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<h1>a73x</h1>
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<h1>Simplifying Interfaces with Function Types</h1>
<p>In Go, you can define methods on type aliases, which means that we can define a type alias of a function, and then define methods on that function.</p>
<h2 id="example">Example</h2>
<p>Given the following interface</p>
<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">DB</span> <span class="kd">interface</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="nf">Get</span><span class="p">(</span><span class="kt">string</span><span class="p">)</span> <span class="p">(</span><span class="kt">string</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span>
</span></span></code></pre>
<p>You can fulfill it using a function type like this:</p>
<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="kd">type</span> <span class="nx">GetFn</span> <span class="kd">func</span><span class="p">(</span><span class="kt">string</span><span class="p">)</span> <span class="p">(</span><span class="kt">string</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span>
</span></span><span class="line"><span class="cl">
</span></span><span class="line"><span class="cl"><span class="kd">func</span> <span class="p">(</span><span class="nx">f</span> <span class="nx">GetFn</span><span class="p">)</span> <span class="nf">Get</span><span class="p">(</span><span class="nx">a</span> <span class="kt">string</span><span class="p">)</span> <span class="p">(</span><span class="kt">string</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="k">return</span> <span class="nf">f</span><span class="p">(</span><span class="nx">a</span><span class="p">)</span>
</span></span><span class="line"><span class="cl"><span class="p">}</span>
</span></span></code></pre>
<p>Now you can use GetFn whenever a DB is required:</p>
<pre tabindex="0" 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">storeFn</span> <span class="nx">DB</span> <span class="p">=</span> <span class="nf">GetFn</span><span class="p">(</span><span class="kd">func</span><span class="p">(</span><span class="nx">s</span> <span class="kt">string</span><span class="p">)</span> <span class="p">(</span><span class="kt">string</span><span class="p">,</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="k">return</span> <span class="s">"bar"</span><span class="p">,</span> <span class="kc">nil</span>
</span></span><span class="line"><span class="cl"> <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="nx">storeFn</span><span class="p">.</span><span class="nf">Get</span><span class="p">(</span><span class="s">"Foo"</span><span class="p">))</span> <span class="c1">// Outputs: bar
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="p">}</span>
</span></span></code></pre>
<p>You can try this example in this [Go Playground](<a href="https://go.dev/play/p/hyBNIMblafs">https://go.dev/play/p/hyBNIMblafs</a></p>
<h2 id="how-it-works">How it works</h2>
<p>In Go, interfaces are implicitly through method sets, which means any type (including a function type) that defines the required methods satisfies the interface. By defining the <code>Get</code> method on the <code>GetFn</code> type, the compiler treats <code>GetFn</code> as a valid implementation of the DB interface.</p>
<p>This flexibility allows you to use function types as lightweight, dynamic implementations of interfaces, without the need for full struct definitions.</p>
<h2 id="application">Application</h2>
<p>One common use case for this pattern is testing. Instead of implementing a full mock, you can use an inline function to provide test specific behavior.</p>
<pre tabindex="0" class="chroma"><code><span class="line"><span class="cl"><span class="kd">func</span> <span class="nf">TestHandler</span><span class="p">()</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="nx">mockDB</span> <span class="o">:=</span> <span class="nf">GetFn</span><span class="p">(</span><span class="kd">func</span><span class="p">(</span><span class="nx">key</span> <span class="kt">string</span><span class="p">)</span> <span class="p">(</span><span class="kt">string</span><span class="p">,</span> <span class="kt">error</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">key</span> <span class="o">==</span> <span class="s">"foo"</span> <span class="p">{</span>
</span></span><span class="line"><span class="cl"> <span class="k">return</span> <span class="s">"bar"</span><span class="p">,</span> <span class="kc">nil</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="s">""</span><span class="p">,</span> <span class="nx">fmt</span><span class="p">.</span><span class="nf">Errorf</span><span class="p">(</span><span class="s">"not found"</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></span><span class="line"><span class="cl"> <span class="nx">result</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">mockDB</span><span class="p">.</span><span class="nf">Get</span><span class="p">(</span><span class="s">"foo"</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="nx">result</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span> <span class="c1">// Outputs: bar, <nil>
</span></span></span><span class="line"><span class="cl"><span class="c1"></span><span class="p">}</span>
</span></span></code></pre>
<p>This approach is not limited to testing. It’s also useful for dependency injection, where you can pass in lightweight or context specific implementations of an interface.</p>
<p>This pattern is similar to how <code>http.HandleFunc</code> works. In the HTTP package, <code>http.HandlerFunc</code> is a function type that fulfills the <code>http.Handler</code> interface by implementing its <code>ServeHTTP</code> method. This allows functions to act as handlers, providing great flexibility in designing web servers.</p>
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