//! Definition of the `Result` (immediately finished) combinator

use core::result;

use {Future, Poll, Async};

/// A future representing a value that is immediately ready.
///
/// Created by the `result` function.
#[derive(Debug)]
#[must_use = "futures do nothing unless polled"]
// TODO: rename this to `Result` on the next major version
pub struct FutureResult<T, E> {
    inner: Option<result::Result<T, E>>,
}

/// Creates a new "leaf future" which will resolve with the given result.
///
/// The returned future represents a computation which is finshed immediately.
/// This can be useful with the `finished` and `failed` base future types to
/// convert an immediate value to a future to interoperate elsewhere.
///
/// # Examples
///
/// ```
/// use futures::future::*;
///
/// let future_of_1 = result::<u32, u32>(Ok(1));
/// let future_of_err_2 = result::<u32, u32>(Err(2));
/// ```
pub fn result<T, E>(r: result::Result<T, E>) -> FutureResult<T, E> {
    FutureResult { inner: Some(r) }
}

/// Creates a "leaf future" from an immediate value of a finished and
/// successful computation.
///
/// The returned future is similar to `result` where it will immediately run a
/// scheduled callback with the provided value.
///
/// # Examples
///
/// ```
/// use futures::future::*;
///
/// let future_of_1 = ok::<u32, u32>(1);
/// ```
pub fn ok<T, E>(t: T) -> FutureResult<T, E> {
    result(Ok(t))
}

/// Creates a "leaf future" from an immediate value of a failed computation.
///
/// The returned future is similar to `result` where it will immediately run a
/// scheduled callback with the provided value.
///
/// # Examples
///
/// ```
/// use futures::future::*;
///
/// let future_of_err_1 = err::<u32, u32>(1);
/// ```
pub fn err<T, E>(e: E) -> FutureResult<T, E> {
    result(Err(e))
}

impl<T, E> Future for FutureResult<T, E> {
    type Item = T;
    type Error = E;

    fn poll(&mut self) -> Poll<T, E> {
        self.inner.take().expect("cannot poll Result twice").map(Async::Ready)
    }
}
