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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> Functions and Indentation</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.3.</strong> Uniform Function Call Syntax</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.4.</strong> Basic Types</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.5.</strong> Conditionals and Pattern Matching</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> Blocks and Loops</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.8.</strong> Module System</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 System and Threading</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.11.</strong> Memory Management</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.12.</strong> Types System</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.13.</strong> Structs and Tuples</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.14.</strong> Unions and Enums</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">1.15.</strong> Classes</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> Call Syntax</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">2.2.</strong> Indentation Rules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">2.3.</strong> Expression Rules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">2.4.</strong> Reserved Keywords</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">2.5.</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"><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> What are modules?</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.2.</strong> Using modules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.3.</strong> Implicit modules</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.4.</strong> Defining interfaces [todo]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">4.5.</strong> Defining an external API [todo]</div></li></ol></li><li class="chapter-item expanded "><a href="MEMORY_MANAGEMENT.html"><strong aria-hidden="true">5.</strong> Memory Management [todo]</a></li><li class="chapter-item expanded "><a href="METAPROGRAMMING.html" class="active"><strong aria-hidden="true">6.</strong> Metaprogramming</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">6.1.</strong> Scope</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">6.2.</strong> Usage</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">6.3.</strong> Quoting [todo]</div></li></ol></li><li class="chapter-item expanded "><a href="ERRORS.html"><strong aria-hidden="true">7.</strong> Error Handling</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">7.1.</strong> Errors as monads</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">7.2.</strong> Errors as checked exceptions</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">7.3.</strong> Errors as effects [todo]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">7.4.</strong> Unrecoverable Exceptions</div></li></ol></li><li class="chapter-item expanded "><a href="ASYNC.html"><strong aria-hidden="true">8.</strong> Async System</a></li><li><ol class="section"><li class="chapter-item expanded "><div><strong aria-hidden="true">8.1.</strong> Effects System [todo]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">8.2.</strong> Threading [todo]</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</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">9.2.</strong> Swift, Nim</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">9.3.</strong> Java, Kotlin</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">9.4.</strong> Python, Racket, C</div></li></ol></li><li class="chapter-item expanded "><div><strong aria-hidden="true">10.</strong> Effects System [draft]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">11.</strong> Refinement Types [draft]</div></li><li class="chapter-item expanded "><div><strong aria-hidden="true">12.</strong> Dependent Types [draft]</div></li><li class="chapter-item expanded "><a href="EXAMPLES.html"><strong aria-hidden="true">13.</strong> Examples</a></li></ol>
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<h1 class="menu-title">The Puck Programming Language</h1>
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<h1 id="metaprogramming"><a class="header" href="#metaprogramming">Metaprogramming</a></h1>
<p>Puck has rich metaprogramming support, heavily inspired by Nim. Many features that would have to be at the compiler level in most languages (error propagation <code>?</code>, <code>std.fmt.print</code>, <code>?</code>, <code>!</code>, <code>-></code> type sugar, <code>=></code> closure sugar, <code>async</code>/<code>await</code>) are instead implemented as macros within the standard library.</p>
<p>Macros take in fragments of the AST within their scope, transform them with arbitrary compile-time code, and spit back out transformed AST fragments to be injected and checked for validity. This is similar to what the Lisp family of languages do. It has a number of benefits: there is no separate metaprogramming language, it is syntactically and semantically hygienic, and the underlying framework can be reused for all kinds of compile-time code execution.</p>
<p>By keeping an intentionally minimal AST, some things not possible to express in literal code may be expressible in the AST: in particular, bindings can be injected in many places they could not be injected in ordinarily. (A minimal AST also has the benefit of being quite predictable.)</p>
<h2 id="scope"><a class="header" href="#scope">Scope</a></h2>
<p>Macros may not change Puck's syntax: the syntax is flexible enough. They have the same scope as other routines, that is:</p>
<p><strong>function scope</strong>: takes the arguments within or following a function call</p>
<pre><code class="language-puck">macro print(params: varargs) =
var res = Call("write", [stdout])
for param in params do
res.params.add(param)
print(1, 2, 3, 4)
print "hello", " ", "world", "!"
</code></pre>
<p><strong>block scope</strong>: takes the expression following a colon as a single argument</p>
<pre><code class="language-puck">macro my_macro(body)
my_macro
1
2
3
4
</code></pre>
<p><strong>operator scope</strong>: takes one or two parameters either as an infix (two parameters) or a postfix (one parameter) operator</p>
<pre><code class="language-puck"># operators are restricted to punctuation
macro +=(a, b) =
Call("=", [a, Call("+", [a, b])])
a += b
</code></pre>
<h2 id="usage"><a class="header" href="#usage">Usage</a></h2>
<p>Macros typically take a list of parameters <em>without</em> types, but they optionally may be given a type to constrain the usage of a macro. Regardless: as macros operate at compile time, their parameters are not instances of a type, but rather an <code>Expr</code> expression representing a portion of the <em>abstract syntax tree</em>.
Similarly, macros always return an <code>Expr</code> to be injected into the abstract syntax tree despite the usual absence of an explicit return type, but the return type may be specified to additionally typecheck the returned <code>Expr</code>.</p>
<pre><code class="language-puck"></code></pre>
<p>As macros operate at compile time, they may not inspect the <em>values</em> that their parameters evaluate to. However, parameters may be marked <code>const</code>: in which case they will be treated like parameters in functions: as values. (note constant parameters may be written as <code>const[T]</code> or <code>const T</code>.)</p>
<pre><code class="language-puck">macro ?[T, E](self: Result[T, E]) =
quote
match `self`
of Okay(x) then x
of Error(e) then return Error(e)
func meow: Result[bool, ref Err] =
let a = stdin.get()?
</code></pre>
<h2 id="quoting"><a class="header" href="#quoting">Quoting</a></h2>
<p>The <code>quote</code> macro is special. It takes in literal code and returns that code <strong>as the AST</strong>. Within quoted data, backticks may be used to break out in order to evaluate and inject arbitrary code: though the code must evaluate to an expression of type <code>Expr</code>. Thus, quoting is <em>structured</em>: one cannot simply quote any arbitrary section. Quoting is very powerful: most macros are implemented using it.</p>
<pre><code class="language-puck"></code></pre>
<p>The <code>Expr</code> type is available from <code>std.ast</code>, as are many helpers, and combined they provide the construction of arbitrary syntax trees (indeed, <code>quote</code> relies on and emits types of it). It is a <code>union</code> type with its variants directly corresponding to the variants of the internal AST of Puck.</p>
<pre><code class="language-puck"></code></pre>
<p>Construction of macros can be difficult: and so several helpers are provided to ease debugging. The <code>Debug</code> and <code>Display</code> interfaces are implemented for abstract syntax trees: <code>dbg</code> will print a representation of the passed syntax tree as an object, and <code>print</code> will print a best-effort representation as literal code. Together with <code>quote</code> and optionally with <code>const</code>, these can be used to quickly get the representation of arbitrary code.</p>
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