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<title data-rh="true">24.1 System Construction Concepts | Common Lisp (New) Language Reference</title><meta data-rh="true" name="viewport" content="width=device-width,initial-scale=1"><meta data-rh="true" name="twitter:card" content="summary_large_image"><meta data-rh="true" property="og:image" content="https://lisp-docs.github.io/cl-language-reference/img/1024px-Lisp_logo.svg.png"><meta data-rh="true" name="twitter:image" content="https://lisp-docs.github.io/cl-language-reference/img/1024px-Lisp_logo.svg.png"><meta data-rh="true" property="og:url" content="https://lisp-docs.github.io/cl-language-reference/chap-24/ce-b-system-construction-concepts"><meta data-rh="true" property="og:locale" content="en"><meta data-rh="true" name="docusaurus_locale" content="en"><meta data-rh="true" name="docsearch:language" content="en"><meta data-rh="true" name="google-site-verification" content="Vzaw013_bfdKeUVG89Ch3W1zC9_vH9ID2dPB9Dz0vr0"><meta data-rh="true" name="docusaurus_version" content="current"><meta data-rh="true" name="docusaurus_tag" content="docs-default-current"><meta data-rh="true" name="docsearch:version" content="current"><meta data-rh="true" name="docsearch:docusaurus_tag" content="docs-default-current"><meta data-rh="true" property="og:title" content="24.1 System Construction Concepts | Common Lisp (New) Language Reference"><meta data-rh="true" name="description" content="24.1.1 Loading"><meta data-rh="true" property="og:description" content="24.1.1 Loading"><link data-rh="true" rel="icon" href="../img/favicon.ico"><link data-rh="true" rel="canonical" href="ce-b-system-construction-concepts.html"><link data-rh="true" rel="alternate" href="ce-b-system-construction-concepts.html" hreflang="en"><link data-rh="true" rel="alternate" href="ce-b-system-construction-concepts.html" hreflang="x-default"><link data-rh="true" rel="preconnect" href="https://C1F2Q5VM6X-dsn.algolia.net" crossorigin="anonymous"><link rel="preconnect" href="https://www.google-analytics.com">
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="2411-loading">24.1.1 Loading<a href="ce-b-system-construction-concepts.html#2411-loading" class="hash-link" aria-label="Direct link to 24.1.1 Loading" title="Direct link to 24.1.1 Loading"></a></h2>
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<p>To <span><b>load</b></span> a <span><i>file</i></span> is to treat its contents as <span><i>code</i></span> and <span><i>execute</i></span> that <span><i>code</i></span>. The <span><i>file</i></span> may contain <span><i>source code</i></span> or <span><i>compiled code</i></span>.</p>
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<p>A <span><i>file</i></span> containing <span><i>source code</i></span> is called a <span><i>source file</i></span>. <em>Loading</em> a <span><i>source file</i></span> is accomplished essentially by sequentially <em>reading</em><sub>2</sub> the <span><i>forms</i></span> in the file, <em>evaluating</em> each immediately after it is <span><i>read</i></span>.</p>
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<p>A <span><i>file</i></span> containing <span><i>compiled code</i></span> is called a <span><i>compiled file</i></span>. <em>Loading</em> a <span><i>compiled file</i></span> is similar to <em>loading</em> a <span><i>source file</i></span>, except that the <span><i>file</i></span> does not contain text but rather an <em>implementation dependent</em> representation of pre-digested <span><i>expressions</i></span> created by the <span><i>compiler</i></span> . Often, a <span><i>compiled file</i></span> can be <em>loaded</em> more quickly than a <span><i>source file</i></span>. See Section 3.2 (Compilation).</p>
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<p>The way in which a <span><i>source file</i></span> is distinguished from a <span><i>compiled file</i></span> is <span><i>implementation-dependent</i></span>.</p>
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<h2 class="anchor anchorWithStickyNavbar_LWe7" id="2412-features">24.1.2 Features<a href="ce-b-system-construction-concepts.html#2412-features" class="hash-link" aria-label="Direct link to 24.1.2 Features" title="Direct link to 24.1.2 Features"></a></h2>
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<p>A <span><i>feature</i></span> is an aspect or attribute of Common Lisp, of the <span><i>implementation</i></span>, or of the <span><i>environment</i></span>. A <span><i>feature</i></span> is identified by a <span><i>symbol</i></span>.</p>
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<p>A <span><i>feature</i></span> is said to be <span><i>present</i></span> in a <em>Lisp image</em> if and only if the <span><i>symbol</i></span> naming it is an <span><i>element</i></span> of the <span><i>list</i></span> held by the <span><i>variable</i></span> <span><b>*features*</b></span>, which is called the <span><i>features list</i></span>.</p>
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<h3 class="anchor anchorWithStickyNavbar_LWe7" id="24121-feature-expressions">24.1.2.1 Feature Expressions<a href="ce-b-system-construction-concepts.html#24121-feature-expressions" class="hash-link" aria-label="Direct link to 24.1.2.1 Feature Expressions" title="Direct link to 24.1.2.1 Feature Expressions"></a></h3>
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<p>Boolean combinations of <span><i>features</i></span>, called <span><i>feature expressions</i></span>, are used by the #+ and #- <span><i>reader macros</i></span> in order to direct conditional <em>reading</em> of <span><i>expressions</i></span> by the <em>Lisp reader</em> .</p>
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<p>The rules for interpreting a <span><i>feature expression</i></span> are as follows:</p>
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<span><i>feature</i></span>
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<p>If a <span><i>symbol</i></span> naming a <span><i>feature</i></span> is used as a <span><i>feature expression</i></span>, the <span><i>feature expression</i></span> succeeds if that <span><i>feature</i></span> is <span><i>present</i></span>; otherwise it fails.</p>
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<p>(not <em>feature-conditional</em>)</p>
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<p>A <span><b>not</b></span> <span><i>feature expression</i></span> succeeds if its argument <em>feature-conditional</em> fails; otherwise, it succeeds.</p>
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<p>(and <em>{feature-conditional}</em>*)</p>
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<p>An <span><b>and</b></span> <span><i>feature expression</i></span> succeeds if all of its argument <em>feature-conditionals</em> succeed; otherwise, it fails.</p>
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<p>System</p>
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<p>(or <em>{feature-conditional}</em>*)</p>
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<p>An <span><b>or</b></span> <span><i>feature expression</i></span> succeeds if any of its argument <em>feature-conditionals</em> succeed; otherwise, it fails.</p>
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<h4 class="anchor anchorWithStickyNavbar_LWe7" id="241211-examples-of-feature-expressions">24.1.2.1.1 Examples of Feature Expressions<a href="ce-b-system-construction-concepts.html#241211-examples-of-feature-expressions" class="hash-link" aria-label="Direct link to 24.1.2.1.1 Examples of Feature Expressions" title="Direct link to 24.1.2.1.1 Examples of Feature Expressions"></a></h4>
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<p>For example, suppose that in <span><i>implementation</i></span> A, the <span><i>features</i></span> spice and perq are <span><i>present</i></span>, but the <span><i>feature</i></span> lispm is not <span><i>present</i></span>; in <span><i>implementation</i></span> B, the feature lispm is <span><i>present</i></span>, but the <span><i>features</i></span> spice and perq are not <span><i>present</i></span>; and in <span><i>implementation</i></span> C, none of the features spice, <em>lispm</em>, or perq are <span><i>present</i></span>. Figure 24–1 shows some sample <span><i>expressions</i></span>, and how they would be <span><i>read</i></span> <sub>2</sub> in these <span><i>implementations</i></span>.</p>
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<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><div class="codeBlockTitle_Ktv7">Figure 24–1. Features examples</div><div class="codeBlockContent_biex"><pre tabindex="0" class="prism-code language-lisp codeBlock_bY9V thin-scrollbar" style="color:#393A34;background-color:#f6f8fa"><code class="codeBlockLines_e6Vv"><span class="token-line" style="color:#393A34"><span class="token plain"><p></span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">cons</span><span class="token plain"> #+spice </span><span class="token string" style="color:#e3116c">"Spice"</span><span class="token plain"> #-spice </span><span class="token string" style="color:#e3116c">"Lispm"</span><span class="token plain"> x</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p>in *implementation* A *. </span><span class="token punctuation" style="color:#393A34">.</span><span class="token plain"> .* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">CONS</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Spice"</span><span class="token plain"> X</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p>in *implementation* B *. </span><span class="token punctuation" style="color:#393A34">.</span><span class="token plain"> .* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">CONS</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Lispm"</span><span class="token plain"> X</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p>in *implementation* C *. </span><span class="token punctuation" style="color:#393A34">.</span><span class="token plain"> .* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">CONS</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Lispm"</span><span class="token plain"> X</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p></span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">cons</span><span class="token plain"> #+spice </span><span class="token string" style="color:#e3116c">"Spice"</span><span class="token plain"> #+LispM </span><span class="token string" style="color:#e3116c">"Lispm"</span><span class="token plain"> x</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p>in *implementation* A *. </span><span class="token punctuation" style="color:#393A34">.</span><span class="token plain"> .* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">CONS</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Spice"</span><span class="token plain"> X</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p>in *implementation* B *. </span><span class="token punctuation" style="color:#393A34">.</span><span class="token plain"> .* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">CONS</span><span class="token plain"> </span><span class="token string" style="color:#e3116c">"Lispm"</span><span class="token plain"> X</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </p><p>in *implementation* C *. </span><span class="token punctuation" style="color:#393A34">.</span><span class="token plain"> .* </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">C
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