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(Version1),
//!         2 => Ok(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 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
//! fail, by converting to an `Option` with `ok`, then asserting
//! success with `expect`. This will fail 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.
//!
//! # `Result` and `Option`
//!
//! The `Result` and [`Option`](../option/index.html) types are
//! similar and complementary: they are often employed to indicate a
//! lack of a return value; and they are trivially converted between
//! each other, so `Result`s are often handled by first converting to
//! `Option` with the [`ok`](type.Result.html#method.ok) and
//! [`err`](type.Result.html#method.ok) methods.
//!
//! Whereas `Option` only indicates the lack of a value, `Result` is
//! specifically for error reporting, and carries with it an error
//! value.  Sometimes `Option` is used for indicating errors, but this
//! is only for simple cases and is generally discouraged. Even when
//! there is no useful error value to return, prefer `Result<T, ()>`.
//!
//! Converting to an `Option` with `ok()` to handle an error:
//!
//! ~~~
//! use std::io::Timer;
//! let mut t = Timer::new().ok().expect("failed to create timer!");
//! ~~~
//!
//! # `Result` vs. `fail!`
//!
//! `Result` is for recoverable errors; `fail!` is for unrecoverable
//! errors. Callers should always be able to avoid failure if they
//! take the proper precautions, for example, calling `is_some()`
//! on an `Option` type before calling `unwrap`.
//!
//! The suitability of `fail!` as an error handling mechanism is
//! limited by Rust's lack of any way to "catch" and resume execution
//! from a thrown exception. Therefore using failure for error
//! handling requires encapsulating fallible code in a task. Calling
//! the `fail!` macro, or invoking `fail!` indirectly should be
//! avoided as an error reporting strategy. Failure is only for
//! unrecoverable errors and a failing task is typically the sign of
//! a bug.
//!
//! A module that instead returns `Results` is alerting the caller
//! that failure is possible, and providing precise control over how
//! it is handled.
//!
//! Furthermore, failure may not be recoverable at all, depending on
//! the context. The caller of `fail!` should assume that execution
//! will not resume after failure, that failure is catastrophic.

#![stable]

use clone::Clone;
use cmp::PartialEq;
use std::fmt::Show;
use slice;
use slice::Slice;
use iter::{Iterator, DoubleEndedIterator, FromIterator, ExactSize};
use option::{None, Option, Some};

/// `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)]
#[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
    ///
    /// ~~~
    /// use std::io::{File, Open, Write};
    ///
    /// # fn do_not_run_example() { // creates a file
    /// let mut file = File::open_mode(&Path::new("secret.txt"), Open, Write);
    /// assert!(file.write_line("it's cold in here").is_ok());
    /// # }
    /// ~~~
    #[inline]
    #[stable]
    pub fn is_ok(&self) -> bool {
        match *self {
            Ok(_) => true,
            Err(_) => false
        }
    }

    /// Returns true if the result is `Err`
    ///
    /// # Example
    ///
    /// ~~~
    /// use std::io::{File, Open, Read};
    ///
    /// // When opening with `Read` access, if the file does not exist
    /// // then `open_mode` returns an error.
    /// let bogus = File::open_mode(&Path::new("not_a_file.txt"), Open, Read);
    /// assert!(bogus.is_err());
    /// ~~~
    #[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.
    ///
    /// To convert to an `Option` without discarding the error value,
    /// use `as_ref` to first convert the `Result<T, E>` into a
    /// `Result<&T, &E>`.
    ///
    /// # Examples
    ///
    /// ~~~{.should_fail}
    /// use std::io::{File, IoResult};
    ///
    /// let bdays: IoResult<File> = File::open(&Path::new("important_birthdays.txt"));
    /// let bdays: File = bdays.ok().expect("unable to open birthday file");
    /// ~~~
    #[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.
    #[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.
    #[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>`
    #[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)
    #[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.
    ///
    /// # Examples
    ///
    /// Sum the lines of a buffer by mapping strings to numbers,
    /// ignoring I/O and parse errors:
    ///
    /// ~~~
    /// use std::io::{BufReader, IoResult};
    ///
    /// let buffer = "1\n2\n3\n4\n";
    /// let mut reader = BufReader::new(buffer.as_bytes());
    ///
    /// let mut sum = 0;
    ///
    /// while !reader.eof() {
    ///     let line: IoResult<String> = reader.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>(self, op: |T| -> U) -> 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.
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    pub fn map_err<F>(self, op: |E| -> F) -> Result<T,F> {
        match self {
          Ok(t) => Ok(t),
          Err(e) => Err(op(e))
        }
    }


    /////////////////////////////////////////////////////////////////////////
    // Iterator constructors
    /////////////////////////////////////////////////////////////////////////

    /// Returns an iterator over the possibly contained value.
    #[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.
    #[inline]
    #[unstable = "waiting for iterator conventions"]
    pub fn mut_iter<'r>(&'r mut self) -> Item<&'r mut T> {
        Item{opt: self.as_mut().ok()}
    }

    /// Returns a consuming iterator over the possibly contained value.
    #[inline]
    #[unstable = "waiting for iterator conventions"]
    pub fn move_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`.
    #[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
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    pub fn and_then<U>(self, op: |T| -> Result<U, E>) -> 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`.
    #[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
    #[inline]
    #[unstable = "waiting for unboxed closures"]
    pub fn or_else<F>(self, op: |E| -> Result<T, F>) -> 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`.
    #[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.
    #[inline]
    #[unstable = "waiting for conventions"]
    pub fn unwrap_or_else(self, op: |E| -> T) -> T {
        match self {
            Ok(t) => t,
            Err(e) => op(e)
        }
    }

    /// Deprecated name for `unwrap_or_else()`.
    #[deprecated = "replaced by .unwrap_or_else()"]
    #[inline]
    pub fn unwrap_or_handle(self, op: |E| -> T) -> T {
        self.unwrap_or_else(op)
    }
}

impl<T, E: Show> Result<T, E> {
    /// Unwraps a result, yielding the content of an `Ok`.
    ///
    /// # Failure
    ///
    /// Fails if the value is an `Err`, with a custom failure message provided
    /// by the `Err`'s value.
    #[inline]
    #[unstable = "waiting for conventions"]
    pub fn unwrap(self) -> T {
        match self {
            Ok(t) => t,
            Err(e) =>
                fail!("called `Result::unwrap()` on an `Err` value: {}", e)
        }
    }
}

impl<T: Show, E> Result<T, E> {
    /// Unwraps a result, yielding the content of an `Err`.
    ///
    /// # Failure
    ///
    /// Fails if the value is an `Ok`, with a custom failure message provided
    /// by the `Ok`'s value.
    #[inline]
    #[unstable = "waiting for conventions"]
    pub fn unwrap_err(self) -> E {
        match self {
            Ok(t) =>
                fail!("called `Result::unwrap_err()` on an `Ok` value: {}", t),
            Err(e) => e
        }
    }
}

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

impl<T, E> Slice<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`, `mut_iter` and `move_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> ExactSize<A> for Item<A> {}

/////////////////////////////////////////////////////////////////////////////
// Free functions
/////////////////////////////////////////////////////////////////////////////

/// Deprecated: use `Iterator::collect`.
#[inline]
#[deprecated = "use Iterator::collect instead"]
pub fn collect<T, E, Iter: Iterator<Result<T, E>>, V: FromIterator<T>>(mut iter: Iter)
                                                                       -> Result<V, E> {
    iter.collect()
}

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),
        }
    }
}

/// 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,
            Iter: Iterator<Result<T, E>>>(
            mut iterator: Iter,
            mut init: V,
            f: |V, T| -> V)
            -> Result<V, E> {
    for t in iterator {
        match t {
            Ok(v) => init = f(init, v),
            Err(u) => return Err(u)
        }
    }
    Ok(init)
}

/// Deprecated.
///
/// Perform a trivial fold operation over the result values
/// from an iterator.
///
/// If an `Err` is encountered, it is immediately returned.
/// Otherwise, a simple `Ok(())` is returned.
#[inline]
#[deprecated = "use fold instead"]
pub fn fold_<T,E,Iter:Iterator<Result<T,E>>>(iterator: Iter) -> Result<(),E> {
    fold(iterator, (), |_, _| ())
}