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<h1 id="errors-and-exceptions"><a class="header" href="#errors-and-exceptions">Errors and exceptions</a></h1>
<ul>
<li><a href="#exceptions">Exceptions</a></li>
<li><a href="#checked-exceptions">Checked exceptions</a></li>
<li><a href="#the-exception-type">The <code>Exception</code> type</a></li>
<li><a href="#compatibility-modes">Compatibility modes</a></li>
</ul>
<h2 id="exceptions"><a class="header" href="#exceptions">Exceptions</a></h2>
<p>Exceptions inheriting from <a href="https://nim-lang.org/docs/system.html#CatchableError"><code>CatchableError</code></a>
interrupt execution of an <code>async</code> procedure. The exception is placed in the
<code>Future.error</code> field while changing the status of the <code>Future</code> to <code>Failed</code>
and callbacks are scheduled.</p>
<p>When a future is read or awaited the exception is re-raised, traversing the
<code>async</code> execution chain until handled.</p>
<pre><code class="language-nim">proc p1() {.async.} =
await sleepAsync(1.seconds)
raise newException(ValueError, &quot;ValueError inherits from CatchableError&quot;)
proc p2() {.async.} =
await sleepAsync(1.seconds)
proc p3() {.async.} =
let
fut1 = p1()
fut2 = p2()
await fut1
echo &quot;unreachable code here&quot;
await fut2
# `waitFor()` would call `Future.read()` unconditionally, which would raise the
# exception in `Future.error`.
let fut3 = p3()
while not(fut3.finished()):
poll()
echo &quot;fut3.state = &quot;, fut3.state # &quot;Failed&quot;
if fut3.failed():
echo &quot;p3() failed: &quot;, fut3.error.name, &quot;: &quot;, fut3.error.msg
# prints &quot;p3() failed: ValueError: ValueError inherits from CatchableError&quot;
</code></pre>
<p>You can put the <code>await</code> in a <code>try</code> block, to deal with that exception sooner:</p>
<pre><code class="language-nim">proc p3() {.async.} =
let
fut1 = p1()
fut2 = p2()
try:
await fut1
except CachableError:
echo &quot;p1() failed: &quot;, fut1.error.name, &quot;: &quot;, fut1.error.msg
echo &quot;reachable code here&quot;
await fut2
</code></pre>
<p>Because <code>chronos</code> ensures that all exceptions are re-routed to the <code>Future</code>,
<code>poll</code> will not itself raise exceptions.</p>
<p><code>poll</code> may still panic / raise <code>Defect</code> if such are raised in user code due to
undefined behavior.</p>
<h2 id="checked-exceptions"><a class="header" href="#checked-exceptions">Checked exceptions</a></h2>
<p>By specifying a <code>raises</code> list to an async procedure, you can check which
exceptions can be raised by it:</p>
<pre><code class="language-nim">proc p1(): Future[void] {.async: (raises: [IOError]).} =
assert not (compiles do: raise newException(ValueError, &quot;uh-uh&quot;))
raise newException(IOError, &quot;works&quot;) # Or any child of IOError
proc p2(): Future[void] {.async, (raises: [IOError]).} =
await p1() # Works, because await knows that p1
# can only raise IOError
</code></pre>
<p>Under the hood, the return type of <code>p1</code> will be rewritten to an internal type
which will convey raises informations to <code>await</code>.</p>
<pre><code class="language-admonition note">Most `async` include `CancelledError` in the list of `raises`, indicating that
the operation they implement might get cancelled resulting in neither value nor
error!
</code></pre>
<p>When using checked exceptions, the <code>Future</code> type is modified to include
<code>raises</code> information - it can be constructed with the <code>Raising</code> helper:</p>
<pre><code class="language-nim"># Create a variable of the type that will be returned by a an async function
# raising `[CancelledError]`:
var fut: Future[int].Raising([CancelledError])
</code></pre>
<pre><code class="language-admonition note">`Raising` creates a specialization of `InternalRaisesFuture` type - as the name
suggests, this is an internal type whose implementation details are likely to
change in future `chronos` versions.
</code></pre>
<h2 id="the-exception-type"><a class="header" href="#the-exception-type">The <code>Exception</code> type</a></h2>
<p>Exceptions deriving from <code>Exception</code> are not caught by default as these may
include <code>Defect</code> and other forms undefined or uncatchable behavior.</p>
<p>Because exception effect tracking is turned on for <code>async</code> functions, this may
sometimes lead to compile errors around forward declarations, methods and
closures as Nim conservatively asssumes that any <code>Exception</code> might be raised
from those.</p>
<p>Make sure to explicitly annotate these with <code>{.raises.}</code>:</p>
<pre><code class="language-nim"># Forward declarations need to explicitly include a raises list:
proc myfunction() {.raises: [ValueError].}
# ... as do `proc` types
type MyClosure = proc() {.raises: [ValueError].}
proc myfunction() =
raise (ref ValueError)(msg: &quot;Implementation here&quot;)
let closure: MyClosure = myfunction
</code></pre>
<h2 id="compatibility-modes"><a class="header" href="#compatibility-modes">Compatibility modes</a></h2>
<p><strong>Individual functions.</strong> For compatibility, <code>async</code> functions can be instructed
to handle <code>Exception</code> as well, specifying <code>handleException: true</code>. Any
<code>Exception</code> that is not a <code>Defect</code> and not a <code>CatchableError</code> will then be
caught and remapped to <code>AsyncExceptionError</code>:</p>
<pre><code class="language-nim">proc raiseException() {.async: (handleException: true, raises: [AsyncExceptionError]).} =
raise (ref Exception)(msg: &quot;Raising Exception is UB&quot;)
proc callRaiseException() {.async: (raises: []).} =
try:
await raiseException()
except AsyncExceptionError as exc:
# The original Exception is available from the `parent` field
echo exc.parent.msg
</code></pre>
<p><strong>Global flag.</strong> This mode can be enabled globally with
<code>-d:chronosHandleException</code> as a help when porting code to <code>chronos</code>. The
behavior in this case will be that:</p>
<ol>
<li>
<p>old-style functions annotated with plain <code>async</code> will behave as if they had
been annotated with <code>async: (handleException: true)</code>.</p>
<p>This is functionally equivalent to
<code>async: (handleException: true, raises: [CatchableError])</code> and will, as
before, remap any <code>Exception</code> that is not <code>Defect</code> into
<code>AsyncExceptionError</code>, while also allowing any <code>CatchableError</code> (including
<code>AsyncExceptionError</code>) to get through without compilation errors.</p>
</li>
<li>
<p>New-style functions with <code>async: (raises: [...])</code> annotations or their own
<code>handleException</code> annotations will not be affected.</p>
</li>
</ol>
<p>The rationale here is to allow one to incrementally introduce exception
annotations and get compiler feedback while not requiring that every bit of
legacy code is updated at once.</p>
<p>This should be used sparingly and with care, however, as global configuration
settings may interfere with libraries that use <code>chronos</code> leading to unexpected
behavior.</p>
<footer id="open-on-gh">Found a bug? <a href="https://github.com/status-im/nim-chronos//edit/master/docs/src/error_handling.md">Edit this page on GitHub.</a></footer>
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