src/libextra/future.rs

// Copyright 2012-2013 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.

/*!
 * A type representing values that may be computed concurrently and
 * operations for working with them.
 *
 * # Example
 *
 * ```rust
 * # fn fib(n: uint) -> uint {42};
 * # fn make_a_sandwich() {};
 * let mut delayed_fib = extra::future::spawn (|| fib(5000) );
 * make_a_sandwich();
 * printfln!("fib(5000) = %?", delayed_fib.get())
 * ```
 */

#[allow(missing_doc)];

use std::cell::Cell;
use std::comm::{PortOne, oneshot};
use std::task;
use std::util::replace;

/// A type encapsulating the result of a computation which may not be complete
pub struct Future<A> {
    priv state: FutureState<A>,
}

enum FutureState<A> {
    Pending(~fn() -> A),
    Evaluating,
    Forced(A)
}

/// Methods on the `future` type
impl<A:Clone> Future<A> {
    pub fn get(&mut self) -> A {
        //! Get the value of the future.
        (*(self.get_ref())).clone()
    }
}

impl<A> Future<A> {
    /// Gets the value from this future, forcing evaluation.
    pub fn unwrap(self) -> A {
        let mut this = self;
        this.get_ref();
        let state = replace(&mut this.state, Evaluating);
        match state {
            Forced(v) => v,
            _ => fail!( "Logic error." ),
        }
    }

    pub fn get_ref<'a>(&'a mut self) -> &'a A {
        /*!
        * Executes the future's closure and then returns a borrowed
        * pointer to the result.  The borrowed pointer lasts as long as
        * the future.
        */
        match self.state {
            Forced(ref v) => return v,
            Evaluating => fail!("Recursive forcing of future!"),
            Pending(_) => {
                match replace(&mut self.state, Evaluating) {
                    Forced(_) | Evaluating => fail!("Logic error."),
                    Pending(f) => {
                        self.state = Forced(f());
                        self.get_ref()
                    }
                }
            }
        }
    }

    pub fn from_value(val: A) -> Future<A> {
        /*!
         * Create a future from a value.
         *
         * The value is immediately available and calling `get` later will
         * not block.
         */

        Future {state: Forced(val)}
    }

    pub fn from_fn(f: ~fn() -> A) -> Future<A> {
        /*!
         * Create a future from a function.
         *
         * The first time that the value is requested it will be retrieved by
         * calling the function.  Note that this function is a local
         * function. It is not spawned into another task.
         */

        Future {state: Pending(f)}
    }
}

impl<A:Send> Future<A> {
    pub fn from_port(port: PortOne<A>) -> Future<A> {
        /*!
         * Create a future from a port
         *
         * The first time that the value is requested the task will block
         * waiting for the result to be received on the port.
         */

        let port = Cell::new(port);
        do Future::from_fn {
            port.take().recv()
        }
    }

    pub fn spawn(blk: ~fn() -> A) -> Future<A> {
        /*!
         * Create a future from a unique closure.
         *
         * The closure will be run in a new task and its result used as the
         * value of the future.
         */

        let (port, chan) = oneshot();

        do task::spawn_with(chan) |chan| {
            chan.send(blk());
        }

        Future::from_port(port)
    }

    pub fn spawn_with<B: Send>(v: B, blk: ~fn(B) -> A) -> Future<A> {
        /*!
         * Create a future from a unique closure taking one argument.
         *
         * The closure and its argument will be moved into a new task. The
         * closure will be run and its result used as the value of the future.
         */

         let (port, chan) = oneshot();

         do task::spawn_with((v, chan)) |(v, chan)| {
            chan.send(blk(v));
         }

         Future::from_port(port)
    }
}

#[cfg(test)]
mod test {
    use future::Future;

    use std::cell::Cell;
    use std::comm::oneshot;
    use std::task;

    #[test]
    fn test_from_value() {
        let mut f = Future::from_value(~"snail");
        assert_eq!(f.get(), ~"snail");
    }

    #[test]
    fn test_from_port() {
        let (po, ch) = oneshot();
        ch.send(~"whale");
        let mut f = Future::from_port(po);
        assert_eq!(f.get(), ~"whale");
    }

    #[test]
    fn test_from_fn() {
        let mut f = Future::from_fn(|| ~"brail");
        assert_eq!(f.get(), ~"brail");
    }

    #[test]
    fn test_interface_get() {
        let mut f = Future::from_value(~"fail");
        assert_eq!(f.get(), ~"fail");
    }

    #[test]
    fn test_interface_unwrap() {
        let f = Future::from_value(~"fail");
        assert_eq!(f.unwrap(), ~"fail");
    }

    #[test]
    fn test_get_ref_method() {
        let mut f = Future::from_value(22);
        assert_eq!(*f.get_ref(), 22);
    }

    #[test]
    fn test_spawn() {
        let mut f = Future::spawn(|| ~"bale");
        assert_eq!(f.get(), ~"bale");
    }

    #[test]
    fn test_spawn_with() {
        let mut f = Future::spawn_with(~"gale", |s| { s });
        assert_eq!(f.get(), ~"gale");
    }

    #[test]
    #[should_fail]
    fn test_futurefail() {
        let mut f = Future::spawn(|| fail!());
        let _x: ~str = f.get();
    }

    #[test]
    fn test_sendable_future() {
        let expected = "schlorf";
        let f = Cell::new(do Future::spawn { expected });
        do task::spawn {
            let mut f = f.take();
            let actual = f.get();
            assert_eq!(actual, expected);
        }
    }
}