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<ol class="chapter"><li class="chapter-item expanded affix "><a href="../index.html">The Puck Programming Language</a></li><li class="chapter-item expanded "><a href="OVERVIEW.html"><strong aria-hidden="true">1.</strong> Basic Usage</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">1.1.</strong> Variables and Comments</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.2.</strong> Basic Types</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.3.</strong> Functions and Calls</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.4.</strong> Boolean and Integer Operations</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.5.</strong> Conditionals and Control Flow</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.6.</strong> Error Handling</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.7.</strong> Loops and Iterators</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.8.</strong> Modules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.9.</strong> Compile-time Programming</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.10.</strong> Async and Threading</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.11.</strong> Advanced Types</div></li></ol></li><li class="chapter-item expanded "><a href="SYNTAX.html"><strong aria-hidden="true">2.</strong> Syntax</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">2.1.</strong> Indentation Rules [todo]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">2.2.</strong> Reserved Keywords</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">2.3.</strong> A Formal Grammar</div></li></ol></li><li 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Nim</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">9.2.</strong> Java, Kotlin</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">9.3.</strong> Python, Racket, C</div></li></ol></li><li class="chapter-item expanded "><div><strong aria-hidden="true">10.</strong> Refinement Types [draft]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">11.</strong> Dependent Types [draft]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">12.</strong> Effects System [draft]</div></li></ol>
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<h1 class="menu-title">The Puck Programming Language</h1>
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<h1 id="error-handling"><a class="header" href="#error-handling">Error Handling</a></h1>
<p>Puck's error handling is shamelessly stolen from Swift. It uses a combination of <code>Option</code>/<code>Result</code> types and <code>try</code>/<code>catch</code> statements, and leans somewhat on Puck's metaprogramming capabilities.</p>
<p>There are several ways to handle errors in Puck. If the error is encoded in the type, one can:</p>
<ol>
<li><code>match</code> on the error</li>
<li>compactly match on the error with <code>if ... of</code></li>
<li>propagate the error with <code>?</code></li>
<li>throw the error with <code>!</code></li>
</ol>
<p>If an error is thrown, one <strong>must</strong> explicitly handle (or disregard) it with a <code>try/catch</code> block or risk runtime failure. This method of error handling may feel more familiar to Java programmers.</p>
<h2 id="errors-as-monads"><a class="header" href="#errors-as-monads">Errors as Monads</a></h2>
<p>Puck provides <a href="std/default/options.pk"><code>Option[T]</code></a> and a <a href="std/default/results.pk"><code>Result[T, E]</code></a> types, imported by default. These are <code>union</code> types and so must be pattern matched upon to be useful: but the standard library provides <a href="std/default/results.pk">a bevy of helper functions</a>.
Two in particular are of note. The <code>?</code> operator unwraps a Result or propagates its error up a function call (and may only be used in type-appropriate contexts). The <code>!</code> operator unwraps an Option or Result directly or throws an exception in the case of None or Error.</p>
<pre><code class="language-puck">pub macro `?`[T, E](self: Result[T, E]) =
quote:
match `self`
of Okay(x): x
of Error(e): return Error(e)
</code></pre>
<pre><code class="language-puck">pub func `!`[T](self: Option[T]): T =
match self
of Some(x): x
of None: raise EmptyValue
pub func `!`[T, E](self: Result[T, E]): T =
of Okay(x): x
of Error(e): raise e
</code></pre>
<p>The utility of the provided helpers in <a href="std/default/options.pk"><code>std.options</code></a> and <a href="std/default/results.pk"><code>std.results</code></a> should not be understated. While encoding errors into the type system may appear restrictive at first glance, some syntactic sugar goes a long way in writing compact and idiomatic code. Java programmers in particular are urged to give type-first errors a try, before falling back on unwraps and <code>try</code>/<code>catch</code>.</p>
<p>A notable helpful type is the aliasing of <code>Result[T]</code> to <code>Result[T, ref Err]</code>, for when the particular error does not matter. This breaks <code>try</code>/<code>catch</code> exhaustion (as <code>ref Err</code> denotes a reference to <em>any</em> Error), but is particularly useful when used in conjunction with the propagation operator.</p>
<h2 id="errors-as-catchable-exceptions"><a class="header" href="#errors-as-catchable-exceptions">Errors as Catchable Exceptions</a></h2>
<p>Errors raised by <code>raise</code>/<code>throw</code> (or subsequently the <code>!</code> operator) must be explicitly caught and handled via a <code>try</code>/<code>catch</code>/<code>finally</code> statement.
If an exception is not handled within a function body, the function must be explicitly marked as a throwing function via the <code>yeet</code> prefix (name to be determined). The compiler will statically determine which exceptions in particular are thrown from any given function, and enforce them to be explicitly handled or explicitly ignored.</p>
<p>Despite functioning here as exceptions: errors remain types. An error thrown from an unwrapped <code>Result[T, E]</code> is of type <code>E</code>. <code>catch</code> statements, then, may pattern match upon possible errors, behaving similarly to <code>of</code> branches.</p>
<pre><code class="language-puck">try:
...
catch "Error":
...
finally:
...
</code></pre>
<p>This creates a distinction between two types of error handling, working in sync: functional error handling with <a href="https://en.wikipedia.org/wiki/Option_type">Option</a> and <a href="https://en.wikipedia.org/wiki/Result_type">Result</a> types, and object-oriented error handling with <a href="https://en.wikipedia.org/wiki/Exception_handling">catchable exceptions</a>. These styles may be swapped between with minimal syntactic overhead. Libraries, however, should universally use <code>Option</code>/<code>Result</code>, as this provides the best support for both styles.</p>
<!-- [nullable types](https://en.wikipedia.org/wiki/Nullable_type)?? -->
<h2 id="errors-and-void-functions"><a class="header" href="#errors-and-void-functions">Errors and Void Functions</a></h2>
<p>Some functions do not return a value but can still fail: for example, setters.
This can make it difficult to do monadic error handling elegantly: one could return a <code>Result[void, E]</code>, but...</p>
<pre><code class="language-puck">pub func set[T](self: list[T], i: uint, val: T) =
if i > self.length:
raise IndexOutOfBounds
self.data.raw_set(offset = i, val)
</code></pre>
<h2 id="unrecoverable-exceptions"><a class="header" href="#unrecoverable-exceptions">Unrecoverable Exceptions</a></h2>
<p>There exist errors from which a program can not reasonably recover. These are the following:</p>
<ul>
<li><code>Assertation Failure</code>: a call to an <code>assert</code> function has returned false at runtime.</li>
<li><code>Out of Memory</code>: the executable is out of memory.</li>
<li><code>Stack Overflow</code>: the executable has overflowed the stack.</li>
<li>any others?</li>
</ul>
<p>They are not recoverable, but the user should be aware of them as possible failure conditions.</p>
<p>References: <a href="https://docs.swift.org/swift-book/documentation/the-swift-programming-language/errorhandling">Error Handling in Swift</a></p>
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