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    <div class="example" id="generics">
      <h2><a href="./">Go by Example</a>: Generics</h2>
      
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          <td class="docs">
            <p>Starting with version 1.18, Go has added support for
<em>generics</em>, also known as <em>type parameters</em>.</p>

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            <a href="https://go.dev/play/p/uXlb-AyeYmQ"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">package</span> <span class="nx">main</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kn">import</span> <span class="s">&#34;fmt&#34;</span></span></span></code></pre>
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          <td class="docs">
            <p>As an example of a generic function, <code>MapKeys</code> takes
a map of any type and returns a slice of its keys.
This function has two type parameters - <code>K</code> and <code>V</code>;
<code>K</code> has the <code>comparable</code> <em>constraint</em>, meaning that
we can compare values of this type with the <code>==</code> and
<code>!=</code> operators. This is required for map keys in Go.
<code>V</code> has the <code>any</code> constraint, meaning that it&rsquo;s not
restricted in any way (<code>any</code> is an alias for <code>interface{}</code>).</p>

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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nx">MapKeys</span><span class="p">[</span><span class="nx">K</span> <span class="nx">comparable</span><span class="p">,</span> <span class="nx">V</span> <span class="nx">any</span><span class="p">](</span><span class="nx">m</span> <span class="kd">map</span><span class="p">[</span><span class="nx">K</span><span class="p">]</span><span class="nx">V</span><span class="p">)</span> <span class="p">[]</span><span class="nx">K</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="nx">r</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="nx">K</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="nb">len</span><span class="p">(</span><span class="nx">m</span><span class="p">))</span>
</span></span><span class="line"><span class="cl">    <span class="k">for</span> <span class="nx">k</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">m</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <span class="nx">r</span> <span class="p">=</span> <span class="nb">append</span><span class="p">(</span><span class="nx">r</span><span class="p">,</span> <span class="nx">k</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="p">}</span>
</span></span><span class="line"><span class="cl">    <span class="k">return</span> <span class="nx">r</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            <p>As an example of a generic type, <code>List</code> is a
singly-linked list with values of any type.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">List</span><span class="p">[</span><span class="nx">T</span> <span class="nx">any</span><span class="p">]</span> <span class="kd">struct</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="nx">head</span><span class="p">,</span> <span class="nx">tail</span> <span class="o">*</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">element</span><span class="p">[</span><span class="nx">T</span> <span class="nx">any</span><span class="p">]</span> <span class="kd">struct</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="nx">next</span> <span class="o">*</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]</span>
</span></span><span class="line"><span class="cl">    <span class="nx">val</span>  <span class="nx">T</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            <p>We can define methods on generic types just like we
do on regular types, but we have to keep the type
parameters in place. The type is <code>List[T]</code>, not <code>List</code>.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="p">(</span><span class="nx">lst</span> <span class="o">*</span><span class="nx">List</span><span class="p">[</span><span class="nx">T</span><span class="p">])</span> <span class="nf">Push</span><span class="p">(</span><span class="nx">v</span> <span class="nx">T</span><span class="p">)</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="k">if</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span> <span class="o">==</span> <span class="kc">nil</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <span class="nx">lst</span><span class="p">.</span><span class="nx">head</span> <span class="p">=</span> <span class="o">&amp;</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]{</span><span class="nx">val</span><span class="p">:</span> <span class="nx">v</span><span class="p">}</span>
</span></span><span class="line"><span class="cl">        <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span> <span class="p">=</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">head</span>
</span></span><span class="line"><span class="cl">    <span class="p">}</span> <span class="k">else</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span><span class="p">.</span><span class="nx">next</span> <span class="p">=</span> <span class="o">&amp;</span><span class="nx">element</span><span class="p">[</span><span class="nx">T</span><span class="p">]{</span><span class="nx">val</span><span class="p">:</span> <span class="nx">v</span><span class="p">}</span>
</span></span><span class="line"><span class="cl">        <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span> <span class="p">=</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">tail</span><span class="p">.</span><span class="nx">next</span>
</span></span><span class="line"><span class="cl">    <span class="p">}</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="p">(</span><span class="nx">lst</span> <span class="o">*</span><span class="nx">List</span><span class="p">[</span><span class="nx">T</span><span class="p">])</span> <span class="nf">GetAll</span><span class="p">()</span> <span class="p">[]</span><span class="nx">T</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">var</span> <span class="nx">elems</span> <span class="p">[]</span><span class="nx">T</span>
</span></span><span class="line"><span class="cl">    <span class="k">for</span> <span class="nx">e</span> <span class="o">:=</span> <span class="nx">lst</span><span class="p">.</span><span class="nx">head</span><span class="p">;</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span><span class="p">;</span> <span class="nx">e</span> <span class="p">=</span> <span class="nx">e</span><span class="p">.</span><span class="nx">next</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">        <span class="nx">elems</span> <span class="p">=</span> <span class="nb">append</span><span class="p">(</span><span class="nx">elems</span><span class="p">,</span> <span class="nx">e</span><span class="p">.</span><span class="nx">val</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="p">}</span>
</span></span><span class="line"><span class="cl">    <span class="k">return</span> <span class="nx">elems</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl">    <span class="kd">var</span> <span class="nx">m</span> <span class="p">=</span> <span class="kd">map</span><span class="p">[</span><span class="kt">int</span><span class="p">]</span><span class="kt">string</span><span class="p">{</span><span class="mi">1</span><span class="p">:</span> <span class="s">&#34;2&#34;</span><span class="p">,</span> <span class="mi">2</span><span class="p">:</span> <span class="s">&#34;4&#34;</span><span class="p">,</span> <span class="mi">4</span><span class="p">:</span> <span class="s">&#34;8&#34;</span><span class="p">}</span></span></span></code></pre>
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          <td class="docs">
            <p>When invoking generic functions, we can often rely
on <em>type inference</em>. Note that we don&rsquo;t have to
specify the types for <code>K</code> and <code>V</code> when
calling <code>MapKeys</code> - the compiler infers them
automatically.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;keys:&#34;</span><span class="p">,</span> <span class="nf">MapKeys</span><span class="p">(</span><span class="nx">m</span><span class="p">))</span></span></span></code></pre>
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            <p>&hellip; though we could also specify them explicitly.</p>

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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">_</span> <span class="p">=</span> <span class="nx">MapKeys</span><span class="p">[</span><span class="kt">int</span><span class="p">,</span> <span class="kt">string</span><span class="p">](</span><span class="nx">m</span><span class="p">)</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl">    <span class="nx">lst</span> <span class="o">:=</span> <span class="nx">List</span><span class="p">[</span><span class="kt">int</span><span class="p">]{}</span>
</span></span><span class="line"><span class="cl">    <span class="nx">lst</span><span class="p">.</span><span class="nf">Push</span><span class="p">(</span><span class="mi">10</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="nx">lst</span><span class="p">.</span><span class="nf">Push</span><span class="p">(</span><span class="mi">13</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="nx">lst</span><span class="p">.</span><span class="nf">Push</span><span class="p">(</span><span class="mi">23</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">    <span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">&#34;list:&#34;</span><span class="p">,</span> <span class="nx">lst</span><span class="p">.</span><span class="nf">GetAll</span><span class="p">())</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span></span></span></code></pre>
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          <pre class="chroma"><code><span class="line"><span class="cl"><span class="gp">$</span> go run generics.go
</span></span><span class="line"><span class="cl"><span class="go">keys: [4 1 2]
</span></span></span><span class="line"><span class="cl"><span class="go">list: [10 13 23]</span></span></span></code></pre>
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            <p>Note: The order of iteration over map keys is not
defined in Go, so different invocations may
result in different orders.</p>

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      <p class="next">
        Next example: <a href="errors">Errors</a>.
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      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>
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