Macro select 

macro_rules! select {
    (biased; $($token:tt)*) => { ... };
    ($($token:tt)*) => { ... };
}

Select multiplex asynchronous futures simultaneously

select! supports three different clauses:

• pattern = future [, if condition] => code,
• default => code,
• complete => code,

Evaluation order

• All conditions and futures are evaluated before selection.
• Future expression is not evaluated if corresponding condition evaluated to false.
• Whenever a branch is ready, its clause is executed. And the whole select returns.
• Fail to match a refutable pattern will disable that branch.
• default clause is executed if no futures are ready. That is non blocking mode.
• If all branches are disabled by conditions or refutable pattern match, it resort to complete or default in case of no complete.

Panics

• Panic when all futures are disabled or completed and there is no default or complete.

Comparing with tokio::select!

• Future expression is only evaluated if condition meets.

  use std::future::ready;
  use async_select::select;
  
  async fn guard_future_by_condition() {
      let opt: Option<i32> = None;
      let r = select! {
          v = ready(opt.unwrap()), if opt.is_some() => v,
          v = ready(6) => v,
      };
      assert_eq!(r, 6);
  }

  This will panic in tokio::select! as it evaluates ready(opt.unwrap()) irrespective of corresponding condition. See https://github.com/tokio-rs/tokio/pull/6555.
• There is no default counterpart in tokio::select!. But it could be emulated with biased and ready(()).complete is same as else in tokio::select!.
• async_select::select! depends only on proc_macro macros and hence the generated code is no_std compatible.

Polling order

By default, the polling order of each branch is indeterminate. Use biased; to poll sequentially if desired.

use async_select::select;
use core::future::{pending, ready};

async fn poll_sequentially() {
    let r = select! {
        biased;
        default => unreachable!(),
        _ = pending() => unreachable!(),
        _ = pending() => unreachable!(),
        v = ready(5) => v,
        v = ready(6) => v,
        v = ready(7) => v,
    };
    assert_eq!(r, 5);
}

Efficiency

select! blindly Future:poll all enabled futures without checking for waking branch.

Examples

use async_select::select;
use core::future::{pending, ready};

async fn on_ready() {
    let r = select! {
        _ = pending() => unreachable!(),
        v = ready(5) => v,
        default => unreachable!(),
    };
    assert_eq!(r, 5);
}