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283 lines
10 KiB
HTML
<!DOCTYPE html>
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<html>
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<head>
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<meta http-equiv="Content-type" content="text/html;charset=UTF-8" />
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<title>Functions – Wren</title>
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</head>
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<body id="top">
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<header>
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<div class="page">
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<div class="main-column">
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<h1><a href="./">wren</a></h1>
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<h2>a classy little scripting language</h2>
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</div>
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</div>
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</header>
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<div class="page">
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<nav class="big">
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<a href="./"><img src="./wren.svg" class="logo"></a>
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<ul>
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<li><a href="getting-started.html">Getting Started</a></li>
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<li><a href="contributing.html">Contributing</a></li>
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<li><a href="blog">Blog</a></li>
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<li><a href="try">Try it!</a></li>
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</ul>
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<section>
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<h2>guides</h2>
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<ul>
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<li><a href="syntax.html">Syntax</a></li>
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<li><a href="values.html">Values</a></li>
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<li><a href="lists.html">Lists</a></li>
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<li><a href="maps.html">Maps</a></li>
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<li><a href="method-calls.html">Method Calls</a></li>
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<li><a href="control-flow.html">Control Flow</a></li>
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<li><a href="variables.html">Variables</a></li>
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<li><a href="functions.html">Functions</a></li>
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<li><a href="classes.html">Classes</a></li>
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<li><a href="concurrency.html">Concurrency</a></li>
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<li><a href="error-handling.html">Error Handling</a></li>
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<li><a href="modularity.html">Modularity</a></li>
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</ul>
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</section>
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<section>
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<h2>API docs</h2>
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<ul>
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<li><a href="modules">Modules</a></li>
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</ul>
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</section>
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<section>
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<h2>reference</h2>
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<ul>
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<li><a href="cli">Wren CLI</a></li>
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<li><a href="embedding">Embedding</a></li>
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<li><a href="performance.html">Performance</a></li>
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<li><a href="qa.html">Q & A</a></li>
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</ul>
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</section>
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</nav>
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<nav class="small">
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<table>
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<tr>
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<div><a href="getting-started.html">Getting Started</a></div>
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<div><a href="contributing.html">Contributing</a></div>
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<div><a href="blog">Blog</a></div>
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<div><a href="try">Try it!</a></div>
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</tr>
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<tr>
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<td colspan="2"><h2>guides</h2></td>
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<td><h2>reference</h2></td>
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</tr>
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<tr>
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<td>
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<ul>
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<li><a href="syntax.html">Syntax</a></li>
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<li><a href="values.html">Values</a></li>
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<li><a href="lists.html">Lists</a></li>
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<li><a href="maps.html">Maps</a></li>
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<li><a href="method-calls.html">Method Calls</a></li>
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<li><a href="control-flow.html">Control Flow</a></li>
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</ul>
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</td>
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<td>
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<ul>
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<li><a href="variables.html">Variables</a></li>
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<li><a href="functions.html">Functions</a></li>
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<li><a href="classes.html">Classes</a></li>
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<li><a href="concurrency.html">Concurrency</a></li>
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<li><a href="error-handling.html">Error Handling</a></li>
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<li><a href="modularity.html">Modularity</a></li>
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</ul>
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</td>
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<td>
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<ul>
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<li><a href="modules">API/Modules</a></li>
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<li><a href="embedding">Embedding</a></li>
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<li><a href="performance.html">Performance</a></li>
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<li><a href="qa.html">Q & A</a></li>
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</ul>
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</td>
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</tr>
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</table>
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</nav>
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<main>
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<h2>Functions</h2>
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<p>No self-respecting language today can get by without functions—first
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class little bundles of code. Since Wren is object-oriented, most of your code
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will live in methods on classes, but free-floating functions are still
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eminently handy.</p>
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<p>Functions are objects like everything else in Wren, instances of the <code>Fn</code>
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class.</p>
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<h2>Block arguments <a href="#block-arguments" name="block-arguments" class="header-anchor">#</a></h2>
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<p>Most of the time you create a function just to pass it to some method. For
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example, if you want to filter a <a href="lists.html">list</a> by some criteria, you’ll
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call its <code>where</code> method, passing in a function that defines the predicate
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you’re filtering on.</p>
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<p>Since that’s the most common usage pattern, Wren’s syntax optimizes for that.
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Taking a page from Ruby, a function is created by passing a <em>block argument</em> to
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a method. At its simplest, it looks like this:</p>
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<pre class="snippet">
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blondie.callMe {
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System.print("This is the body!")
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}
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</pre>
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<p>Here we’re invoking the <code>callMe</code> method on <code>blondie</code>. We’re passing one
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argument, a function whose body is the
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following <a href="syntax.html#blocks">block</a>—everything between that pair of
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curly braces.</p>
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<p>Methods that take a block argument receive it as a normal parameter. <code>callMe</code>
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could be defined like so:</p>
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<pre class="snippet">
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class Blondie {
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callMe(fn) {
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// Call it...
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}
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}
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var blondie = Blondie.new()
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</pre>
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<p>A method can take other arguments in addition to the block. They appear before
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the block just like a regular argument list. For example:</p>
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<pre class="snippet">
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blondie.callMeAt(867, 5309) {
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System.print("This is the body!")
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}
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</pre>
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<p>Of course, you don’t <em>have</em> to use a block argument to pass a function to a
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method. If you already have a function object, you can pass it like a regular
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argument:</p>
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<pre class="snippet">
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var someFn = // Get a function...
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blondie.callMe(someFn)
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</pre>
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<p>Block arguments are purely sugar for creating a function and passing it in one
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little blob of syntax. There are some times when you want to create a function
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but <em>don’t</em> need to pass it to a method. For that, you can call the <code>Fn</code>
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class’s constructor:</p>
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<pre class="snippet">
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var someFn = Fn.new {
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System.print("Hi!")
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}
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</pre>
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<p>As you can see it takes a block argument too! All the constructor does it
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return that, so this exists purely as a convenience method for you.</p>
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<h2>Calling functions <a href="#calling-functions" name="calling-functions" class="header-anchor">#</a></h2>
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<p>Once you have a function, how do you invoke it? Like everything in Wren, you do
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so by calling a method on it:</p>
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<pre class="snippet">
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class Blondie {
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callMe(fn) {
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fn.call()
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}
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}
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</pre>
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<p>Functions expose a <code>call()</code> method that executes the body of the function. This
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method is dynamically-dispatched like any other, so you can define your own
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“function-like” classes and pass them to methods that expect “real” functions.</p>
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<pre class="snippet">
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class FakeFn {
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call() {
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System.print("I'm feeling functional!")
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}
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}
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blondie.callMe(FakeFn.new())
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</pre>
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<h2>Function parameters <a href="#function-parameters" name="function-parameters" class="header-anchor">#</a></h2>
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<p>Of course, functions aren’t very useful if you can’t pass values to them. The
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functions that we’ve seen so far take no arguments. To change that, you can
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provide a parameter list surrounded by <code>|</code> immediately after the opening brace
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of the body, like so:</p>
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<pre class="snippet">
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blondie.callMe {|first, last|
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System.print("Hi, " + first + " " + last + "!")
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}
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</pre>
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<p>Here we’re passing a function to <code>callMe</code> that takes two parameters, <code>first</code> and
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<code>last</code>. They are passed to the function when it’s called:</p>
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<pre class="snippet">
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class Blondie {
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callMe(fn) {
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fn.call("Debbie", "Harry")
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}
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}
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</pre>
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<p>It’s an error to call a function with fewer arguments than its parameter list
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expects. If you pass too <em>many</em> arguments, the extras are ignored.</p>
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<h2>Returning values <a href="#returning-values" name="returning-values" class="header-anchor">#</a></h2>
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<p>The body of a function is a <a href="syntax.html#blocks">block</a>. If it is a single
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expression—more precisely if there is no newline after the <code>{</code> or
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parameter list—then the function implicitly returns the value of the
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expression.</p>
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<p>Otherwise, the body returns <code>null</code> by default. You can explicitly return a
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value using a <code>return</code> statement. In other words, these two functions do the
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same thing:</p>
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<pre class="snippet">
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Fn.new { "return value" }
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Fn.new {
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return "return value"
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}
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</pre>
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<h2>Closures <a href="#closures" name="closures" class="header-anchor">#</a></h2>
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<p>As you expect, functions are closures—they can access variables defined
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outside of their scope. They will hold onto closed-over variables even after
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leaving the scope where the function is defined:</p>
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<pre class="snippet">
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class Counter {
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static create() {
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var i = 0
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return Fn.new { i = i + 1 }
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}
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}
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</pre>
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<p>Here, the <code>create</code> method returns the function created on its second line. That
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function references a variable <code>i</code> declared outside of the function. Even after
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the function is returned from <code>create</code>, it is still able to read and assign
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to<code>i</code>:</p>
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<pre class="snippet">
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var counter = Counter.create()
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System.print(counter.call()) //> 1
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System.print(counter.call()) //> 2
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System.print(counter.call()) //> 3
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</pre>
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<p><br><hr>
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<a class="right" href="classes.html">Classes →</a>
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<a href="variables.html">← Variables</a></p>
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</main>
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</div>
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<footer>
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<div class="page">
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<div class="main-column">
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<p>Wren lives
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<a href="https://github.com/wren-lang/wren">on GitHub</a>
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— Made with ❤ by
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<a href="http://journal.stuffwithstuff.com/">Bob Nystrom</a> and
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<a href="https://github.com/wren-lang/wren/blob/main/AUTHORS">friends</a>.
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</p>
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<div class="main-column">
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</div>
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</footer>
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</body>
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</html>
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