &E>`
    ///
    /// Produces a new `Result`, containing a reference
    /// into the original, leaving the original in place.
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(2);
    /// assert_eq!(x.as_ref(), Ok(&2));
    ///
    /// let x: Result<uint, &str> = Err("Error");
    /// assert_eq!(x.as_ref(), Err(&"Error"));
    /// ```
    #[inline]
    #[stable]
    pub fn as_ref<'r>(&'r self) -> Result<&'r T, &'r E> {
        match *self {
            Ok(ref x) => Ok(x),
            Err(ref x) => Err(x),
        }
    }

    /// Convert from `Result<T, E>` to `Result<&mut T, &mut E>`
    ///
    /// ```
    /// fn mutate(r: &mut Result<int, int>) {
    ///     match r.as_mut() {
    ///         Ok(&ref mut v) => *v = 42,
    ///         Err(&ref mut e) => *e = 0,
    ///     }
    /// }
    ///
    /// let mut x: Result<int, int> = Ok(2);
    /// mutate(&mut x);
    /// assert_eq!(x.unwrap(), 42);
    ///
    /// let mut x: Result<int, int> = Err(13);
    /// mutate(&mut x);
    /// assert_eq!(x.unwrap_err(), 0);
    /// ```
    #[inline]
    #[unstable = "waiting for mut conventions"]
    pub fn as_mut<'r>(&'r mut self) -> Result<&'r mut T, &'r mut E> {
        match *self {
            Ok(ref mut x) => Ok(x),
            Err(ref mut x) => Err(x),
        }
    }

    /// Convert from `Result<T, E>` to `&mut [T]` (without copying)
    ///
    /// ```
    /// let mut x: Result<&str, uint> = Ok("Gold");
    /// {
    ///     let v = x.as_mut_slice();
    ///     assert!(v == ["Gold"]);
    ///     v[0] = "Silver";
    ///     assert!(v == ["Silver"]);
    /// }
    /// assert_eq!(x, Ok("Silver"));
    ///
    /// let mut x: Result<&str, uint> = Err(45);
    /// assert!(x.as_mut_slice().is_empty());
    /// ```
    #[inline]
    #[unstable = "waiting for mut conventions"]
    pub fn as_mut_slice<'r>(&'r mut self) -> &'r mut [T] {
        match *self {
            Ok(ref mut x) => slice::mut_ref_slice(x),
            Err(_) => {
                // work around lack of implicit coercion from fixed-size array to slice
                let emp: &mut [_] = &mut [];
                emp
            }
        }
    }

    /////////////////////////////////////////////////////////////////////////
    // Transforming contained values
    /////////////////////////////////////////////////////////////////////////

    /// Maps a `Result<T, E>` to `Result<U, E>` by applying a function to an
    /// contained `Ok` value, leaving an `Err` value untouched.
    ///
    /// This function can be used to compose the results of two functions.
    ///
    /// # Example
    ///
    /// Sum the lines of a buffer by mapping strings to numbers,
    /// ignoring I/O and parse errors:
    ///
    /// ```
    /// use std::io::IoResult;
    ///
    /// let mut buffer = &mut b"1\n2\n3\n4\n";
    ///
    /// let mut sum = 0;
    ///
    /// while !buffer.is_empty() {
    ///     let line: IoResult<String> = buffer.read_line();
    ///     // Convert the string line to a number using `map` and `from_str`
    ///     let val: IoResult<int> = line.map(|line| {
    ///         from_str::<int>(line.as_slice().trim_right()).unwrap_or(0)
    ///     });
    ///     // Add the value if there were no errors, otherwise add 0
    ///     sum += val.ok().unwrap_or(0);
    /// }
    ///
    /// assert!(sum == 10);
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    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 an
    /// contained `Err` value, leaving an `Ok` value untouched.
    ///
    /// This function can be used to pass through a successful result while handling
    /// an error.
    ///
    /// # Example
    ///
    /// ```
    /// fn stringify(x: uint) -> String { format!("error code: {}", x) }
    ///
    /// let x: Result<uint, uint> = Ok(2u);
    /// assert_eq!(x.map_err(stringify), Ok(2u));
    ///
    /// let x: Result<uint, uint> = Err(13);
    /// assert_eq!(x.map_err(stringify), Err("error code: 13".to_string()));
    /// ```
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    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.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(7);
    /// assert_eq!(x.iter().next(), Some(&7));
    ///
    /// let x: Result<uint, &str> = Err("nothing!");
    /// assert_eq!(x.iter().next(), None);
    /// ```
    #[inline]
    #[unstable = "waiting for iterator conventions"]
    pub fn iter<'r>(&'r self) -> Item<&'r T> {
        Item{opt: self.as_ref().ok()}
    }

    /// Returns a mutable iterator over the possibly contained value.
    ///
    /// # Example
    ///
    /// ```
    /// let mut x: Result<uint, &str> = Ok(7);
    /// match x.iter_mut().next() {
    ///     Some(&ref mut x) => *x = 40,
    ///     None => {},
    /// }
    /// assert_eq!(x, Ok(40));
    ///
    /// let mut x: Result<uint, &str> = Err("nothing!");
    /// assert_eq!(x.iter_mut().next(), None);
    /// ```
    #[inline]
    #[unstable = "waiting for iterator conventions"]
    pub fn iter_mut<'r>(&'r mut self) -> Item<&'r mut T> {
        Item{opt: self.as_mut().ok()}
    }

    /// Returns a consuming iterator over the possibly contained value.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(5);
    /// let v: Vec<uint> = x.into_iter().collect();
    /// assert_eq!(v, vec![5u]);
    ///
    /// let x: Result<uint, &str> = Err("nothing!");
    /// let v: Vec<uint> = x.into_iter().collect();
    /// assert_eq!(v, vec![]);
    /// ```
    #[inline]
    #[unstable = "waiting for iterator conventions"]
    pub fn into_iter(self) -> Item<T> {
        Item{opt: self.ok()}
    }

    ////////////////////////////////////////////////////////////////////////
    // Boolean operations on the values, eager and lazy
    /////////////////////////////////////////////////////////////////////////

    /// Returns `res` if the result is `Ok`, otherwise returns the `Err` value of `self`.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(2);
    /// let y: Result<&str, &str> = Err("late error");
    /// assert_eq!(x.and(y), Err("late error"));
    ///
    /// let x: Result<uint, &str> = Err("early error");
    /// let y: Result<&str, &str> = Ok("foo");
    /// assert_eq!(x.and(y), Err("early error"));
    ///
    /// let x: Result<uint, &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<uint, &str> = Ok(2);
    /// let y: Result<&str, &str> = Ok("different result type");
    /// assert_eq!(x.and(y), Ok("different result type"));
    /// ```
    #[inline]
    #[stable]
    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.
    ///
    /// # Example
    ///
    /// ```
    /// fn sq(x: uint) -> Result<uint, uint> { Ok(x * x) }
    /// fn err(x: uint) -> Result<uint, uint> { 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]
    #[unstable = "waiting for unboxed closures"]
    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`.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(2);
    /// let y: Result<uint, &str> = Err("late error");
    /// assert_eq!(x.or(y), Ok(2));
    ///
    /// let x: Result<uint, &str> = Err("early error");
    /// let y: Result<uint, &str> = Ok(2);
    /// assert_eq!(x.or(y), Ok(2));
    ///
    /// let x: Result<uint, &str> = Err("not a 2");
    /// let y: Result<uint, &str> = Err("late error");
    /// assert_eq!(x.or(y), Err("late error"));
    ///
    /// let x: Result<uint, &str> = Ok(2);
    /// let y: Result<uint, &str> = Ok(100);
    /// assert_eq!(x.or(y), Ok(2));
    /// ```
    #[inline]
    #[stable]
    pub fn or(self, res: Result<T, E>) -> Result<T, E> {
        match self {
            Ok(_) => self,
            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.
    ///
    /// # Example
    ///
    /// ```
    /// fn sq(x: uint) -> Result<uint, uint> { Ok(x * x) }
    /// fn err(x: uint) -> Result<uint, uint> { 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]
    #[unstable = "waiting for unboxed closures"]
    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`.
    ///
    /// # Example
    ///
    /// ```
    /// let optb = 2u;
    /// let x: Result<uint, &str> = Ok(9u);
    /// assert_eq!(x.unwrap_or(optb), 9u);
    ///
    /// let x: Result<uint, &str> = Err("error");
    /// assert_eq!(x.unwrap_or(optb), optb);
    /// ```
    #[inline]
    #[unstable = "waiting for conventions"]
    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.
    ///
    /// # Example
    ///
    /// ```
    /// fn count(x: &str) -> uint { x.len() }
    ///
    /// assert_eq!(Ok(2u).unwrap_or_else(count), 2u);
    /// assert_eq!(Err("foo").unwrap_or_else(count), 3u);
    /// ```
    #[inline]
    #[unstable = "waiting for conventions"]
    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: Show> Result<T, E> {
    /// Unwraps a result, yielding the content of an `Ok`.
    ///
    /// # Panics
    ///
    /// Panics if the value is an `Err`, with a custom panic message provided
    /// by the `Err`'s value.
    ///
    /// # Example
    ///
    /// ```
    /// let x: Result<uint, &str> = Ok(2u);
    /// assert_eq!(x.unwrap(), 2u);
    /// ```
    ///
    /// ```{.should_fail}
    /// let x: Result<uint, &str> = Err("emergency failure");
    /// x.unwrap(); // panics with `emergency failure`
    /// ```
    #[inline]
    #[unstable = "waiting for conventions"]
    pub fn unwrap(self) -> T {
        match self {
            Ok(t) => t,
            Err(e) =>
                panic!("called `Result::unwrap()` on an `Err` value: {}", e)
        }
    }
}

impl<T: Show, 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.
    ///
    /// # Example
    ///
    /// ```{.should_fail}
    /// let x: Result<uint, &str> = Ok(2u);
    /// x.unwrap_err(); // panics with `2`
    /// ```
    ///
    /// ```
    /// let x: Result<uint, &str> = Err("emergency failure");
    /// assert_eq!(x.unwrap_err(), "emergency failure");
    /// ```
    #[inline]
    #[unstable = "waiting for conventions"]
    pub fn unwrap_err(self) -> E {
        match self {
            Ok(t) =>
                panic!("called `Result::unwrap_err()` on an `Ok` value: {}", t),
            Err(e) => e
        }
    }
}

/////////////////////////////////////////////////////////////////////////////
// Trait implementations
/////////////////////////////////////////////////////////////////////////////

impl<T, E> AsSlice<T> for Result<T, E> {
    /// Convert from `Result<T, E>` to `&[T]` (without copying)
    #[inline]
    #[stable]
    fn as_slice<'a>(&'a self) -> &'a [T] {
        match *self {
            Ok(ref x) => slice::ref_slice(x),
            Err(_) => {
                // work around lack of implicit coercion from fixed-size array to slice
                let emp: &[_] = &[];
                emp
            }
        }
    }
}

/////////////////////////////////////////////////////////////////////////////
// The Result Iterator
/////////////////////////////////////////////////////////////////////////////

/// A `Result` iterator that yields either one or zero elements
///
/// The `Item` iterator is returned by the `iter`, `iter_mut` and `into_iter`
/// methods on `Result`.
#[deriving(Clone)]
#[unstable = "waiting for iterator conventions"]
pub struct Item<T> {
    opt: Option<T>
}

impl<T> Iterator<T> for Item<T> {
    #[inline]
    fn next(&mut self) -> Option<T> {
        self.opt.take()
    }

    #[inline]
    fn size_hint(&self) -> (uint, Option<uint>) {
        match self.opt {
            Some(_) => (1, Some(1)),
            None => (0, Some(0)),
        }
    }
}

impl<A> DoubleEndedIterator<A> for Item<A> {
    #[inline]
    fn next_back(&mut self) -> Option<A> {
        self.opt.take()
    }
}

impl<A> ExactSizeIterator<A> for Item<A> {}

/////////////////////////////////////////////////////////////////////////////
// FromIterator
/////////////////////////////////////////////////////////////////////////////

#[stable]
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:
    ///
    /// ```rust
    /// use std::uint;
    ///
    /// let v = vec!(1u, 2u);
    /// let res: Result<Vec<uint>, &'static str> = v.iter().map(|&x: &uint|
    ///     if x == uint::MAX { Err("Overflow!") }
    ///     else { Ok(x + 1) }
    /// ).collect();
    /// assert!(res == Ok(vec!(2u, 3u)));
    /// ```
    #[inline]
    fn from_iter<I: Iterator<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<Result<T, E>>> Iterator<T> for Adapter<Iter, E> {
            #[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,
                }
            }
        }

        let mut adapter = Adapter { iter: iter, err: None };
        let v: V = FromIterator::from_iter(adapter.by_ref());

        match adapter.err {
            Some(err) => Err(err),
            None => Ok(v),
        }
    }
}

/////////////////////////////////////////////////////////////////////////////
// FromIterator
/////////////////////////////////////////////////////////////////////////////

/// Perform a fold operation over the result values from an iterator.
///
/// If an `Err` is encountered, it is immediately returned.
/// Otherwise, the folded value is returned.
#[inline]
#[experimental]
pub fn fold<T,
            V,
            E,
            F: FnMut(V, T) -> V,
            Iter: Iterator<Result<T, E>>>(
            mut iterator: Iter,
            mut init: V,
            mut f: F)
            -> Result<V, E> {
    for t in iterator {
        match t {
            Ok(v) => init = f(init, v),
            Err(u) => return Err(u)
        }
    }
    Ok(init)
}

impl<T:Copy,U:Copy> Copy for Result<T,U> {}