use Future;
use SpawnError;
/// A value that executes futures.
///
/// The [`spawn`] function is used to submit a future to an executor. Once
/// submitted, the executor takes ownership of the future and becomes
/// responsible for driving the future to completion.
///
/// The strategy employed by the executor to handle the future is less defined
/// and is left up to the `Executor` implementation. The `Executor` instance is
/// expected to call [`poll`] on the future once it has been notified, however
/// the "when" and "how" can vary greatly.
///
/// For example, the executor might be a thread pool, in which case a set of
/// threads have already been spawned up and the future is inserted into a
/// queue. A thread will acquire the future and poll it.
///
/// The `Executor` trait is only for futures that **are** `Send`. These are most
/// common. There currently is no trait that describes executors that operate
/// entirely on the current thread (i.e., are able to spawn futures that are not
/// `Send`). Note that single threaded executors can still implement `Executor`,
/// but only futures that are `Send` can be spawned via the trait.
///
/// This trait is primarily intended to implemented by executors and used to
/// back `tokio::spawn`. Libraries and applications **may** use this trait to
/// bound generics, but doing so will limit usage to futures that implement
/// `Send`. Instead, libraries and applications are recommended to use
/// [`TypedExecutor`] as a bound.
///
/// # Errors
///
/// The [`spawn`] function returns `Result` with an error type of `SpawnError`.
/// This error type represents the reason that the executor was unable to spawn
/// the future. The two current represented scenarios are:
///
/// * An executor being at capacity or full. As such, the executor is not able
/// to accept a new future. This error state is expected to be transient.
/// * An executor has been shutdown and can no longer accept new futures. This
/// error state is expected to be permanent.
///
/// If a caller encounters an at capacity error, the caller should try to shed
/// load. This can be as simple as dropping the future that was spawned.
///
/// If the caller encounters a shutdown error, the caller should attempt to
/// gracefully shutdown.
///
/// # Examples
///
/// ```rust
/// # extern crate futures;
/// # extern crate tokio_executor;
/// # use tokio_executor::Executor;
/// # fn docs(my_executor: &mut Executor) {
/// use futures::future::lazy;
/// my_executor.spawn(Box::new(lazy(|| {
/// println!("running on the executor");
/// Ok(())
/// }))).unwrap();
/// # }
/// # fn main() {}
/// ```
///
/// [`spawn`]: #tymethod.spawn
/// [`poll`]: https://docs.rs/futures/0.1/futures/future/trait.Future.html#tymethod.poll
/// [`TypedExecutor`]: ../trait.TypedExecutor.html