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<h1 id="async-procedures"><a class="header" href="#async-procedures">Async procedures</a></h1>
<p>Async procedures are those that interact with <code>chronos</code> to cooperatively
suspend and resume their execution depending on the completion of other
async procedures, timers, tasks on other threads or asynchronous I/O scheduled
with the operating system.</p>
<p>Async procedures are marked with the <code>{.async.}</code> pragma and return a <code>Future</code>
indicating the state of the operation.</p>
<ul>
<li><a href="#the-async-pragma">The <code>async</code> pragma</a></li>
<li><a href="#await-keyword"><code>await</code> keyword</a></li>
<li><a href="#raw-async-procedures">Raw async procedures</a></li>
<li><a href="#callbacks-and-closures">Callbacks and closures</a></li>
</ul>
<h2 id="the-async-pragma"><a class="header" href="#the-async-pragma">The <code>async</code> pragma</a></h2>
<p>The <code>{.async.}</code> pragma will transform a procedure (or a method) returning a
<code>Future</code> into a closure iterator. If there is no return type specified,
<code>Future[void]</code> is returned.</p>
<pre><code class="language-nim">proc p() {.async.} =
await sleepAsync(100.milliseconds)
echo p().type # prints &quot;Future[system.void]&quot;
</code></pre>
<h2 id="await-keyword"><a class="header" href="#await-keyword"><code>await</code> keyword</a></h2>
<p>The <code>await</code> keyword operates on <code>Future</code> instances typically returned from an
<code>async</code> procedure.</p>
<p>Whenever <code>await</code> is encountered inside an async procedure, control is given
back to the dispatcher for as many steps as it's necessary for the awaited
future to complete, fail or be cancelled. <code>await</code> calls the
equivalent of <code>Future.read()</code> on the completed future to return the
encapsulated value when the operation finishes.</p>
<pre><code class="language-nim">proc p1() {.async.} =
await sleepAsync(1.seconds)
proc p2() {.async.} =
await sleepAsync(1.seconds)
proc p3() {.async.} =
let
fut1 = p1()
fut2 = p2()
# Just by executing the async procs, both resulting futures entered the
# dispatcher queue and their &quot;clocks&quot; started ticking.
await fut1
await fut2
# Only one second passed while awaiting them both, not two.
waitFor p3()
</code></pre>
<pre><code class="language-admonition warning">Because `async` procedures are executed concurrently, they are subject to many
of the same risks that typically accompany multithreaded programming.
In particular, if two `async` procedures have access to the same mutable state,
the value before and after `await` might not be the same as the order of execution is not guaranteed!
</code></pre>
<h2 id="raw-async-procedures"><a class="header" href="#raw-async-procedures">Raw async procedures</a></h2>
<p>Raw async procedures are those that interact with <code>chronos</code> via the <code>Future</code>
type but whose body does not go through the async transformation.</p>
<p>Such functions are created by adding <code>raw: true</code> to the <code>async</code> parameters:</p>
<pre><code class="language-nim">proc rawAsync(): Future[void] {.async: (raw: true).} =
let fut = newFuture[void](&quot;rawAsync&quot;)
fut.complete()
fut
</code></pre>
<p>Raw functions must not raise exceptions directly - they are implicitly declared
as <code>raises: []</code> - instead they should store exceptions in the returned <code>Future</code>:</p>
<pre><code class="language-nim">proc rawFailure(): Future[void] {.async: (raw: true).} =
let fut = newFuture[void](&quot;rawAsync&quot;)
fut.fail((ref ValueError)(msg: &quot;Oh no!&quot;))
fut
</code></pre>
<p>Raw procedures can also use checked exceptions:</p>
<pre><code class="language-nim">proc rawAsyncRaises(): Future[void] {.async: (raw: true, raises: [IOError]).} =
let fut = newFuture[void]()
assert not (compiles do: fut.fail((ref ValueError)(msg: &quot;uh-uh&quot;)))
fut.fail((ref IOError)(msg: &quot;IO&quot;))
fut
</code></pre>
<h2 id="callbacks-and-closures"><a class="header" href="#callbacks-and-closures">Callbacks and closures</a></h2>
<p>Callback/closure types are declared using the <code>async</code> annotation as usual:</p>
<pre><code class="language-nim">type MyCallback = proc(): Future[void] {.async.}
proc runCallback(cb: MyCallback) {.async: (raises: []).} =
try:
await cb()
except CatchableError:
discard # handle errors as usual
</code></pre>
<p>When calling a callback, it is important to remember that it may raise exceptions that need to be handled.</p>
<p>Checked exceptions can be used to limit the exceptions that a callback can
raise:</p>
<pre><code class="language-nim">type MyEasyCallback = proc(): Future[void] {.async: (raises: []).}
proc runCallback(cb: MyEasyCallback) {.async: (raises: [])} =
await cb()
</code></pre>
<footer id="open-on-gh">Found a bug? <a href="https://github.com/status-im/nim-chronos//edit/master/docs/src/async_procs.md">Edit this page on GitHub.</a></footer>
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