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//! The `select` macro. use proc_macro_hack::proc_macro_hack; 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. /// /// Note that `select!` relies on `proc-macro-hack`, and may require to set the /// compiler's recursion limit very high, e.g. `#![recursion_limit="1024"]`. /// /// # 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 $select_biased }; } #[cfg(feature = "std")] #[doc(hidden)] #[proc_macro_hack(support_nested, only_hack_old_rustc)] pub use futures_macro::select_internal; #[doc(hidden)] #[proc_macro_hack(support_nested, only_hack_old_rustc)] pub use futures_macro::select_biased_internal; document_select_macro! { #[cfg(feature = "std")] #[macro_export] macro_rules! select { ($($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 )* } }} } }