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<h1 id="higher-order-functions">Higher order functions</h1>
<div class="sourceCode" id="cb1"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb1-1" title="1">(<span class="kw">mapcar</span> #'<span class="dv">1</span>+ '(<span class="dv">1</span> <span class="dv">2</span> <span class="dv">3</span>))</a>
<a class="sourceLine" id="cb1-2" title="2"><span class="co">;; => (2 3 4)</span></a></code></pre></div>
<ul>
<li><code>mapcar</code> is a higher-order-function since it takes a function and applies it to each element in the list</li>
</ul>
<div class="sourceCode" id="cb2"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb2-1" title="1">(<span class="kw">function</span> <span class="op">1+</span>)</a>
<a class="sourceLine" id="cb2-2" title="2"><span class="co">;; => #<FUNCTION 1+></span></a>
<a class="sourceLine" id="cb2-3" title="3"></a>
<a class="sourceLine" id="cb2-4" title="4">#'<span class="dv">1</span>+</a>
<a class="sourceLine" id="cb2-5" title="5"><span class="co">;; => #<FUNCTION 1+></span></a></code></pre></div>
<ul>
<li>Here we can see how to reference functions differently.
<ul>
<li>“Sharp quote” (<code>#'</code>) is how you can reference a function</li>
<li>The function, <code>function</code> is the equivalent of sharp quote</li>
</ul></li>
</ul>
<div class="sourceCode" id="cb3"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb3-1" title="1">(<span class="kw">defun</span><span class="fu"> plus-one </span>(a) (<span class="op">+</span> <span class="dv">1</span> a))</a>
<a class="sourceLine" id="cb3-2" title="2"><span class="co">;; => PLUS-ONE</span></a>
<a class="sourceLine" id="cb3-3" title="3"></a>
<a class="sourceLine" id="cb3-4" title="4">(<span class="kw">mapcar</span> (<span class="kw">function</span> plus-one) '(<span class="dv">1</span> <span class="dv">2</span> <span class="dv">3</span>))</a>
<a class="sourceLine" id="cb3-5" title="5"><span class="co">;; => (2 3 4)</span></a></code></pre></div>
<ul>
<li>Just like calling standard library functions you can also use your own.</li>
</ul>
<div class="sourceCode" id="cb4"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb4-1" title="1">(<span class="kw">mapcar</span> #'(<span class="kw">lambda</span> (a) (<span class="op">+</span> a <span class="dv">1</span>)) '(<span class="dv">1</span> <span class="dv">2</span> <span class="dv">3</span>))</a>
<a class="sourceLine" id="cb4-2" title="2"><span class="co">;; => (2 3 4)</span></a></code></pre></div>
<ul>
<li><code>lambda</code> allows you to define an anonymous (i.e. unnamed) function inline.</li>
<li>It is valid to provide an unquoted lambda but this is perhaps inadvisable.</li>
</ul>
<div class="sourceCode" id="cb5"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb5-1" title="1"></a>
<a class="sourceLine" id="cb5-2" title="2">(<span class="kw">defun</span><span class="fu"> my-map </span>(f <span class="kw">list</span>)</a>
<a class="sourceLine" id="cb5-3" title="3"> (<span class="kw">cond</span></a>
<a class="sourceLine" id="cb5-4" title="4"> ((<span class="kw">null</span> <span class="kw">list</span>) <span class="kw">nil</span>)</a>
<a class="sourceLine" id="cb5-5" title="5"> (<span class="kw">t</span> (<span class="kw">cons</span> (<span class="kw">funcall</span> f (<span class="kw">first</span> <span class="kw">list</span>))</a>
<a class="sourceLine" id="cb5-6" title="6"> (my-map f (<span class="kw">rest</span> <span class="kw">list</span>))))))</a>
<a class="sourceLine" id="cb5-7" title="7"><span class="co">;; => MY-MAP</span></a>
<a class="sourceLine" id="cb5-8" title="8"></a>
<a class="sourceLine" id="cb5-9" title="9">(my-map (<span class="kw">lambda</span> (a) (<span class="op">+</span> a <span class="dv">1</span>)) '(<span class="dv">1</span> <span class="dv">2</span> <span class="dv">3</span>))</a>
<a class="sourceLine" id="cb5-10" title="10"><span class="co">;; => (2 3 4)</span></a></code></pre></div>
<ul>
<li>When defining our own higher-order-functions we need to use <code>funcall</code>. This is because the first argument passed into our function is in the “value namespace” which means we cant just call it as if its a function. What <code>funcall</code> does is let us “lift” the function from the value namespace into the function namespace.</li>
</ul>
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