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//! The `select` macro.
macro_rules! document_select_macro {
// This branch is required for `futures 0.3.1`, from before select_biased was introduced
($select:item) => {
/// Polls multiple futures and streams simultaneously, executing the branch
/// for the future that finishes first. If multiple futures are ready,
/// one will be pseudo-randomly selected at runtime. Futures directly
/// passed to `select!` must be `Unpin` and implement `FusedFuture`.
///
/// If an expression which yields a `Future` is passed to `select!`
/// (e.g. an `async fn` call) instead of a `Future` by name the `Unpin`
/// requirement is relaxed, since the macro will pin the resulting `Future`
/// on the stack. However the `Future` returned by the expression must
/// still implement `FusedFuture`.
///
/// Futures and streams which are not already fused can be fused using the
/// `.fuse()` method. Note, though, that fusing a future or stream directly
/// in the call to `select!` will not be enough to prevent it from being
/// polled after completion if the `select!` call is in a loop, so when
/// `select!`ing in a loop, users should take care to `fuse()` outside of
/// the loop.
///
/// `select!` can be used as an expression and will return the return
/// value of the selected branch. For this reason the return type of every
/// branch in a `select!` must be the same.
///
/// This macro is only usable inside of async functions, closures, and blocks.
/// It is also gated behind the `async-await` feature of this library, which is
/// activated by default.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select;
/// let mut a = future::ready(4);
/// let mut b = future::pending::<()>();
///
/// let res = select! {
/// a_res = a => a_res + 1,
/// _ = b => 0,
/// };
/// assert_eq!(res, 5);
/// # });
/// ```
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::stream::{self, StreamExt};
/// use futures::select;
/// let mut st = stream::iter(vec![2]).fuse();
/// let mut fut = future::pending::<()>();
///
/// select! {
/// x = st.next() => assert_eq!(Some(2), x),
/// _ = fut => panic!(),
/// };
/// # });
/// ```
///
/// As described earlier, `select` can directly select on expressions
/// which return `Future`s - even if those do not implement `Unpin`:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -> usize {
/// arg
/// }
///
/// let res = select! {
/// a_res = async_identity_fn(62).fuse() => a_res + 1,
/// b_res = async_identity_fn(13).fuse() => b_res,
/// };
/// assert!(res == 63 || res == 13);
/// # });
/// ```
///
/// If a similar async function is called outside of `select` to produce
/// a `Future`, the `Future` must be pinned in order to be able to pass
/// it to `select`. This can be achieved via `Box::pin` for pinning a
/// `Future` on the heap or the `pin_mut!` macro for pinning a `Future`
/// on the stack.
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select;
/// use futures::pin_mut;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -> usize {
/// arg
/// }
///
/// let fut_1 = async_identity_fn(1).fuse();
/// let fut_2 = async_identity_fn(2).fuse();
/// let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
/// pin_mut!(fut_2); // Pins the Future on the stack
///
/// let res = select! {
/// a_res = fut_1 => a_res,
/// b_res = fut_2 => b_res,
/// };
/// assert!(res == 1 || res == 2);
/// # });
/// ```
///
/// `select` also accepts a `complete` branch and a `default` branch.
/// `complete` will run if all futures and streams have already been
/// exhausted. `default` will run if no futures or streams are
/// immediately ready. `complete` takes priority over `default` in
/// the case where all futures have completed.
/// A motivating use-case for passing `Future`s by name as well as for
/// `complete` blocks is to call `select!` in a loop, which is
/// demonstrated in the following example:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select;
/// let mut a_fut = future::ready(4);
/// let mut b_fut = future::ready(6);
/// let mut total = 0;
///
/// loop {
/// select! {
/// a = a_fut => total += a,
/// b = b_fut => total += b,
/// complete => break,
/// default => panic!(), // never runs (futures run first, then complete)
/// };
/// }
/// assert_eq!(total, 10);
/// # });
/// ```
///
/// Note that the futures that have been matched over can still be mutated
/// from inside the `select!` block's branches. This can be used to implement
/// more complex behavior such as timer resets or writing into the head of
/// a stream.
$select
};
($select:item $select_biased:item) => {
document_select_macro!($select);
/// Polls multiple futures and streams simultaneously, executing the branch
/// for the future that finishes first. Unlike [`select!`], if multiple futures are ready,
/// one will be selected in order of declaration. Futures directly
/// passed to `select_biased!` must be `Unpin` and implement `FusedFuture`.
///
/// If an expression which yields a `Future` is passed to `select_biased!`
/// (e.g. an `async fn` call) instead of a `Future` by name the `Unpin`
/// requirement is relaxed, since the macro will pin the resulting `Future`
/// on the stack. However the `Future` returned by the expression must
/// still implement `FusedFuture`.
///
/// Futures and streams which are not already fused can be fused using the
/// `.fuse()` method. Note, though, that fusing a future or stream directly
/// in the call to `select_biased!` will not be enough to prevent it from being
/// polled after completion if the `select_biased!` call is in a loop, so when
/// `select_biased!`ing in a loop, users should take care to `fuse()` outside of
/// the loop.
///
/// `select_biased!` can be used as an expression and will return the return
/// value of the selected branch. For this reason the return type of every
/// branch in a `select_biased!` must be the same.
///
/// This macro is only usable inside of async functions, closures, and blocks.
/// It is also gated behind the `async-await` feature of this library, which is
/// activated by default.
///
/// # Examples
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select_biased;
/// let mut a = future::ready(4);
/// let mut b = future::pending::<()>();
///
/// let res = select_biased! {
/// a_res = a => a_res + 1,
/// _ = b => 0,
/// };
/// assert_eq!(res, 5);
/// # });
/// ```
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::stream::{self, StreamExt};
/// use futures::select_biased;
/// let mut st = stream::iter(vec![2]).fuse();
/// let mut fut = future::pending::<()>();
///
/// select_biased! {
/// x = st.next() => assert_eq!(Some(2), x),
/// _ = fut => panic!(),
/// };
/// # });
/// ```
///
/// As described earlier, `select_biased` can directly select on expressions
/// which return `Future`s - even if those do not implement `Unpin`:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select_biased;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -> usize {
/// arg
/// }
///
/// let res = select_biased! {
/// a_res = async_identity_fn(62).fuse() => a_res + 1,
/// b_res = async_identity_fn(13).fuse() => b_res,
/// };
/// assert!(res == 63 || res == 12);
/// # });
/// ```
///
/// If a similar async function is called outside of `select_biased` to produce
/// a `Future`, the `Future` must be pinned in order to be able to pass
/// it to `select_biased`. This can be achieved via `Box::pin` for pinning a
/// `Future` on the heap or the `pin_mut!` macro for pinning a `Future`
/// on the stack.
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future::FutureExt;
/// use futures::select_biased;
/// use futures::pin_mut;
///
/// // Calling the following async fn returns a Future which does not
/// // implement Unpin
/// async fn async_identity_fn(arg: usize) -> usize {
/// arg
/// }
///
/// let fut_1 = async_identity_fn(1).fuse();
/// let fut_2 = async_identity_fn(2).fuse();
/// let mut fut_1 = Box::pin(fut_1); // Pins the Future on the heap
/// pin_mut!(fut_2); // Pins the Future on the stack
///
/// let res = select_biased! {
/// a_res = fut_1 => a_res,
/// b_res = fut_2 => b_res,
/// };
/// assert!(res == 1 || res == 2);
/// # });
/// ```
///
/// `select_biased` also accepts a `complete` branch and a `default` branch.
/// `complete` will run if all futures and streams have already been
/// exhausted. `default` will run if no futures or streams are
/// immediately ready. `complete` takes priority over `default` in
/// the case where all futures have completed.
/// A motivating use-case for passing `Future`s by name as well as for
/// `complete` blocks is to call `select_biased!` in a loop, which is
/// demonstrated in the following example:
///
/// ```
/// # futures::executor::block_on(async {
/// use futures::future;
/// use futures::select_biased;
/// let mut a_fut = future::ready(4);
/// let mut b_fut = future::ready(6);
/// let mut total = 0;
///
/// loop {
/// select_biased! {
/// a = a_fut => total += a,
/// b = b_fut => total += b,
/// complete => break,
/// default => panic!(), // never runs (futures run first, then complete)
/// };
/// }
/// assert_eq!(total, 10);
/// # });
/// ```
///
/// Note that the futures that have been matched over can still be mutated
/// from inside the `select_biased!` block's branches. This can be used to implement
/// more complex behavior such as timer resets or writing into the head of
/// a stream.
///
/// [`select!`]: macro.select.html
$select_biased
};
}
#[cfg(feature = "std")]
#[allow(unreachable_pub)]
#[doc(hidden)]
pub use futures_macro::select_internal;
#[allow(unreachable_pub)]
#[doc(hidden)]
pub use futures_macro::select_biased_internal;
document_select_macro! {
#[cfg(feature = "std")]
#[macro_export]
macro_rules! select {
($($tokens:tt)*) => {{
use $crate::__private as __futures_crate;
$crate::select_internal! {
$( $tokens )*
}
}}
}
#[macro_export]
macro_rules! select_biased {
($($tokens:tt)*) => {{
use $crate::__private as __futures_crate;
$crate::select_biased_internal! {
$( $tokens )*
}
}}
}
}