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closures
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Closures</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'variadic-functions';
}
if (e.key == "ArrowRight") {
window.location.href = 'recursion';
}
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<body>
<div class="example" id="closures">
<h2><a href="./">Go by Example</a>: Closures</h2>
<table>
<tr>
<td class="docs">
<p>Go supports <a href="http://en.wikipedia.org/wiki/Anonymous_function"><em>anonymous functions</em></a>,
which can form <a href="http://en.wikipedia.org/wiki/Closure_(computer_science)"><em>closures</em></a>.
Anonymous functions are useful when you want to define
a function inline without having to name it.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/66Lgw9iIIch"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="s">"fmt"</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>This function <code>intSeq</code> returns another function, which
we define anonymously in the body of <code>intSeq</code>. The
returned function <em>closes over</em> the variable <code>i</code> to
form a closure.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">intSeq</span><span class="p">()</span> <span class="kd">func</span><span class="p">()</span> <span class="kt">int</span> <span class="p">{</span>
<span class="nx">i</span> <span class="o">:=</span> <span class="mi">0</span>
<span class="k">return</span> <span class="kd">func</span><span class="p">()</span> <span class="kt">int</span> <span class="p">{</span>
<span class="nx">i</span><span class="o">++</span>
<span class="k">return</span> <span class="nx">i</span>
<span class="p">}</span>
<span class="p">}</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>We call <code>intSeq</code>, assigning the result (a function)
to <code>nextInt</code>. This function value captures its
own <code>i</code> value, which will be updated each time
we call <code>nextInt</code>.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">nextInt</span> <span class="o">:=</span> <span class="nf">intSeq</span><span class="p">()</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>See the effect of the closure by calling <code>nextInt</code>
a few times.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">nextInt</span><span class="p">())</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">nextInt</span><span class="p">())</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">nextInt</span><span class="p">())</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>To confirm that the state is unique to that
particular function, create and test a new one.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="nx">newInts</span> <span class="o">:=</span> <span class="nf">intSeq</span><span class="p">()</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="nf">newInts</span><span class="p">())</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="gp">$</span> go run closures.go
<span class="go">1
</span><span class="go">2
</span><span class="go">3
</span><span class="go">1</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>The last feature of functions we’ll look at for now is
recursion.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="recursion">Recursion</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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