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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 class="chapter-item expanded "><a href="TYPES.html"><strong aria-hidden="true">3.</strong> Type System</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">3.1.</strong> Basic Types</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">3.2.</strong> Parameter Types</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">3.3.</strong> Reference Types</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">3.4.</strong> Abstract Types</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">3.5.</strong> Advanced Types</div></li></ol></li><li class="chapter-item expanded "><a href="MODULES.html" class="active"><strong aria-hidden="true">4.</strong> Module System</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">4.1.</strong> Using Modules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.2.</strong> Implicit Modules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.3.</strong> Defining Module Interfaces [todo]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.4.</strong> Defining an External API [todo]</div></li></ol></li><li class="chapter-item expanded "><a href="ERRORS.html"><strong aria-hidden="true">5.</strong> Error Handling</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">5.1.</strong> Errors as Monads</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">5.2.</strong> Errors as Catchable Exceptions</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">5.3.</strong> Errors and Void Functions [todo]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">5.4.</strong> Unrecoverable Exceptions</div></li></ol></li><li class="chapter-item expanded "><a href="ASYNC.html"><strong aria-hidden="true">6.</strong> Async System</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">6.1.</strong> Threading [todo]</div></li></ol></li><li class="chapter-item expanded "><a href="METAPROGRAMMING.html"><strong aria-hidden="true">7.</strong> Metaprogramming</a></li><li class="chapter-item expanded "><div><strong aria-hidden="true">8.</strong> Memory Management [todo]</div></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">8.1.</strong> Reference Counting Optimizations</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">8.2.</strong> Annotations and Ownership</div></li></ol></li><li class="chapter-item expanded "><a href="INTEROP.html"><strong aria-hidden="true">9.</strong> Language Interop [draft]</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">9.1.</strong> Rust, Swift, 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="modules-and-namespacing"><a class="header" href="#modules-and-namespacing">Modules and Namespacing</a></h1>
<blockquote>
<p>! This section is <strong>incomplete</strong>. Proceed with caution.</p>
</blockquote>
<p>Puck has a first-class module system, inspired by such expressive designs in the ML family.</p>
<h2 id="what-are-modules"><a class="header" href="#what-are-modules">What are Modules?</a></h2>
<pre><code class="language-puck">pub mod stack =
pub type Stack[T] = class
init(static type Self): Stack[T]
push(mut Self, val: T)
pop(mut Self): T?
peek(lent Self): lent T?
pub mod list =
type ListStack[T] = list[T]
pub func init[T](self: static type ListStack[T]): Stack[T] = []
pub func push[T](self: mut ListStack[T], val: T) = self.push(T)
pub func pop[T](self: mut ListStack[T]): T? = self.pop
pub func peek[T](self: lent ListStack[T]): lent T? =
if self.len == 0 then None else Some(self.last)
use stack.list
let a = ListStack[int].init
print a.len # error: unable to access method on private type outside its module
a.push(5)
print a.pop # Some(5)
</code></pre>
<p>Modules package up code for use by others. Identifiers known at compile time may be part of a module: these being constants, functions, macros, types, and other modules themselves. Such identifiers may be made accessible outside of the module by prefixing them with the <code>pub</code> keyword.</p>
<p>Importantly, <em>files</em> are implicitly modules, public and named with their filename. The <code>mod</code> keyword followed by an identifier and an indented block of code explicitly defines a module, inside of the current module. Modules are first class: they may be bound to constants (having the type <code>: mod</code>) and publicly exported, or bound to local variables and passed into functions for who knows what purpose.</p>
<h2 id="using-modules"><a class="header" href="#using-modules">Using modules</a></h2>
<p>The <code>use</code> keyword lets you use other modules.</p>
<p>The <code>use</code> keyword imports public symbols from the specified module into the current scope <em>unqualified</em>. This runs contrary to expectations coming from most other languages: from Python to Standard ML, the standard notion of an "import" puts the imported symbols behind another symbol to avoid "polluting the namespace". As Puck is strongly typed and allows overloading, however, we see no reason for namespace pollution to be of concern. These unqualified imports have the added benefit of making <em>uniform function call syntax</em> more widely accessible. It is inevitable that identifier conflicts will exist on occasion, of course: when this happens, the compiler will force qualification (this then does restrict uniform function call syntax). We discuss this more later.</p>
<p>Nonetheless, if qualification of imports is so desired, an alternative approach is available - binding a module to a constant. Both the standard library and external libraries are available behind identifiers without use of <code>use</code>: <code>std</code> and <code>lib</code>, respectively. (FFI and local modules will likely use more identifiers, but this is not hammered out yet.) A submodule - for example, <code>std.net</code> - may be bound in a constant as <code>const net = std.net</code>, providing all of the modules' public identifiers for use, as fields of the constant <code>net</code>. We will see this construction to be extraordinarily helpful in crafting high-level public APIs for libraries later on.</p>
<pre><code class="language-puck">use std.[logs, test]
use lib.crypto, lib.http
</code></pre>
<p>Multiple modules can be imported at once. The standard namespaces deserve more than a passing mention. There are several of these: <code>std</code> for the standard library, <code>lib</code> for all external libraries, <code>pkg</code> for the top-level namespace of a project, <code>this</code> for the current containing module... In addition: there are a suite of <em>language</em> namespaces, for FFI - <code>rust</code>, <code>nim</code>, and <code>swift</code> preliminarily - that give access to libraries from other languages. Recall that imports are unqualified - so <code>use std</code> will allow use of the standard library without the <code>std</code> qualifier (not recommended: several modules have common names), and <code>use lib</code> will dump the name of every library it can find into the global namespace (even less recommended).</p>
<h2 id="implicit-modules"><a class="header" href="#implicit-modules">Implicit Modules</a></h2>
<p>A major goal of Puck's module system is to allow the same level of expressiveness as the ML family, while cutting down on the extraneous syntax and boilerplate needed to do so. As such, access modifiers are written directly inline with their declaration, and the file system structure is reused to form an implicit module system for internal use. This - particularly the former - <em>limits</em> the structure a module can expose at first glance, but we will see later that classes recoup much of this lost specificity.</p>
<p>We mentioned that the filesystem forms an implicit module structure. This begets a couple of design choices. Module names <strong>must</strong> be lowercase, for compatibility with case-insensitive filesystems. Both a file and a folder with the same name can exist. Files within the aforementioned folder are treated as submodules of the aforementioned file. This again restricts the sorts of module structures we can build, but we will see later that this restriction can be bypassed.</p>
<p>The <code>this</code> and <code>pkg</code> modules are useful for this implicit structure...</p>
<h2 id="defining-interfaces"><a class="header" href="#defining-interfaces">Defining interfaces</a></h2>
<p>...</p>
<h2 id="defining-an-external-api"><a class="header" href="#defining-an-external-api">Defining an External API</a></h2>
<p>The filesystem provides an implicit module structure, but it may not be the one you want to expose to users.</p>
<p>...</p>
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