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<title>symbols</title>
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<p>#Symbols</p>
<p>Common Lisp is a Lisp 2. The only difference between a Lisp 1 and Lisp 2 is that Lisp 2 languages have two separate namespaces for values and functions. In Common Lisp this is managed using symbols. We can demonstrate this as follows, in the REPL:</p>
<div class="sourceCode" id="cb1"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb1-1" title="1">CL-USER> (<span class="kw">inspect</span> <span class="dv">42</span>)</a>
<a class="sourceLine" id="cb1-2" title="2"></a>
<a class="sourceLine" id="cb1-3" title="3">The object is an ATOM:</a>
<a class="sourceLine" id="cb1-4" title="4"> <span class="dv">42</span></a></code></pre></div>
<p><code>inspect</code> is a function which reports on the internal representation of the object that is passed to it. In the example above, the number <code>42</code> which is an <code>ATOM</code>.</p>
<div class="sourceCode" id="cb2"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb2-1" title="1">CL-USER> (<span class="kw">inspect</span> 'foo)</a>
<a class="sourceLine" id="cb2-2" title="2"></a>
<a class="sourceLine" id="cb2-3" title="3">The object is a SYMBOL.</a>
<a class="sourceLine" id="cb2-4" title="4"><span class="fl">0.</span> Name: <span class="st">"FOO"</span></a>
<a class="sourceLine" id="cb2-5" title="5"><span class="fl">1.</span> Package: #<PACKAGE <span class="st">"COMMON-LISP-USER"</span>></a>
<a class="sourceLine" id="cb2-6" title="6"><span class="fl">2.</span> Value: <span class="st">"unbound"</span></a>
<a class="sourceLine" id="cb2-7" title="7"><span class="fl">3.</span> Function: <span class="st">"unbound"</span></a>
<a class="sourceLine" id="cb2-8" title="8"><span class="fl">4.</span> Plist: NIL</a></code></pre></div>
<p>Here we <code>inspect</code> a symbol <code>foo</code> and we can see 5 “slots”. The <code>Name</code>, which is a unique identifier for the symbol. <code>Package</code>, which as you might expect defines a namespace in which the symbol exists. <code>Plist</code> is out of scope for this chapter, but we will look more deeply at <code>Value</code> and <code>Function</code>.</p>
<p>When we define a value “foo” we fill the <code>Value</code> slot on the symbol <code>foo</code>. For example:</p>
<div class="sourceCode" id="cb3"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb3-1" title="1"><span class="op">></span> (<span class="kw">defparameter</span><span class="fu"> foo </span><span class="st">"some value"</span>)</a>
<a class="sourceLine" id="cb3-2" title="2"></a>
<a class="sourceLine" id="cb3-3" title="3">FOO</a>
<a class="sourceLine" id="cb3-4" title="4"></a>
<a class="sourceLine" id="cb3-5" title="5">CL-USER> (<span class="kw">inspect</span> 'foo)</a>
<a class="sourceLine" id="cb3-6" title="6"></a>
<a class="sourceLine" id="cb3-7" title="7">The object is a SYMBOL.</a>
<a class="sourceLine" id="cb3-8" title="8"><span class="fl">0.</span> Name: <span class="st">"FOO"</span></a>
<a class="sourceLine" id="cb3-9" title="9"><span class="fl">1.</span> Package: #<PACKAGE <span class="st">"COMMON-LISP-USER"</span>></a>
<a class="sourceLine" id="cb3-10" title="10"><span class="fl">2.</span> Value: <span class="st">"some value"</span></a>
<a class="sourceLine" id="cb3-11" title="11"><span class="fl">3.</span> Function: <span class="st">"unbound"</span></a>
<a class="sourceLine" id="cb3-12" title="12"><span class="fl">4.</span> Plist: NIL</a></code></pre></div>
<p>The <code>Value</code> slot of <code>foo</code> now contains <code>"some value"</code>. Similarly, declaring a function <code>foo</code> will fill the <code>Function</code> slot on <code>foo</code></p>
<div class="sourceCode" id="cb4"><pre class="sourceCode commonlisp"><code class="sourceCode commonlisp"><a class="sourceLine" id="cb4-1" title="1">CL-USER> (<span class="kw">inspect</span> '(a b c))</a>
<a class="sourceLine" id="cb4-2" title="2"></a>
<a class="sourceLine" id="cb4-3" title="3">The object is a proper <span class="kw">list</span> of <span class="kw">length</span> <span class="fl">3.</span></a>
<a class="sourceLine" id="cb4-4" title="4"><span class="fl">0.</span> <span class="dv">0</span>: A</a>
<a class="sourceLine" id="cb4-5" title="5"><span class="fl">1.</span> <span class="dv">1</span>: B</a>
<a class="sourceLine" id="cb4-6" title="6"><span class="fl">2.</span> <span class="dv">2</span>: C</a></code></pre></div>
<p>In this example, a list is passed to <code>inspect</code>. We can see that the list has a numbered slot for each item, each item has an index and also a value.</p>
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