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<!DOCTYPE html>
<html>
<head>
<meta http-equiv="Content-type" content="text/html;charset=UTF-8" />
<title>Classes &ndash; Wren</title>
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<h1><a href="./">wren</a></h1>
<h2>a classy little scripting language</h2>
</div>
</div>
</header>
<div class="page">
<nav class="big">
<a href="./"><img src="./wren.svg" class="logo"></a>
<ul>
<li><a href="getting-started.html">Getting Started</a></li>
<li><a href="contributing.html">Contributing</a></li>
<li><a href="blog">Blog</a></li>
</ul>
<section>
<h2>guides</h2>
<ul>
<li><a href="syntax.html">Syntax</a></li>
<li><a href="values.html">Values</a></li>
<li><a href="lists.html">Lists</a></li>
<li><a href="maps.html">Maps</a></li>
<li><a href="method-calls.html">Method Calls</a></li>
<li><a href="control-flow.html">Control Flow</a></li>
<li><a href="variables.html">Variables</a></li>
<li><a href="functions.html">Functions</a></li>
<li><a href="classes.html">Classes</a></li>
<li><a href="concurrency.html">Concurrency</a></li>
<li><a href="error-handling.html">Error Handling</a></li>
<li><a href="modularity.html">Modularity</a></li>
</ul>
</section>
<section>
<h2>reference</h2>
<ul>
<li><a href="modules">Modules</a></li>
<li><a href="embedding">Embedding</a></li>
<li><a href="performance.html">Performance</a></li>
<li><a href="qa.html">Q &amp; A</a></li>
</ul>
</section>
</nav>
<nav class="small">
<table>
<tr>
<div><a href="getting-started.html">Getting Started</a></div>
<div><a href="contributing.html">Contributing</a></div>
<div><a href="blog">Blog</a></div>
</tr>
<tr>
<td colspan="2"><h2>guides</h2></td>
<td><h2>reference</h2></td>
</tr>
<tr>
<td>
<ul>
<li><a href="syntax.html">Syntax</a></li>
<li><a href="values.html">Values</a></li>
<li><a href="lists.html">Lists</a></li>
<li><a href="maps.html">Maps</a></li>
<li><a href="method-calls.html">Method Calls</a></li>
<li><a href="control-flow.html">Control Flow</a></li>
</ul>
</td>
<td>
<ul>
<li><a href="variables.html">Variables</a></li>
<li><a href="functions.html">Functions</a></li>
<li><a href="classes.html">Classes</a></li>
<li><a href="concurrency.html">Concurrency</a></li>
<li><a href="error-handling.html">Error Handling</a></li>
<li><a href="modularity.html">Modularity</a></li>
</ul>
</td>
<td>
<ul>
<li><a href="modules">Modules</a></li>
<li><a href="embedding">Embedding</a></li>
<li><a href="performance.html">Performance</a></li>
<li><a href="qa.html">Q &amp; A</a></li>
</ul>
</td>
</tr>
</table>
</nav>
<main>
<h2>Classes</h2>
<p>Every value in Wren is an object, and every object is an instance of a class.
Even <code>true</code> and <code>false</code> are full-featured objects&mdash;instances of the
<a href="modules/core/bool.html">Bool</a> class. </p>
<p>Classes define an objects <em>behavior</em> and <em>state</em>. Behavior is defined by
<a href="method-calls.html"><em>methods</em></a> which live in the class. Every object of the same
class supports the same methods. State is defined in <em>fields</em>, whose values are
stored in each instance. </p>
<h2>Defining a class <a href="#defining-a-class" name="defining-a-class" class="header-anchor">#</a></h2>
<p>Classes are created using the <code>class</code> keyword, unsurprisingly: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{}</span>
</pre></div>
<p>This creates a class named <code>Unicorn</code> with no methods or fields. </p>
<h2>Methods <a href="#methods" name="methods" class="header-anchor">#</a></h2>
<p>To let our unicorn do stuff, we need to give it methods. </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">prance</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;The unicorn prances in a fancy manner!&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>This defines a <code>prance()</code> method that takes no arguments. To add parameters, put
their names inside the parentheses: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">prance</span><span class="p">(</span><span class="n">where</span><span class="p">,</span> <span class="n">when</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;The unicorn prances in </span><span class="si">%(</span><span class="n">where</span><span class="si">)</span><span class="s"> at </span><span class="si">%(</span><span class="n">when</span><span class="si">)</span><span class="s">.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Since the number of parameters is part of a method&rsquo;s <a href="method-calls.html#signature">signature</a> a class can
define multiple methods with the same name: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">prance</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;The unicorn prances in a fancy manner!&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="n">prance</span><span class="p">(</span><span class="n">where</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;The unicorn prances in </span><span class="si">%(</span><span class="n">where</span><span class="si">)</span><span class="s">.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="n">prance</span><span class="p">(</span><span class="n">where</span><span class="p">,</span> <span class="n">when</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;The unicorn prances in </span><span class="si">%(</span><span class="n">where</span><span class="si">)</span><span class="s"> at </span><span class="si">%(</span><span class="n">when</span><span class="si">)</span><span class="s">.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>It&rsquo;s often natural to have the same conceptual operation work with different
sets of arguments. In other languages, you&rsquo;d define a single method for the
operation and have to check for missing optional arguments. In Wren, they are
different methods that you implement separately. </p>
<p>In addition to named methods with parameter lists, Wren has a bunch of other
different syntaxes for methods. Your classes can define all of them. </p>
<h3>Getters <a href="#getters" name="getters" class="header-anchor">#</a></h3>
<p>A getter leaves off the parameter list and the parentheses: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="c1">// Unicorns are always fancy.</span>
<span class="n">isFancy</span> <span class="p">{</span> <span class="kc">true</span> <span class="p">}</span>
<span class="p">}</span>
</pre></div>
<h3>Setters <a href="#setters" name="setters" class="header-anchor">#</a></h3>
<p>A setter has <code>=</code> after the name, followed by a single parenthesized parameter: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">rider</span><span class="o">=</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;I am being ridden by </span><span class="si">%(</span><span class="n">value</span><span class="si">)</span><span class="s">.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>By convention, the parameter is usually named <code>value</code> but you can call it
whatever makes your heart flutter. </p>
<h3>Operators <a href="#operators" name="operators" class="header-anchor">#</a></h3>
<p>Prefix operators, like getters, have no parameter list: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="o">-</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Negating a unicorn is weird.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Infix operators, like setters, have a single parenthesized parameter for the
right-hand operand: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="o">-</span><span class="p">(</span><span class="n">other</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Subtracting </span><span class="si">%(</span><span class="n">other</span><span class="si">)</span><span class="s"> from a unicorn is weird.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>A subscript operator puts the parameters inside square brackets and can have
more than one: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="p">[</span><span class="n">index</span><span class="p">]</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Unicorns are not lists!&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">[</span><span class="err">x</span><span class="p">,</span> <span class="err">y</span><span class="p">]</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;Unicorns are not matrices either!&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Unlike with named methods, you can&rsquo;t define a subscript operator with an empty
parameter list. </p>
<p>As the name implies, a subscript setter looks like a combination of a subscript
operator and a setter: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="p">[</span><span class="n">index</span><span class="p">]</span><span class="o">=</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;You can&#39;t stuff </span><span class="si">%(</span><span class="n">value</span><span class="si">)</span><span class="s"> into me at </span><span class="si">%(</span><span class="n">index</span><span class="si">)</span><span class="s">!&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<h2>Method Scope <a href="#method-scope" name="method-scope" class="header-anchor">#</a></h2>
<p>Up to this point, &ldquo;<a href="variables.html#scope">scope</a>&rdquo; has been used to talk exclusively about
<a href="variables.html">variables</a>. In a procedural language like C, or a functional one like Scheme,
that&rsquo;s the only kind of scope there is. But object-oriented languages like Wren
introduce another kind of scope: <em>object scope</em>. It contains the methods that
are available on an object. When you write: </p>
<div class="codehilite"><pre><span class="n">unicorn</span><span class="o">.</span><span class="n">isFancy</span>
</pre></div>
<p>You&rsquo;re saying &ldquo;look up the method <code>isFancy</code> in the scope of the object
<code>unicorn</code>&rdquo;. In this case, the fact that you want to look up a <em>method</em>
<code>isFancy</code> and not a <em>variable</em> <code>isFancy</code> is explicit. That&rsquo;s what <code>.</code> does and
the object to the left of the period is the object you want to look up the
method on. </p>
<h3><code>this</code> <a href="#this" name="this" class="header-anchor">#</a></h3>
<p>Things get more interesting when you&rsquo;re inside the body of a method. When the
method is called on some object and the body is being executed, you often need
to access that object itself. You can do that using <code>this</code>. </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">name</span> <span class="p">{</span> <span class="s">&quot;Francis&quot;</span> <span class="p">}</span>
<span class="n">printName</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="nb">this</span><span class="o">.</span><span class="n">name</span><span class="p">)</span> <span class="output">Francis</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>The <code>this</code> keyword works sort of like a variable, but has special behavior. It
always refers to the instance whose method is currently being executed. This
lets you invoke methods on &ldquo;yourself&rdquo;. </p>
<p>It&rsquo;s an error to refer to <code>this</code> outside of a method. However, it&rsquo;s perfectly
fine to use it inside a <a href="functions.html">function</a> declared <em>inside</em> a method. When you do,
<code>this</code> still refers to the instance whose <em>method</em> is being called: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">name</span> <span class="p">{</span> <span class="s">&quot;Francis&quot;</span> <span class="p">}</span>
<span class="n">printNameThrice</span><span class="p">()</span> <span class="p">{</span>
<span class="p">(</span><span class="mi">1</span><span class="o">..</span><span class="mi">3</span><span class="p">)</span><span class="o">.</span><span class="n">each</span> <span class="p">{</span>
<span class="c1">// Use &quot;this&quot; inside the function passed to each().</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="nb">this</span><span class="o">.</span><span class="n">name</span><span class="p">)</span> <span class="output">Francis</span>
<span class="p">}</span> <span class="output">Francis</span>
<span class="p">}</span> <span class="output">Francis</span>
<span class="p">}</span>
</pre></div>
<p>This is unlike Lua and JavaScript which can &ldquo;forget&rdquo; <code>this</code> when you create a
callback inside a method. Wren does what you want here and retains the
reference to the original object. </p>
<p>(In technical terms, a function&rsquo;s closure includes <code>this</code>. Wren can do this
because it makes a distinction between methods and functions.) </p>
<h3>Implicit <code>this</code> <a href="#implicit-this" name="implicit-this" class="header-anchor">#</a></h3>
<p>Using <code>this.</code> every time you want to call a method on yourself works, but it&rsquo;s
tedious and verbose, which is why some languages don&rsquo;t require it. You can do a
&ldquo;self send&rdquo; by calling a method (or getter or setter) without any explicit
receiver: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">name</span> <span class="p">{</span> <span class="s">&quot;Francis&quot;</span> <span class="p">}</span>
<span class="n">printName</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="output">Francis</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Code like this gets tricky when there is also a variable outside of the class
with the same name. Consider: </p>
<div class="codehilite"><pre><span class="k">var</span> <span class="n">name</span> <span class="o">=</span> <span class="s">&quot;variable&quot;</span>
<span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="n">name</span> <span class="p">{</span> <span class="s">&quot;Francis&quot;</span> <span class="p">}</span>
<span class="n">printName</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="c1">// ???</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Should <code>printName()</code> print &ldquo;variable&rdquo; or &ldquo;Francis&rdquo;? A method body has a foot in
each of two worlds. It is surrounded by the lexical scope where it&rsquo;s defined in
the program, but it also has the object scope of the methods on <code>this</code>. </p>
<p>Which scope wins? Every language has to decide how to handle this and there
is a surprising plethora of approaches. Wren&rsquo;s approach to resolving a name
inside a method works like this: </p>
<ol>
<li>If there is a local variable inside the method with that name, that wins. </li>
<li>Else, if the name starts with a lowercase letter, treat it like a method on
<code>this</code>. </li>
<li>Otherwise, look for a variable with that name in the surrounding scope. </li>
</ol>
<p>So, in the above example, we hit case #2 and it prints &ldquo;Francis&rdquo;. Distinguishing
self sends from outer variables based on the <em>case</em> of the first letter in the
name probably seems crazy but it works surprisingly well. Method names are
lowercase in Wren. Class names are capitalized. </p>
<p>Most of the time, when you&rsquo;re in a method and want to access a name from outside
of the class, it&rsquo;s usually the name of some other class. This rule makes that
work. </p>
<p>Here&rsquo;s an example that shows all three cases: </p>
<div class="codehilite"><pre><span class="k">var</span> <span class="n">shadowed</span> <span class="o">=</span> <span class="s">&quot;surrounding&quot;</span>
<span class="k">var</span> <span class="n">lowercase</span> <span class="o">=</span> <span class="s">&quot;surrounding&quot;</span>
<span class="k">var</span> <span class="vg">Capitalized</span> <span class="o">=</span> <span class="s">&quot;surrounding&quot;</span>
<span class="k">class</span> <span class="vg">Scope</span> <span class="p">{</span>
<span class="n">shadowed</span> <span class="p">{</span> <span class="s">&quot;object&quot;</span> <span class="p">}</span>
<span class="n">lowercase</span> <span class="p">{</span> <span class="s">&quot;object&quot;</span> <span class="p">}</span>
<span class="vg">Capitalized</span> <span class="p">{</span> <span class="s">&quot;object&quot;</span> <span class="p">}</span>
<span class="n">test</span><span class="p">()</span> <span class="p">{</span>
<span class="k">var</span> <span class="n">shadowed</span> <span class="o">=</span> <span class="s">&quot;local&quot;</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="n">shadowed</span><span class="p">)</span> <span class="output">local</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="n">lowercase</span><span class="p">)</span> <span class="output">object</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="vg">Capitalized</span><span class="p">)</span> <span class="output">surrounding</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>It&rsquo;s a bit of a strange rule, but Ruby works more or less the same way. </p>
<h2>Constructors <a href="#constructors" name="constructors" class="header-anchor">#</a></h2>
<p>We&rsquo;ve seen how to define kinds of objects and how to declare methods on them.
Our unicorns can prance around, but we don&rsquo;t actually <em>have</em> any unicorns to do
it. To create <em>instances</em> of a class, we need a <em>constructor</em>. You define one
like so: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">(</span><span class="n">name</span><span class="p">,</span> <span class="n">color</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;My name is &quot;</span> <span class="o">+</span> <span class="n">name</span> <span class="o">+</span> <span class="s">&quot; and I am &quot;</span> <span class="o">+</span> <span class="n">color</span> <span class="o">+</span> <span class="s">&quot;.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>The <code>construct</code> keyword says we&rsquo;re defining a constructor, and <code>new</code> is its
name. In Wren, all constructors have names. The word &ldquo;new&rdquo; isn&rsquo;t special to
Wren, it&rsquo;s just a common constructor name. </p>
<p>To make a unicorn now, we call the constructor method on the class itself: </p>
<div class="codehilite"><pre><span class="k">var</span> <span class="n">fred</span> <span class="o">=</span> <span class="vg">Unicorn</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s">&quot;Fred&quot;</span><span class="p">,</span> <span class="s">&quot;palomino&quot;</span><span class="p">)</span>
</pre></div>
<p>Giving constructors names is handy because it means you can have more than one,
and each can clarify how it creates the instance: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">brown</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;My name is &quot;</span> <span class="o">+</span> <span class="n">name</span> <span class="o">+</span> <span class="s">&quot; and I am brown.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">var</span> <span class="n">dave</span> <span class="o">=</span> <span class="vg">Unicorn</span><span class="o">.</span><span class="n">brown</span><span class="p">(</span><span class="s">&quot;Dave&quot;</span><span class="p">)</span>
</pre></div>
<p>Note that we have to declare a constructor because, unlike some other
languages, Wren doesn&rsquo;t give you a default one. This is useful because some
classes aren&rsquo;t designed to be constructed. If you have an abstract base class
that just contains methods to be inherited by other classes, it doesn&rsquo;t need
and won&rsquo;t have a constructor. </p>
<p>Like other methods, constructors can obviously have arguments, and can be
overloaded by <a href="#signature">arity</a>. A constructor <em>must</em> be a named method with
a (possibly empty) argument list. Operators, getters, and setters cannot be
constructors. </p>
<p>A constructor is actually a pair of methods. You get a method on the class: </p>
<div class="codehilite"><pre><span class="vg">Unicorn</span><span class="o">.</span><span class="n">brown</span><span class="p">(</span><span class="s">&quot;Dave&quot;</span><span class="p">)</span>
</pre></div>
<p>That creates the new instance, then it invokes the <em>initializer</em> on that
instance. This is where the constructor body you defined gets run. </p>
<p>This distinction is important because it means inside the body of the
constructor, you can access <code>this</code>, assign <a href="#fields">fields</a>, call superclass
constructors, etc. </p>
<h2>Fields <a href="#fields" name="fields" class="header-anchor">#</a></h2>
<p>All state stored in instances is stored in <em>fields</em>. Each field has a name
that starts with an underscore. </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Rectangle</span> <span class="p">{</span>
<span class="n">area</span> <span class="p">{</span> <span class="vi">_width</span> <span class="o">*</span> <span class="vi">_height</span> <span class="p">}</span>
<span class="c1">// Other stuff...</span>
<span class="p">}</span>
</pre></div>
<p>Here, <code>_width</code> and <code>_height</code> in the <code>area</code> <a href="classes.html#methods">getter</a> refer
to fields on the rectangle instance. You can think of them like <code>this.width</code>
and <code>this.height</code> in other languages. </p>
<p>When a field name appears, Wren looks for the nearest enclosing class and looks
up the field on the instance of that class. Field names cannot be used outside
of an instance method. They <em>can</em> be used inside a <a href="functions.html">function</a>
in a method. Wren will look outside any nested functions until it finds an
enclosing method. </p>
<p>Unlike <a href="variables.html">variables</a>, fields are implicitly declared by simply
assigning to them. If you access a field before it has been initialized, its
value is <code>null</code>. </p>
<h3>Encapsulation <a href="#encapsulation" name="encapsulation" class="header-anchor">#</a></h3>
<p>All fields are <em>private</em> in Wren&mdash;an object&rsquo;s fields can only be directly
accessed from within methods defined on the object&rsquo;s class. </p>
<p>In short, if you want to make a property of an object visible,
<strong>you need to define a getter to expose it</strong>: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Rectangle</span> <span class="p">{</span>
<span class="n">width</span> <span class="p">{</span> <span class="vi">_width</span> <span class="p">}</span>
<span class="n">height</span> <span class="p">{</span> <span class="vi">_height</span> <span class="p">}</span>
<span class="c1">// ...</span>
<span class="p">}</span>
</pre></div>
<p>To allow outside code to modify the field,
<strong>you need to provide setters to provide access</strong>: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Rectangle</span> <span class="p">{</span>
<span class="n">width</span><span class="o">=</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="p">{</span> <span class="vi">_width</span> <span class="o">=</span> <span class="n">value</span> <span class="p">}</span>
<span class="n">height</span><span class="o">=</span><span class="p">(</span><span class="n">value</span><span class="p">)</span> <span class="p">{</span> <span class="vi">_height</span> <span class="o">=</span> <span class="n">value</span> <span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>This might be different from what you&rsquo;re used to, so here are two important facts: </p>
<ul>
<li>You can&rsquo;t access fields from a base class. </li>
<li>You can&rsquo;t access fields on another instance of your own class. </li>
</ul>
<p>Here is an example in code: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Shape</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">()</span> <span class="p">{</span>
<span class="vi">_shape</span> <span class="o">=</span> <span class="s">&quot;none&quot;</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="vg">Rectangle</span> <span class="k">is</span> <span class="vg">Shape</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">()</span> <span class="p">{</span>
<span class="c1">//This will print null!</span>
<span class="c1">//_shape from the parent class is private,</span>
<span class="c1">//we are reading `_shape` from `this`,</span>
<span class="c1">//which has not been set, so returns null.</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;I am a </span><span class="si">%(</span><span class="vi">_shape</span><span class="si">)</span><span class="s">&quot;</span><span class="p">)</span>
<span class="c1">//a local variable, all variables are private</span>
<span class="vi">_width</span> <span class="o">=</span> <span class="mi">10</span>
<span class="k">var</span> <span class="n">other</span> <span class="o">=</span> <span class="vg">Rectangle</span><span class="o">.</span><span class="n">new</span><span class="p">()</span>
<span class="c1">//other._width is not accessible from here,</span>
<span class="c1">//even though we are also a rectangle. The field</span>
<span class="c1">//is private, and other._width is invalid syntax!</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="o">...</span>
</pre></div>
<p>One thing we&rsquo;ve learned in the past forty years of software engineering is that
encapsulating state tends to make code easier to maintain, so Wren defaults to
keeping your object&rsquo;s state pretty tightly bundled up. Don&rsquo;t feel that you have
to or even should define getters or setters for most of your object&rsquo;s fields. </p>
<h2>Metaclasses and static members <a href="#metaclasses-and-static-members" name="metaclasses-and-static-members" class="header-anchor">#</a></h2>
<p><strong>TODO</strong> </p>
<h3>Static fields <a href="#static-fields" name="static-fields" class="header-anchor">#</a></h3>
<p>A name that starts with <em>two</em> underscores is a <em>static</em> field. They work
similar to <a href="#fields">fields</a> except the data is stored on the class itself, and
not the instance. They can be used in <em>both</em> instance and static methods. </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Foo</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">()</span> <span class="p">{}</span>
<span class="k">static</span> <span class="n">setFromStatic</span><span class="p">(</span><span class="err">a</span><span class="p">)</span> <span class="p">{</span> <span class="vc">__a</span> <span class="o">=</span> <span class="err">a</span> <span class="p">}</span>
<span class="n">setFromInstance</span><span class="p">(</span><span class="err">a</span><span class="p">)</span> <span class="p">{</span> <span class="vc">__a</span> <span class="o">=</span> <span class="err">a</span> <span class="p">}</span>
<span class="k">static</span> <span class="n">printFromStatic</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="vc">__a</span><span class="p">)</span>
<span class="p">}</span>
<span class="n">printFromInstance</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="vc">__a</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>Just like instance fields, static fields are initially <code>null</code>: </p>
<div class="codehilite"><pre><span class="vg">Foo</span><span class="o">.</span><span class="n">printFromStatic</span><span class="p">()</span> <span class="output">null</span>
</pre></div>
<p>They can be used from static methods: </p>
<div class="codehilite"><pre><span class="vg">Foo</span><span class="o">.</span><span class="n">setFromStatic</span><span class="p">(</span><span class="s">&quot;first&quot;</span><span class="p">)</span>
<span class="vg">Foo</span><span class="o">.</span><span class="n">printFromStatic</span><span class="p">()</span> <span class="output">first</span>
</pre></div>
<p>And also instance methods. When you do so, there is still only one static field
shared among all instances of the class: </p>
<div class="codehilite"><pre><span class="k">var</span> <span class="n">foo1</span> <span class="o">=</span> <span class="vg">Foo</span><span class="o">.</span><span class="n">new</span><span class="p">()</span>
<span class="k">var</span> <span class="n">foo2</span> <span class="o">=</span> <span class="vg">Foo</span><span class="o">.</span><span class="n">new</span><span class="p">()</span>
<span class="n">foo1</span><span class="o">.</span><span class="n">setFromInstance</span><span class="p">(</span><span class="s">&quot;second&quot;</span><span class="p">)</span>
<span class="n">foo2</span><span class="o">.</span><span class="n">printFromInstance</span><span class="p">()</span> <span class="output">second</span>
</pre></div>
<h2>Inheritance <a href="#inheritance" name="inheritance" class="header-anchor">#</a></h2>
<p>A class can inherit from a &ldquo;parent&rdquo; or <em>superclass</em>. When you invoke a method
on an object of some class, if it can&rsquo;t be found, it walks up the chain of
superclasses looking for it there. </p>
<p>By default, any new class inherits from Object, which is the superclass from
which all other classes ultimately descend. You can specify a different parent
class using <code>is</code> when you declare the class: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Pegasus</span> <span class="k">is</span> <span class="vg">Unicorn</span> <span class="p">{}</span>
</pre></div>
<p>This declares a new class Pegasus that inherits from Unicorn. </p>
<p>Note that you should not create classes that inherit from the built-in types
(Bool, Num, String, Range, List). The built-in types expect their internal bit
representation to be very specific and get horribly confused when you invoke one
of the inherited built-in methods on the derived type. </p>
<p>The metaclass hierarchy does <em>not</em> parallel the regular class hierarchy. So, if
Pegasus inherits from Unicorn, Pegasus&rsquo;s metaclass does not inherit from
Unicorn&rsquo;s metaclass. In more prosaic terms, this means that static methods are
not inherited. </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="c1">// Unicorns cannot fly. :(</span>
<span class="k">static</span> <span class="n">canFly</span> <span class="p">{</span> <span class="kc">false</span> <span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="vg">Pegasus</span> <span class="k">is</span> <span class="vg">Unicorn</span> <span class="p">{}</span>
<span class="vg">Pegasus</span><span class="o">.</span><span class="n">canFly</span> <span class="error">Static methods are not inherited.</span>
</pre></div>
<p>This also means constructors are not inherited: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;My name is &quot;</span> <span class="o">+</span> <span class="n">name</span> <span class="o">+</span> <span class="s">&quot;.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="vg">Pegasus</span> <span class="k">is</span> <span class="vg">Unicorn</span> <span class="p">{}</span>
<span class="vg">Pegasus</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s">&quot;Fred&quot;</span><span class="p">)</span> <span class="error">Pegasus does not define new().</span>
</pre></div>
<p>Each class gets to control how it may be constructed independently of its base
classes. However, constructor <em>initializers</em> are inherited since those are
instance methods on the new object. </p>
<p>This means you can do <code>super</code> calls inside a constructor: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;My name is &quot;</span> <span class="o">+</span> <span class="n">name</span> <span class="o">+</span> <span class="s">&quot;.&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="vg">Pegasus</span> <span class="k">is</span> <span class="vg">Unicorn</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">(</span><span class="n">name</span><span class="p">)</span> <span class="p">{</span>
<span class="k">super</span><span class="p">(</span><span class="n">name</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="vg">Pegasus</span><span class="o">.</span><span class="n">new</span><span class="p">(</span><span class="s">&quot;Fred&quot;</span><span class="p">)</span> <span class="output">My name is Fred</span>
</pre></div>
<h2>Super <a href="#super" name="super" class="header-anchor">#</a></h2>
<p><strong>TODO: Integrate better into page. Should explain this before mentioning
super above.</strong> </p>
<p>Sometimes you want to invoke a method on yourself, but using methods defined in
one of your <a href="classes.html#inheritance">superclasses</a>. You typically do this in
an overridden method when you want to access the original method being
overridden. </p>
<p>To do that, you can use the special <code>super</code> keyword as the receiver in a method
call: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Base</span> <span class="p">{</span>
<span class="n">method</span><span class="p">()</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;base method&quot;</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="vg">Derived</span> <span class="k">is</span> <span class="vg">Base</span> <span class="p">{</span>
<span class="n">method</span><span class="p">()</span> <span class="p">{</span>
<span class="k">super</span><span class="o">.</span><span class="n">method</span><span class="p">()</span> <span class="output">base method</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p>You can also use <code>super</code> without a method name inside a constructor to invoke a
base class constructor: </p>
<div class="codehilite"><pre><span class="k">class</span> <span class="vg">Base</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">(</span><span class="n">arg</span><span class="p">)</span> <span class="p">{</span>
<span class="vg">System</span><span class="o">.</span><span class="n">print</span><span class="p">(</span><span class="s">&quot;base got &quot;</span> <span class="o">+</span> <span class="n">arg</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="k">class</span> <span class="vg">Derived</span> <span class="k">is</span> <span class="vg">Base</span> <span class="p">{</span>
<span class="k">construct</span> <span class="n">new</span><span class="p">()</span> <span class="p">{</span>
<span class="k">super</span><span class="p">(</span><span class="s">&quot;value&quot;</span><span class="p">)</span> <span class="output">base got value</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
<p><br><hr>
<a class="right" href="concurrency.html">Concurrency &rarr;</a>
<a href="functions.html">&larr; Functions</a> </p>
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