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docSidebarContainer_YfHR"><div class="sidebarViewport_aRkj"><div class="sidebar_njMd"><nav aria-label="Docs sidebar" class="menu thin-scrollbar menu_SIkG"><ul class="theme-doc-sidebar-menu menu__list"><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-1 menu__list-item"><a class="menu__link" href="../index.html">Introduction</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-0/intro.html">Contents and Figures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-1/b-b-scope-purpose-and-history.html">1. Introduction</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-2/c-b-character-syntax.html">2. Syntax</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret menu__link--active" aria-expanded="true" href="d-b-evaluation.html">3. Evaluation and Compilation</a></div><ul style="display:block;overflow:visible;height:auto" class="menu__list"><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link menu__link--active" aria-current="page" tabindex="0" href="d-b-evaluation.html">3.1 Evaluation</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="d-c-compilation.html">3.2 Compilation</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="d-d-declarations.html">3.3 Declarations</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="d-e-lambda-lists.html">3.4 Lambda Lists</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="d-f-error-checking-in-function-calls.html">3.5 Error Checking in Function Calls</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="d-g-traversal-rules-and-side-effects.html">3.6 Traversal Rules and Side Effects</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-2 menu__list-item"><a class="menu__link" tabindex="0" href="d-h-destructive-operations.html">3.7 Destructive Operations</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-2 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist" aria-expanded="false" tabindex="0" href="../category/38-evaluation-and-compilation-dictionary.html">3.8 Evaluation and Compilation Dictionary</a><button aria-label="Expand sidebar category &#x27;3.8 Evaluation and Compilation Dictionary&#x27;" type="button" class="clean-btn menu__caret"></button></div></li></ul></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-4/e-b-introduction.html">4. Types and Classes</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-5/f-b-generalized-reference.html">5. Data and Control Flow</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-6/g-b-the-loop-facility.html">6. Iteration</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-7/h-b-object-creation-and-initialization.html">7. Objects</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../category/81-structures-dictionary.html">8. Structures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-9/j-b-condition-system-concepts.html">9. Conditions</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-10/ba-b-symbol-concepts.html">10. Symbols</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-11/bb-b-package-concepts.html">11. Packages</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-12/bc-b-number-concepts.html">12. Numbers</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-13/bd-b-character-concepts.html">13. Characters</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-14/be-b-cons-concepts.html">14. Conses</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-15/bf-b-array-concepts.html">15. Arrays</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-16/bg-b-string-concepts.html">16. Strings</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-17/bh-b-sequence-concepts.html">17. Sequences</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-18/bi-b-hash-table-concepts.html">18. Hash Tables</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-19/bj-b-overview-of-filenames.html">19. Filenames</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-20/ca-b-file-system-concepts.html">20. Files</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-21/cb-b-stream-concepts.html">21. Streams</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-22/cc-b-the-lisp-printer.html">22. Printer</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-23/cd-b-reader-concepts.html">23. Reader</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-24/ce-b-system-construction-concepts.html">24. System Construction</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-25/cf-b-the-external-environment.html">25. External Environment</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../environments.html">26. Environments</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../meta-object-protocol.html">27. Meta Object Protocol</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../data-structures.html">28. Data Structures</a></div></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../portability/trivial-packages.html">29. Portability</a></div></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-1 menu__list-item"><a class="menu__link" href="../dictionary-entries.html">dictionary-entries</a></li><li class="theme-doc-sidebar-item-category theme-doc-sidebar-item-category-level-1 menu__list-item menu__list-item--collapsed"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--sublist-caret" aria-expanded="false" href="../chap-26/intro.html">Glossary</a></div></li></ul></nav></div></div></aside><main class="docMainContainer_TBSr"><div class="container padding-top--md padding-bottom--lg"><div class="row"><div class="col docItemCol_VOVn"><div class="docItemContainer_Djhp"><article><nav class="theme-doc-breadcrumbs breadcrumbsContainer_Z_bl" aria-label="Breadcrumbs"><ul class="breadcrumbs" itemscope="" itemtype="https://schema.org/BreadcrumbList"><li class="breadcrumbs__item"><a aria-label="Home page" class="breadcrumbs__link" href="../index.html"><svg viewBox="0 0 24 24" class="breadcrumbHomeIcon_YNFT"><path d="M10 19v-5h4v5c0 .55.45 1 1 1h3c.55 0 1-.45 1-1v-7h1.7c.46 0 .68-.57.33-.87L12.67 3.6c-.38-.34-.96-.34-1.34 0l-8.36 7.53c-.34.3-.13.87.33.87H5v7c0 .55.45 1 1 1h3c.55 0 1-.45 1-1z" fill="currentColor"></path></svg></a></li><li class="breadcrumbs__item"><span class="breadcrumbs__link">3. Evaluation and Compilation</span><meta itemprop="position" content="1"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">3.1 Evaluation</span><meta itemprop="position" content="2"></li></ul></nav><div class="tocCollapsible_ETCw theme-doc-toc-mobile tocMobile_ITEo"><button type="button" class="clean-btn tocCollapsibleButton_TO0P">On this page</button></div><div class="theme-doc-markdown markdown"><h1>3.1 Evaluation</h1>
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<p><em>Execution</em> of <span><i>code</i></span> can be accomplished by a variety of means ranging from direct interpretation of a <span><i>form</i></span> representing a <span><i>program</i></span> to invocation of <span><i>compiled code</i></span> produced by a <span><i>compiler</i></span> .</p>
<p><span><i>Evaluation</i></span> is the process by which a <span><i>program</i></span> is <em>executed</em> in Common Lisp. The mechanism of <span><i>evaluation</i></span> is manifested both implicitly through the effect of the <em>Lisp read-eval-print loop</em>, and explicitly through the presence of the <span><i>functions</i></span> <span><b>eval</b></span>, <span><b>compile</b></span>, <span><b>compile-file</b></span>, and <span><b>load</b></span>. Any of these facilities might share the same execution strategy, or each might use a different one.</p>
<p>The behavior of a <span><i>conforming program</i></span> processed by <span><b>eval</b></span> and by <span><b>compile-file</b></span> might differ; see Section 3.2.2.3 (Semantic Constraints).</p>
<p><span><i>Evaluation</i></span> can be understood in terms of a model in which an interpreter recursively traverses a <span><i>form</i></span> performing each step of the computation as it goes. This model, which describes the semantics of Common Lisp <span><i>programs</i></span>, is described in Section 3.1.2 (The Evaluation Model).</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="311-introduction-to-environments">3.1.1 Introduction to Environments<a href="d-b-evaluation.html#311-introduction-to-environments" class="hash-link" aria-label="Direct link to 3.1.1 Introduction to Environments" title="Direct link to 3.1.1 Introduction to Environments">​</a></h2>
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<p>A <span><i>binding</i></span> is an association between a <span><i>name</i></span> and that which the name denotes. <span><i>Bindings</i></span> are <em>established</em> in a <span><i>lexical environment</i></span> or a <span><i>dynamic environment</i></span> by particular <span><i>special operators</i></span>.</p>
<p>An <span><i>environment</i></span> is a set of <span><i>bindings</i></span> and other information used during evaluation (<em>e.g.</em>, to associate meanings with names).</p>
<p><span><i>Bindings</i></span> in an <span><i>environment</i></span> are partitioned into <span><i>namespaces</i></span>. A single <span><i>name</i></span> can simultaneously have more than one associated <span><i>binding</i></span> per <span><i>environment</i></span>, but can have only one associated <span><i>binding</i></span> per <span><i>namespace</i></span>.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="3111-the-global-environment">3.1.1.1 The Global Environment<a href="d-b-evaluation.html#3111-the-global-environment" class="hash-link" aria-label="Direct link to 3.1.1.1 The Global Environment" title="Direct link to 3.1.1.1 The Global Environment">​</a></h3>
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<p>The <span><i>global environment</i></span> is that part of an <span><i>environment</i></span> that contains <span><i>bindings</i></span> with both <span><i>indefinite scope</i></span> and <span><i>indefinite extent</i></span>. The <span><i>global environment</i></span> contains, among other things, the following:</p>
<p><em>• bindings</em> of <span><i>dynamic variables</i></span> and <span><i>constant variables</i></span>.</p>
<p><em>• bindings</em> of <span><i>functions</i></span>, <span><i>macros</i></span>, and <span><i>special operators</i></span>.</p>
<p><em>• bindings</em> of <span><i>compiler macros</i></span>.</p>
<p><em>• bindings</em> of <span><i>type</i></span> and <em>class names</em></p>
<p><em>•</em> information about <span><i>proclamations</i></span>.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="3112-dynamic-environments">3.1.1.2 Dynamic Environments<a href="d-b-evaluation.html#3112-dynamic-environments" class="hash-link" aria-label="Direct link to 3.1.1.2 Dynamic Environments" title="Direct link to 3.1.1.2 Dynamic Environments">​</a></h3>
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<p>A <span><i>dynamic environment</i></span> for <span><i>evaluation</i></span> is that part of an <span><i>environment</i></span> that contains <span><i>bindings</i></span> whose duration is bounded by points of <em>establishment</em> and <em>disestablishment</em> within the execution of the <span><i>form</i></span> that established the <span><i>binding</i></span>. A <span><i>dynamic environment</i></span> contains, among other things, the following:</p>
<p><em>• bindings</em> for <span><i>dynamic variables</i></span>.</p>
<p><em>•</em> information about <em>active catch tags</em>.</p>
<p><em>•</em> information about <span><i>exit points</i></span> established by <span><b>unwind-protect</b></span>.</p>
<p><em>•</em> information about <em>active handlers</em> and <span><i>restarts</i></span>.</p>
<p>The <span><i>dynamic environment</i></span> that is active at any given point in the <em>execution</em> of a <span><i>program</i></span> is referred to by definite reference as “the current <span><i>dynamic environment</i></span>,” or sometimes as just “the <span><i>dynamic environment</i></span>.”</p>
<p>Within a given <span><i>namespace</i></span>, a <span><i>name</i></span> is said to be <span><i>bound</i></span> in a <span><i>dynamic environment</i></span> if there is a <span><i>binding</i></span> associated with its <span><i>name</i></span> in the <span><i>dynamic environment</i></span> or, if not, there is a <span><i>binding</i></span> associated with its name in the <span><i>global environment</i></span>.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="3113-lexical-environments">3.1.1.3 Lexical Environments<a href="d-b-evaluation.html#3113-lexical-environments" class="hash-link" aria-label="Direct link to 3.1.1.3 Lexical Environments" title="Direct link to 3.1.1.3 Lexical Environments">​</a></h3>
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<p>A <span><i>lexical environment</i></span> for <span><i>evaluation</i></span> at some position in a <span><i>program</i></span> is that part of the <span><i>environment</i></span> that contains information having <span><i>lexical scope</i></span> within the <span><i>forms</i></span> containing that position. A <span><i>lexical environment</i></span> contains, among other things, the following:</p>
<p><em>• bindings</em> of <span><i>lexical variables</i></span> and <span><i>symbol macros</i></span>.</p>
<p><em>• bindings</em> of <span><i>functions</i></span> and <span><i>macros</i></span>. (Implicit in this is information about those <span><i>compiler macros</i></span> that are locally disabled.)</p>
<p><em>• bindings</em> of <span><i>block tags</i></span>.</p>
<p><em>• bindings</em> of <span><i>go tags</i></span>.</p>
<p><em>•</em> information about <span><i>declarations</i></span>.</p>
<p>The <span><i>lexical environment</i></span> that is active at any given position in a <span><i>program</i></span> being semantically processed is referred to by definite reference as “the current <span><i>lexical environment</i></span>,” or sometimes as just “the <span><i>lexical environment</i></span>.”</p>
<p>Within a given <span><i>namespace</i></span>, a <span><i>name</i></span> is said to be <span><i>bound</i></span> in a <span><i>lexical environment</i></span> if there is a <span><i>binding</i></span> associated with its <span><i>name</i></span> in the <span><i>lexical environment</i></span> or, if not, there is a <span><i>binding</i></span> associated with its name in the <span><i>global environment</i></span>.</p>
<h4 class="anchor anchorWithStickyNavbar_LWe7" id="31131-the-null-lexical-environment">3.1.1.3.1 The Null Lexical Environment<a href="d-b-evaluation.html#31131-the-null-lexical-environment" class="hash-link" aria-label="Direct link to 3.1.1.3.1 The Null Lexical Environment" title="Direct link to 3.1.1.3.1 The Null Lexical Environment">​</a></h4>
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<p>The <span><i>null lexical environment</i></span> is equivalent to the <span><i>global environment</i></span>.</p>
<p>Although in general the representation of an <span><i>environment object</i></span> is <span><i>implementation-dependent</i></span>, <span><b>nil</b></span> can be used in any situation where an <span><i>environment object</i></span> is called for in order to denote the <span><i>null lexical environment</i></span>.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="3114-environment-objects">3.1.1.4 Environment Objects<a href="d-b-evaluation.html#3114-environment-objects" class="hash-link" aria-label="Direct link to 3.1.1.4 Environment Objects" title="Direct link to 3.1.1.4 Environment Objects">​</a></h3>
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<p>Some <span><i>operators</i></span> make use of an <span><i>object</i></span>, called an <span><i>environment object</i></span>, that represents the set of <span><i>lexical bindings</i></span> needed to perform semantic analysis on a <span><i>form</i></span> in a given <span><i>lexical environment</i></span>. The set of <span><i>bindings</i></span> in an <span><i>environment object</i></span> may be a subset of the <span><i>bindings</i></span> that would be needed to actually perform an <span><i>evaluation</i></span>; for example, <span><i>values</i></span> associated with <em>variable names</em> and <span><i>function names</i></span> in the corresponding <span><i>lexical environment</i></span> might not be available in an <span><i>environment object</i></span>.</p>
<p>The <span><i>type</i></span> and nature of an <span><i>environment object</i></span> is <span><i>implementation-dependent</i></span>. The <span><i>values</i></span> of <span><i>environment parameters</i></span> to <span><i>macro functions</i></span> are examples of <span><i>environment objects</i></span>.</p>
<p>The <span><i>object</i></span> <span><b>nil</b></span> when used as an <span><i>environment object</i></span> denotes the <span><i>null lexical environment</i></span>; see Section 3.1.1.3.1 (The Null Lexical Environment).</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="312-the-evaluation-model">3.1.2 The Evaluation Model<a href="d-b-evaluation.html#312-the-evaluation-model" class="hash-link" aria-label="Direct link to 3.1.2 The Evaluation Model" title="Direct link to 3.1.2 The Evaluation Model">​</a></h2>
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<p>A Common Lisp system evaluates <span><i>forms</i></span> with respect to lexical, dynamic, and global <span><i>environments</i></span>. The following sections describe the components of the Common Lisp evaluation model.</p>
<h3 class="anchor anchorWithStickyNavbar_LWe7" id="3121-form-evaluation">3.1.2.1 Form Evaluation<a href="d-b-evaluation.html#3121-form-evaluation" class="hash-link" aria-label="Direct link to 3.1.2.1 Form Evaluation" title="Direct link to 3.1.2.1 Form Evaluation">​</a></h3>
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<p><span><i>Forms</i></span> fall into three categories: <span><i>symbols</i></span>, <em>conses</em>, and <span><i>self-evaluating objects</i></span>. The following sections explain these categories.</p>
<h4 class="anchor anchorWithStickyNavbar_LWe7" id="31211-symbols-as-forms">3.1.2.1.1 Symbols as Forms<a href="d-b-evaluation.html#31211-symbols-as-forms" class="hash-link" aria-label="Direct link to 3.1.2.1.1 Symbols as Forms" title="Direct link to 3.1.2.1.1 Symbols as Forms">​</a></h4>
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<p>If a <span><i>form</i></span> is a <span><i>symbol</i></span>, then it is either a <span><i>symbol macro</i></span> or a <span><i>variable</i></span>.</p>
<p>The <span><i>symbol</i></span> names a <span><i>symbol macro</i></span> if there is a <span><i>binding</i></span> of the <span><i>symbol</i></span> as a <span><i>symbol macro</i></span> in the current <span><i>lexical environment</i></span> (see <span><b>define-symbol-macro</b></span> and <span><b>symbol-macrolet</b></span>). If the <span><i>symbol</i></span> is a <span><i>symbol macro</i></span>, its expansion function is obtained. The expansion function is a function of two arguments, and is invoked by calling the <span><i>macroexpand hook</i></span> with the expansion function as its</p>
<p>first argument, the <span><i>symbol</i></span> as its second argument, and an <span><i>environment object</i></span> (corresponding to the current <span><i>lexical environment</i></span>) as its third argument. The <span><i>macroexpand hook</i></span>, in turn, calls the expansion function with the <span><i>form</i></span> as its first argument and the <span><i>environment</i></span> as its second argument. The <span><i>value</i></span> of the expansion function, which is passed through by the <span><i>macroexpand hook</i></span>, is a <span><i>form</i></span>. This resulting <span><i>form</i></span> is processed in place of the original <span><i>symbol</i></span>.</p>
<p>If a <span><i>form</i></span> is a <span><i>symbol</i></span> that is not a <span><i>symbol macro</i></span>, then it is the <span><i>name</i></span> of a <span><i>variable</i></span>, and the <span><i>value</i></span> of that <span><i>variable</i></span> is returned. There are three kinds of variables: <span><i>lexical variables</i></span>, <span><i>dynamic variables</i></span>,</p>
<p>and <span><i>constant variables</i></span>. A <span><i>variable</i></span> can store one <span><i>object</i></span>. The main operations on a <span><i>variable</i></span> are to <span><i>read</i></span> <sub>1</sub> and to <span><i>write</i></span><sub>1</sub> its <span><i>value</i></span>.</p>
<p>An error of <span><i>type</i></span> <span><b>unbound-variable</b></span> should be signaled if an <span><i>unbound variable</i></span> is referenced.</p>
<p><span><i>Non-constant variables</i></span> can be <em>assigned</em> by using <span><b>setq</b></span> or <span><i>bound</i></span> <sub>3</sub> by using <span><b>let</b></span>. Figure 3–1 lists some <span><i>defined names</i></span> that are applicable to assigning, binding, and defining <span><i>variables</i></span>.</p>
<p>|</p><p><strong>boundp let progv</strong> </p><p><strong>defconstant let* psetq</strong> </p><p><strong>defparameter makunbound set</strong> </p><p><strong>defvar multiple-value-bind setq</strong> </p><p><strong>lambda multiple-value-setq symbol-value</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 3–1. Some Defined Names Applicable to Variables</strong></p>
<p>The following is a description of each kind of variable.</p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312111-lexical-variables">3.1.2.1.1.1 Lexical Variables<a href="d-b-evaluation.html#312111-lexical-variables" class="hash-link" aria-label="Direct link to 3.1.2.1.1.1 Lexical Variables" title="Direct link to 3.1.2.1.1.1 Lexical Variables">​</a></h5>
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<p>A <span><i>lexical variable</i></span> is a <span><i>variable</i></span> that can be referenced only within the <span><i>lexical scope</i></span> of the <span><i>form</i></span> that establishes that <span><i>variable</i></span>; <span><i>lexical variables</i></span> have <span><i>lexical scope</i></span>. Each time a <span><i>form</i></span> creates a <span><i>lexical binding</i></span> of a <span><i>variable</i></span>, a <em>fresh binding</em> is <em>established</em>.</p>
<p>Within the <span><i>scope</i></span> of a <span><i>binding</i></span> for a <em>lexical variable name</em>, uses of that <span><i>name</i></span> as a <span><i>variable</i></span> are considered to be references to that <span><i>binding</i></span> except where the <span><i>variable</i></span> is <em>shadowed</em> <sub>2</sub> by a <span><i>form</i></span> that <em>establishes</em> a <em>fresh binding</em> for that <em>variable name</em>, or by a <span><i>form</i></span> that locally <span><i>declares</i></span> the <span><i>name</i></span> <span><b>special</b></span>.</p>
<p>A <span><i>lexical variable</i></span> always has a <span><i>value</i></span>. There is no <span><i>operator</i></span> that introduces a <span><i>binding</i></span> for a <span><i>lexical variable</i></span> without giving it an initial <span><i>value</i></span>, nor is there any <span><i>operator</i></span> that can make a <span><i>lexical variable</i></span> be <span><i>unbound</i></span>.</p>
<p><span><i>Bindings</i></span> of <span><i>lexical variables</i></span> are found in the <span><i>lexical environment</i></span>.</p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312112-dynamic-variables">3.1.2.1.1.2 Dynamic Variables<a href="d-b-evaluation.html#312112-dynamic-variables" class="hash-link" aria-label="Direct link to 3.1.2.1.1.2 Dynamic Variables" title="Direct link to 3.1.2.1.1.2 Dynamic Variables">​</a></h5>
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<p>A <span><i>variable</i></span> is a <span><i>dynamic variable</i></span> if one of the following conditions hold:</p>
<p><em>•</em> It is locally declared or globally proclaimed <span><b>special</b></span>.</p>
<p><em>•</em> It occurs textually within a <span><i>form</i></span> that creates a <span><i>dynamic binding</i></span> for a <span><i>variable</i></span> of the <em>same name</em>, and the <span><i>binding</i></span> is not <em>shadowed</em> <sub>2</sub> by a <span><i>form</i></span> that creates a <span><i>lexical binding</i></span> of the same <em>variable name</em>.</p>
<p>A <span><i>dynamic variable</i></span> can be referenced at any time in any <span><i>program</i></span>; there is no textual limitation on references to <span><i>dynamic variables</i></span>. At any given time, all <span><i>dynamic variables</i></span> with a given name refer to exactly one <span><i>binding</i></span>, either in the <span><i>dynamic environment</i></span> or in the <span><i>global environment</i></span>.</p>
<p>The <span><i>value</i></span> part of the <span><i>binding</i></span> for a <span><i>dynamic variable</i></span> might be empty; in this case, the <span><i>dynamic variable</i></span> is said to have no <span><i>value</i></span>, or to be <span><i>unbound</i></span>. A <span><i>dynamic variable</i></span> can be made <span><i>unbound</i></span> by using <span><b>makunbound</b></span>.</p>
<p>The effect of <span><i>binding</i></span> a <span><i>dynamic variable</i></span> is to create a new <span><i>binding</i></span> to which all references to that <span><i>dynamic variable</i></span> in any <span><i>program</i></span> refer for the duration of the <span><i>evaluation</i></span> of the <span><i>form</i></span> that creates the <span><i>dynamic binding</i></span>.</p>
<p>A <span><i>dynamic variable</i></span> can be referenced outside the <span><i>dynamic extent</i></span> of a <span><i>form</i></span> that <span><i>binds</i></span> it. Such a <span><i>variable</i></span> is sometimes called a “global variable” but is still in all respects just a <span><i>dynamic variable</i></span> whose <span><i>binding</i></span> happens to exist in the <span><i>global environment</i></span> rather than in some <span><i>dynamic environment</i></span>.</p>
<p>A <span><i>dynamic variable</i></span> is <span><i>unbound</i></span> unless and until explicitly assigned a value, except for those variables whose initial value is defined in this specification or by an <span><i>implementation</i></span>.</p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312113-constant-variables">3.1.2.1.1.3 Constant Variables<a href="d-b-evaluation.html#312113-constant-variables" class="hash-link" aria-label="Direct link to 3.1.2.1.1.3 Constant Variables" title="Direct link to 3.1.2.1.1.3 Constant Variables">​</a></h5>
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<p>Certain variables, called <span><i>constant variables</i></span>, are reserved as “named constants.” The consequences are undefined if an attempt is made to assign a value to, or create a <span><i>binding</i></span> for a <span><i>constant variable</i></span>, except that a ‘compatible’ redefinition of a <span><i>constant variable</i></span> using <span><b>defconstant</b></span> is permitted; see the <span><i>macro</i></span> <span><b>defconstant</b></span>.</p>
<p><span><i>Keywords</i></span>, <span><i>symbols</i></span> defined by Common Lisp or the <span><i>implementation</i></span> as constant (such as <span><b>nil</b></span>, <span><b>t</b></span>, and <strong>pi</strong>), and <span><i>symbols</i></span> declared as constant using <span><b>defconstant</b></span> are <span><i>constant variables</i></span>.</p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312114-symbols-naming-both-lexical-and-dynamic-variables">3.1.2.1.1.4 Symbols Naming Both Lexical and Dynamic Variables<a href="d-b-evaluation.html#312114-symbols-naming-both-lexical-and-dynamic-variables" class="hash-link" aria-label="Direct link to 3.1.2.1.1.4 Symbols Naming Both Lexical and Dynamic Variables" title="Direct link to 3.1.2.1.1.4 Symbols Naming Both Lexical and Dynamic Variables">​</a></h5>
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<p>The same <span><i>symbol</i></span> can name both a <span><i>lexical variable</i></span> and a <span><i>dynamic variable</i></span>, but never in the same <span><i>lexical environment</i></span>.</p>
<p>In the following example, the <span><i>symbol</i></span> x is used, at different times, as the <span><i>name</i></span> of a <span><i>lexical variable</i></span> and as the <span><i>name</i></span> of a <span><i>dynamic variable</i></span>.</p>
<p>(let ((x 1)) ;Binds a special variable X</p>
<p>(declare (special x))</p>
<p>(let ((x 2)) ;Binds a lexical variable X</p>
<p>(+ x ;Reads a lexical variable X</p>
<p>(locally (declare (special x))</p>
<p>x)))) ;Reads a special variable X</p>
<p>→ 3</p>
<h4 class="anchor anchorWithStickyNavbar_LWe7" id="31212-conses-as-forms">3.1.2.1.2 Conses as Forms<a href="d-b-evaluation.html#31212-conses-as-forms" class="hash-link" aria-label="Direct link to 3.1.2.1.2 Conses as Forms" title="Direct link to 3.1.2.1.2 Conses as Forms">​</a></h4>
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<p>A <span><i>cons</i></span> that is used as a <span><i>form</i></span> is called a <span><i>compound form</i></span>.</p>
<p>If the <span><i>car</i></span> of that <span><i>compound form</i></span> is a <span><i>symbol</i></span>, that <span><i>symbol</i></span> is the <span><i>name</i></span> of an <span><i>operator</i></span> , and the <span><i>form</i></span> is either a <span><i>special form</i></span>, a <span><i>macro form</i></span>, or a <span><i>function form</i></span>, depending on the <em>function binding</em> of the <span><i>operator</i></span> in the current <span><i>lexical environment</i></span>. If the <span><i>operator</i></span> is neither a <span><i>special operator</i></span></p>
<p>nor a <span><i>macro name</i></span>, it is assumed to be a <span><i>function name</i></span> (even if there is no definition for such a <span><i>function</i></span>).</p>
<p>If the <span><i>car</i></span> of the <span><i>compound form</i></span> is not a <span><i>symbol</i></span>, then that <span><i>car</i></span> must be a <span><i>lambda expression</i></span>, in which case the <span><i>compound form</i></span> is a <span><i>lambda form</i></span>.</p>
<p>How a <span><i>compound form</i></span> is processed depends on whether it is classified as a <span><i>special form</i></span>, a <span><i>macro form</i></span>, a <span><i>function form</i></span>, or a <span><i>lambda form</i></span>.</p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312121-special-forms">3.1.2.1.2.1 Special Forms<a href="d-b-evaluation.html#312121-special-forms" class="hash-link" aria-label="Direct link to 3.1.2.1.2.1 Special Forms" title="Direct link to 3.1.2.1.2.1 Special Forms">​</a></h5>
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<p>A <span><i>special form</i></span> is a <span><i>form</i></span> with special syntax, special evaluation rules, or both, possibly manipulating the evaluation environment, control flow, or both. A <span><i>special operator</i></span> has access to the current <span><i>lexical environment</i></span> and the current <span><i>dynamic environment</i></span>. Each <span><i>special operator</i></span> defines the manner in which its <span><i>subexpressions</i></span> are treated—which are <span><i>forms</i></span>, which are special syntax, <em>etc.</em></p>
<p>Some <span><i>special operators</i></span> create new lexical or dynamic <span><i>environments</i></span> for use during the <span><i>evaluation</i></span> of <span><i>subforms</i></span> of the <span><i>special form</i></span>. For example, <span><b>block</b></span> creates a new <span><i>lexical environment</i></span> that is the same as the one in force at the point of evaluation of the <span><b>block</b></span> <span><i>form</i></span> with the addition of a <span><i>binding</i></span> of the <span><b>block</b></span> name to an <span><i>exit point</i></span> from the <span><b>block</b></span>.</p>
<p>The set of <em>special operator names</em> is fixed in Common Lisp; no way is provided for the user to define a <span><i>special operator</i></span> . Figure 3–2 lists all of the Common Lisp <span><i>symbols</i></span> that have definitions as <span><i>special operators</i></span>.</p>
<p>|</p><p><strong>block let* return-from</strong> </p><p><strong>catch load-time-value setq</strong> </p><p><strong>eval-when locally symbol-macrolet flet macrolet tagbody</strong> </p><p><strong>function multiple-value-call the</strong> </p><p><strong>go multiple-value-prog1 throw</strong> </p><p><strong>if progn unwind-protect labels progv</strong> </p><p><strong>let quote</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 3–2. Common Lisp Special Operators</strong></p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312122-macro-forms">3.1.2.1.2.2 Macro Forms<a href="d-b-evaluation.html#312122-macro-forms" class="hash-link" aria-label="Direct link to 3.1.2.1.2.2 Macro Forms" title="Direct link to 3.1.2.1.2.2 Macro Forms">​</a></h5>
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<p>If the <span><i>operator</i></span> names a <span><i>macro</i></span>, its associated <span><i>macro function</i></span> is applied to the entire <span><i>form</i></span> and the result of that application is used in place of the original <span><i>form</i></span>.</p>
<p>Specifically, a <span><i>symbol</i></span> names a <span><i>macro</i></span> in a given <span><i>lexical environment</i></span> if <span><b>macro-function</b></span> is <span><i>true</i></span> of the <span><i>symbol</i></span> and that <span><i>environment</i></span>. The <span><i>function</i></span> returned by <span><b>macro-function</b></span> is a <span><i>function</i></span> of two arguments, called the expansion function. The expansion function is invoked by calling the <span><i>macroexpand hook</i></span> with the expansion function as its first argument, the entire <span><i>macro form</i></span> as its</p>
<p>second argument, and an <span><i>environment object</i></span> (corresponding to the current <span><i>lexical environment</i></span>) as its third argument. The <span><i>macroexpand hook</i></span>, in turn, calls the expansion function with the <span><i>form</i></span> as its first argument and the <span><i>environment</i></span> as its second argument. The <span><i>value</i></span> of the expansion function, which is passed through by the <span><i>macroexpand hook</i></span>, is a <span><i>form</i></span>. The returned <span><i>form</i></span> is <em>evaluated</em> in place of the original <span><i>form</i></span>.</p>
<p>The consequences are undefined if a <span><i>macro function</i></span> destructively modifies any part of its <span><i>form</i></span> argument.</p>
<p>A <span><i>macro name</i></span> is not a <span><i>function designator</i></span> , and cannot be used as the <span><i>function</i></span> argument to <span><i>functions</i></span> such as <span><b>apply</b></span>, <span><b>funcall</b></span>, or <span><b>map</b></span>.</p>
<p>An <span><i>implementation</i></span> is free to implement a Common Lisp <span><i>special operator</i></span> as a <span><i>macro</i></span>. An <span><i>implementation</i></span> is free to implement any <em>macro operator</em> as a <span><i>special operator</i></span> , but only if an equivalent definition of the <span><i>macro</i></span> is also provided.</p>
<p>Figure 3–3 lists some <span><i>defined names</i></span> that are applicable to <span><i>macros</i></span>.</p>
<p>|<strong>*macroexpand-hook* macro-function macroexpand-1 defmacro macroexpand macrolet</strong>|</p>
<p>| :- |</p>
<p><strong>Figure 3–3. Defined names applicable to macros</strong></p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312123-function-forms">3.1.2.1.2.3 Function Forms<a href="d-b-evaluation.html#312123-function-forms" class="hash-link" aria-label="Direct link to 3.1.2.1.2.3 Function Forms" title="Direct link to 3.1.2.1.2.3 Function Forms">​</a></h5>
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<p>If the <span><i>operator</i></span> is a <span><i>symbol</i></span> naming a <span><i>function</i></span>, the <span><i>form</i></span> represents a <em>function form</em>, and the <span><i>cdr</i></span> of the list contains the <span><i>forms</i></span> which when evaluated will supply the arguments passed to the <span><i>function</i></span>.</p>
<p>When a <em>function name</em> is not defined, an error of <span><i>type</i></span> <span><b>undefined-function</b></span> should be signaled at run time; see Section 3.2.2.3 (Semantic Constraints).</p>
<p>A <em>function form</em> is evaluated as follows:</p>
<p>The <span><i>subforms</i></span> in the <span><i>cdr</i></span> of the original <span><i>form</i></span> are evaluated in left-to-right order in the current lexical and dynamic <span><i>environments</i></span>. The <em>primary value</em> of each such <span><i>evaluation</i></span> becomes an <span><i>argument</i></span> to the named <span><i>function</i></span>; any additional <span><i>values</i></span> returned by the <span><i>subforms</i></span> are discarded.</p>
<p>The <em>functional value</em> of the <span><i>operator</i></span> is retrieved from the <em>lexical environment</em>, and that <span><i>function</i></span> is invoked with the indicated arguments.</p>
<p>Although the order of <span><i>evaluation</i></span> of the <em>argument subforms</em> themselves is strictly left-to-right, it is not specified whether the definition of the <span><i>operator</i></span> in a <em>function form</em> is looked up before the <span><i>evaluation</i></span> of the <em>argument subforms</em>, after the <span><i>evaluation</i></span> of the <em>argument subforms</em>, or between the <span><i>evaluation</i></span> of any two <em>argument subforms</em> if there is more than one such <em>argument subform</em>. For example, the following might return 23 or 24.</p>
<p>(defun foo (x) (+ x 3))</p>
<p>(defun bar () (setf (symbol-function ’foo) #’(lambda (x) (+ x 4))))</p>
<p>(foo (progn (bar) 20))</p>
<p>A <span><i>binding</i></span> for a <span><i>function name</i></span> can be <em>established</em> in one of several ways. A <em>bind ing</em> for a <span><i>function name</i></span> in the <span><i>global environment</i></span> can be <em>established</em> by <span><b>defun</b></span>, <span><b>setf</b></span> of <span><b>fdefinition</b></span>, <span><b>setf</b></span> of <span><b>symbol-function</b></span>, <span><b>ensure-generic-function</b></span>, <span><b>defmethod</b></span> (implicitly, due to <span><b>ensure-generic-function</b></span>), or <span><b>defgeneric</b></span>. A <span><i>binding</i></span> for a <span><i>function name</i></span> in the <span><i>lexical environment</i></span> can be <em>established</em> by <span><b>flet</b></span> or <span><b>labels</b></span>.</p>
<p>Figure 3–4 lists some <span><i>defined names</i></span> that are applicable to <span><i>functions</i></span>.</p>
<p>|</p><p><strong>apply fdefinition mapcan</strong> </p><p><strong>call-arguments-limit flet mapcar</strong> </p><p><strong>complement fmakunbound mapcon</strong> </p><p><strong>constantly funcall mapl</strong> </p><p><strong>defgeneric function maplist</strong> </p><p><strong>defmethod functionp multiple-value-call defun labels reduce</strong> </p><p><strong>fboundp map symbol-function</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 3–4. Some function-related defined names</strong></p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312124-lambda-forms">3.1.2.1.2.4 Lambda Forms<a href="d-b-evaluation.html#312124-lambda-forms" class="hash-link" aria-label="Direct link to 3.1.2.1.2.4 Lambda Forms" title="Direct link to 3.1.2.1.2.4 Lambda Forms">​</a></h5>
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<p>A <span><i>lambda form</i></span> is similar to a <span><i>function form</i></span>, except that the <span><i>function name</i></span> is replaced by a <span><i>lambda expression</i></span>.</p>
<p>A <span><i>lambda form</i></span> is equivalent to using <em>funcall</em> of a <span><i>lexical closure</i></span> of the <span><i>lambda expression</i></span> on the given <span><i>arguments</i></span>. (In practice, some compilers are more likely to produce inline code for a <span><i>lambda form</i></span> than for an arbitrary named function that has been declared <span><b>inline</b></span>; however, such a difference is not semantic.)</p>
<p>For further information, see Section 3.1.3 (Lambda Expressions).</p>
<h4 class="anchor anchorWithStickyNavbar_LWe7" id="31213-self">3.1.2.1.3 Self<a href="d-b-evaluation.html#31213-self" class="hash-link" aria-label="Direct link to 3.1.2.1.3 Self" title="Direct link to 3.1.2.1.3 Self">​</a></h4>
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<p>A <span><i>form</i></span> that is neither a <span><i>symbol</i></span> nor a <span><i>cons</i></span> is defined to be a <em>self-evaluating object<span><i>. </i></span>Evaluating</em> such an <em>object yields</em> the <em>same object</em> as a result.</p>
<p>Certain specific <span><i>symbols</i></span> and <em>conses</em> might also happen to be “self-evaluating” but only as a special case of a more general set of rules for the <span><i>evaluation</i></span> of <span><i>symbols</i></span> and <em>conses</em>; such <span><i>objects</i></span> are not considered to be <span><i>self-evaluating objects</i></span>.</p>
<p>The consequences are undefined if <em>literal objects</em> (including <span><i>self-evaluating objects</i></span>) are destructively modified.</p>
<h5 class="anchor anchorWithStickyNavbar_LWe7" id="312131-examples-of-self">3.1.2.1.3.1 Examples of Self<a href="d-b-evaluation.html#312131-examples-of-self" class="hash-link" aria-label="Direct link to 3.1.2.1.3.1 Examples of Self" title="Direct link to 3.1.2.1.3.1 Examples of Self">​</a></h5>
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<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><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">*Numbers*, *pathnames*, and *arrays* are examples of *self-evaluating objects*. </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token number" style="color:#36acaa">3</span><span class="token plain"> → </span><span class="token number" style="color:#36acaa">3</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#c</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">2/3 5/8</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> → #C</span><span class="token punctuation" style="color:#393A34">(</span><span class="token plain">2/3 5/8</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#p</span><span class="token string" style="color:#e3116c">&quot;S:[BILL]</span><span class="token string argument" style="color:#e3116c">OTHELLO</span><span class="token string" style="color:#e3116c">.TXT&quot;</span><span class="token plain"> → #P</span><span class="token string" style="color:#e3116c">&quot;S:[BILL]</span><span class="token string argument" style="color:#e3116c">OTHELLO</span><span class="token string" style="color:#e3116c">.TXT&quot;</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">#</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">a</span><span class="token plain"> b 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">A</span><span class="token plain"> B C</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token string" style="color:#e3116c">&quot;fred smith&quot;</span><span class="token plain"> → </span><span class="token string" style="color:#e3116c">&quot;fred smith&quot;</span><span class="token plain"> </span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="313-lambda-expressions">3.1.3 Lambda Expressions<a href="d-b-evaluation.html#313-lambda-expressions" class="hash-link" aria-label="Direct link to 3.1.3 Lambda Expressions" title="Direct link to 3.1.3 Lambda Expressions">​</a></h2>
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<p>In a <span><i>lambda expression</i></span>, the body is evaluated in a lexical <span><i>environment</i></span> that is formed by adding the <span><i>binding</i></span> of each <span><i>parameter</i></span> in the <span><i>lambda list</i></span> with the corresponding <span><i>value</i></span> from the <span><i>arguments</i></span> to the current lexical <span><i>environment</i></span>.</p>
<p>For further discussion of how <span><i>bindings</i></span> are <em>established</em> based on the <span><i>lambda list</i></span>, see Section 3.4 (Lambda Lists).</p>
<p>The body of a <span><i>lambda expression</i></span> is an <span><i>implicit progn</i></span>; the <span><i>values</i></span> it returns are returned by the <span><i>lambda expression</i></span>.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="314-closures-and-lexical-binding">3.1.4 Closures and Lexical Binding<a href="d-b-evaluation.html#314-closures-and-lexical-binding" class="hash-link" aria-label="Direct link to 3.1.4 Closures and Lexical Binding" title="Direct link to 3.1.4 Closures and Lexical Binding">​</a></h2>
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<p>A <span><i>lexical closure</i></span> is a <span><i>function</i></span> that can refer to and alter the values of <em>lexical bindings established</em> by <em>binding forms</em> that textually include the function definition.</p>
<p>Consider this code, where x is not declared <span><b>special</b></span>:</p>
<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><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 punctuation" style="color:#393A34">(</span><span class="token defun keyword" style="color:#00009f">defun</span><span class="token defun"> </span><span class="token defun function" style="color:#d73a49">two-funs</span><span class="token defun"> </span><span class="token defun punctuation" style="color:#393A34">(</span><span class="token defun arguments argument variable" style="color:#36acaa">x</span><span class="token defun punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">list</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lambda keyword" style="color:#00009f">lambda</span><span class="token lambda"> </span><span class="token lambda punctuation" style="color:#393A34">(</span><span class="token lambda arguments"></span><span class="token lambda punctuation" style="color:#393A34">)</span><span class="token plain"> x</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lambda keyword" style="color:#00009f">lambda</span><span class="token lambda"> </span><span class="token lambda punctuation" style="color:#393A34">(</span><span class="token lambda arguments argument variable" style="color:#36acaa">y</span><span class="token lambda punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">setq</span><span class="token plain"> x y</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">setq</span><span class="token plain"> funs </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">two-funs</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">6</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">funcall</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">car</span><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> → </span><span class="token number" style="color:#36acaa">6</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">funcall</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">cadr</span><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">43</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> → </span><span class="token number" style="color:#36acaa">43</span><span class="token plain"> </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">funcall</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">car</span><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> → </span><span class="token number" style="color:#36acaa">43</span><span class="token plain"> </span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path 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<p>The <span><b>function</b></span> <span><i>special form</i></span> coerces a <span><i>lambda expression</i></span> into a <span><i>closure</i></span> in which the <span><i>lexical environment</i></span> in effect when the <span><i>special form</i></span> is evaluated is captured along with the <span><i>lambda expression</i></span>.</p>
<p>The function two-funs returns a <span><i>list</i></span> of two <span><i>functions</i></span>, each of which refers to the <span><i>binding</i></span> of the variable x created on entry to the function two-funs when it was called. This variable has the value 6 initially, but <span><b>setq</b></span> can alter this <span><i>binding</i></span>. The <span><i>lexical closure</i></span> created for the first <span><i>lambda expression</i></span> does not “snapshot” the <span><i>value</i></span> 6 for x when the <span><i>closure</i></span> is created; rather it captures the <span><i>binding</i></span> of x. The second <span><i>function</i></span> can be used to alter the <span><i>value</i></span> in the same (captured) <span><i>binding</i></span> (to 43, in the example), and this altered variable binding then affects the value returned by the first <span><i>function</i></span>.</p>
<p>In situations where a <span><i>closure</i></span> of a <span><i>lambda expression</i></span> over the same set of <span><i>bindings</i></span> may be produced more than once, the various resulting <span><i>closures</i></span> may or may not be <span><i>identical</i></span>, at the discretion of the <span><i>implementation</i></span>. That is, two <span><i>functions</i></span> that are behaviorally indistinguishable might or might not be <span><i>identical</i></span>. Two <span><i>functions</i></span> that are behaviorally distinguishable are <span><i>distinct</i></span>. For example:</p>
<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><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 punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">x</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">5</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">funs</span><span class="token plain"> ’</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">dotimes</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">j</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">10</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">push</span><span class="token plain"> #’</span><span class="token punctuation" style="color:#393A34">(</span><span class="token lambda keyword" style="color:#00009f">lambda</span><span class="token lambda"> </span><span class="token lambda punctuation" style="color:#393A34">(</span><span class="token lambda arguments argument variable" style="color:#36acaa">z</span><span class="token lambda punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">null</span><span class="token plain"> z</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 keyword" style="color:#00009f">setq</span><span class="token plain"> x </span><span class="token number" style="color:#36acaa">0</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">+</span><span class="token plain"> x z</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>The result of the above <span><i>form</i></span> is a <span><i>list</i></span> of ten <span><i>closures</i></span>. Each requires only the <span><i>binding</i></span> of x. It is the same <span><i>binding</i></span> in each case, but the ten <em>closure objects</em> might or might not be <span><i>identical</i></span>. On the other hand, the result of the <span><i>form</i></span></p>
<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><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 punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">funs</span><span class="token plain"> ’</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">dotimes</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">j</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">10</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">x</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">5</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">push</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lambda keyword" style="color:#00009f">lambda</span><span class="token lambda"> </span><span class="token lambda punctuation" style="color:#393A34">(</span><span class="token lambda arguments argument variable" style="color:#36acaa">z</span><span class="token lambda punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">if</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">null</span><span class="token plain"> z</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 keyword" style="color:#00009f">setq</span><span class="token plain"> x </span><span class="token number" style="color:#36acaa">0</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">+</span><span class="token plain"> x z</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>is also a <span><i>list</i></span> of ten <span><i>closures</i></span>. However, in this case no two of the <em>closure objects</em> can be <span><i>identical</i></span> because each <span><i>closure</i></span> is closed over a distinct <span><i>binding</i></span> of x, and these <span><i>bindings</i></span> can be behaviorally distinguished because of the use of <span><b>setq</b></span>.</p>
<p>The result of the <span><i>form</i></span></p>
<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><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 punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">funs</span><span class="token plain"> ’</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">dotimes</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">j</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">10</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">x</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">5</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">push</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lambda keyword" style="color:#00009f">lambda</span><span class="token lambda"> </span><span class="token lambda punctuation" style="color:#393A34">(</span><span class="token lambda arguments argument variable" style="color:#36acaa">z</span><span class="token lambda punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">+</span><span class="token plain"> x z</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>is a <span><i>list</i></span> of ten <em>closure objects</em> that might or might not be <span><i>identical</i></span>. A different <span><i>binding</i></span> of x is involved for each <span><i>closure</i></span>, but the <span><i>bindings</i></span> cannot be distinguished because their values are the <span><i>same</i></span> and immutable (there being no occurrence of <span><b>setq</b></span> on x). A compiler could internally transform the <span><i>form</i></span> to</p>
<div class="language-lisp codeBlockContainer_Ckt0 theme-code-block" style="--prism-color:#393A34;--prism-background-color:#f6f8fa"><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 punctuation" style="color:#393A34">(</span><span class="token keyword" style="color:#00009f">let</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">funs</span><span class="token plain"> ’</span><span class="token punctuation" style="color:#393A34">(</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">dotimes</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">j</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">10</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">push</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">function</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lambda keyword" style="color:#00009f">lambda</span><span class="token lambda"> </span><span class="token lambda punctuation" style="color:#393A34">(</span><span class="token lambda arguments argument variable" style="color:#36acaa">z</span><span class="token lambda punctuation" style="color:#393A34">)</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">+</span><span class="token plain"> </span><span class="token number" style="color:#36acaa">5</span><span class="token plain"> z</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"> funs</span><span class="token punctuation" style="color:#393A34">)</span><span class="token plain"> </span><br></span></code></pre><div class="buttonGroup__atx"><button type="button" aria-label="Copy code to clipboard" title="Copy" class="clean-btn"><span class="copyButtonIcons_eSgA" aria-hidden="true"><svg viewBox="0 0 24 24" class="copyButtonIcon_y97N"><path fill="currentColor" d="M19,21H8V7H19M19,5H8A2,2 0 0,0 6,7V21A2,2 0 0,0 8,23H19A2,2 0 0,0 21,21V7A2,2 0 0,0 19,5M16,1H4A2,2 0 0,0 2,3V17H4V3H16V1Z"></path></svg><svg viewBox="0 0 24 24" class="copyButtonSuccessIcon_LjdS"><path fill="currentColor" d="M21,7L9,19L3.5,13.5L4.91,12.09L9,16.17L19.59,5.59L21,7Z"></path></svg></span></button></div></div></div>
<p>where the <span><i>closures</i></span> may be <span><i>identical</i></span>.</p>
<p>It is possible that a <span><i>closure</i></span> does not close over any variable bindings. In the code fragment (mapcar (function (lambda (x) (+ x 2))) y)</p>
<p>the function (lambda (x) (+ x 2)) contains no references to any outside object. In this case, the same <span><i>closure</i></span> might be returned for all evaluations of the <span><b>function</b></span> <span><i>form</i></span>.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="315-shadowing">3.1.5 Shadowing<a href="d-b-evaluation.html#315-shadowing" class="hash-link" aria-label="Direct link to 3.1.5 Shadowing" title="Direct link to 3.1.5 Shadowing">​</a></h2>
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<p>If two <span><i>forms</i></span> that <em>establish lexical bindings</em> with the same <em>name N</em> are textually nested, then references to <em>N</em> within the inner <span><i>form</i></span> refer to the <span><i>binding</i></span> established by the inner <span><i>form</i></span>; the inner <span><i>binding</i></span> for <em>N shadows</em> the outer <span><i>binding</i></span> for <em>N</em>. Outside the inner <span><i>form</i></span> but inside the outer one, references to <em>N</em> refer to the <span><i>binding</i></span> established by the outer <span><i>form</i></span>. For example:</p>
<p>(defun test (x z)</p>
<p>(let ((z (* x 2)))</p>
<p>(print z))</p>
<p>z)</p>
<p>The <span><i>binding</i></span> of the variable z by <span><b>let</b></span> shadows the <span><i>parameter</i></span> binding for the function test. The reference to the variable z in the <span><b>print</b></span> <span><i>form</i></span> refers to the <span><b>let</b></span> binding. The reference to z at the end of the function test refers to the <span><i>parameter</i></span> named z.</p>
<p>Constructs that are lexically scoped act as if new names were generated for each <span><i>object</i></span> on each execution. Therefore, dynamic shadowing cannot occur. For example:</p>
<p>(defun contorted-example (f g x)</p>
<p>(if (= x 0)</p>
<p>(funcall f)</p>
<p>(block here</p>
<p>(+ 5 (contorted-example g</p>
<p>#’(lambda () (return-from here 4))</p>
<p>(- x 1))))))</p>
<p>Consider the call (contorted-example nil nil 2). This produces 4. During the course of execution, there are three calls to contorted-example, interleaved with two blocks:</p>
<p>(contorted-example nil nil 2)</p>
<p>(block here<sub>1</sub> ...)</p>
<p>(contorted-example nil #’(lambda () (return-from here<sub>1</sub> 4)) 1)</p>
<p>(block here<sub>2</sub> ...)</p>
<p>(contorted-example #’(lambda () (return-from here<sub>1</sub> 4))</p>
<p>#’(lambda () (return-from here<sub>2</sub> 4))</p>
<p>\0)</p>
<p>(funcall f)</p>
<p>where f → #’(lambda () (return-from here<sub>1</sub> 4))</p>
<p>(return-from here<sub>1</sub> 4)</p>
<p>At the time the funcall is executed there are two <span><b>block</b></span> <span><i>exit points</i></span> outstanding, each apparently named here. The <span><b>return-from</b></span> <span><i>form</i></span> executed as a result of the funcall operation refers to the outer outstanding <span><i>exit point</i></span> (here<sub>1</sub>), not the inner one (here<sub>2</sub>). It refers to that <span><i>exit point</i></span> textually visible at the point of execution of <span><b>function</b></span> (here abbreviated by the #’ syntax) that resulted in creation of the <em>function object</em> actually invoked by <span><b>funcall</b></span>.</p>
<p>If, in this example, one were to change the (funcall f) to (funcall g), then the value of the call (contorted-example nil nil 2) would be 9. The value would change because <span><b>funcall</b></span> would cause the execution of (return-from here<sub>2</sub> 4), thereby causing a return from the inner <span><i>exit point</i></span> (here<sub>2</sub>).</p>
<p>When that occurs, the value 4 is returned from the middle invocation of contorted-example, 5 is added to that to get 9, and that value is returned from the outer block and the outermost call to contorted-example. The point is that the choice of <span><i>exit point</i></span> returned from has nothing to do with its being innermost or outermost; rather, it depends on the lexical environment that is packaged up with a <span><i>lambda expression</i></span> when <span><b>function</b></span> is executed.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="316-extent">3.1.6 Extent<a href="d-b-evaluation.html#316-extent" class="hash-link" aria-label="Direct link to 3.1.6 Extent" title="Direct link to 3.1.6 Extent">​</a></h2>
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<p>Contorted-example works only because the <span><i>function</i></span> named by f is invoked during the <span><i>extent</i></span> of the <span><i>exit point</i></span>. Once the flow of execution has left the block, the <span><i>exit point</i></span> is <em>disestablished</em>. For example:</p>
<p>(defun invalid-example ()</p>
<p>(let ((y (block here #’(lambda (z) (return-from here z)))))</p>
<p>(if (numberp y) y (funcall y 5))))</p>
<p>One might expect the call (invalid-example) to produce 5 by the following incorrect reasoning: <span><b>let</b></span> binds y to the value of <span><b>block</b></span>; this value is a <span><i>function</i></span> resulting from the <span><i>lambda expression</i></span>. Because y is not a number, it is invoked on the value 5. The <span><b>return-from</b></span> should then return this value from the <span><i>exit point</i></span> named here, thereby exiting from the block again and giving y the value 5 which, being a number, is then returned as the value of the call to invalid-example.</p>
<p>The argument fails only because <span><i>exit points</i></span> have <span><i>dynamic extent</i></span>. The argument is correct up to the execution of <span><b>return-from</b></span>. The execution of <span><b>return-from</b></span> should signal an error of <span><i>type</i></span> <span><b>control-error</b></span>, however, not because it cannot refer to the <span><i>exit point</i></span>, but because it does correctly refer to an <span><i>exit point</i></span> and that <span><i>exit point</i></span> has been <em>disestablished</em>.</p>
<p>A reference by name to a dynamic <span><i>exit point</i></span> binding such as a <span><i>catch tag</i></span> refers to the most recently <em>established binding</em> of that name that has not been <em>disestablished</em>. For example:</p>
<p>(defun fun1 (x)</p>
<p>(catch ’trap (+ 3 (fun2 x))))</p>
<p>(defun fun2 (y)</p>
<p>(catch ’trap (* 5 (fun3 y))))</p>
<p>(defun fun3 (z)</p>
<p>(throw ’trap z))</p>
<p>Consider the call (fun1 7). The result is 10. At the time the <span><b>throw</b></span> is executed, there are two outstanding catchers with the name trap: one established within procedure fun1, and the other within procedure fun2. The latter is the more recent, and so the value 7 is returned from <span><b>catch</b></span> in fun2. Viewed from within fun3, the <span><b>catch</b></span> in fun2 shadows the one in fun1. Had fun2 been defined as</p>
<p>(defun fun2 (y)</p>
<p>(catch ’snare (* 5 (fun3 y))))</p>
<p>then the two <span><i>exit points</i></span> would have different <span><i>names</i></span>, and therefore the one in fun1 would not be shadowed. The result would then have been 7.</p>
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="317-return-values">3.1.7 Return Values<a href="d-b-evaluation.html#317-return-values" class="hash-link" aria-label="Direct link to 3.1.7 Return Values" title="Direct link to 3.1.7 Return Values">​</a></h2>
<!-- -->
<!-- -->
<p>Ordinarily the result of calling a <span><i>function</i></span> is a single <span><i>object</i></span>. Sometimes, however, it is convenient for a function to compute several <span><i>objects</i></span> and return them.</p>
<p>In order to receive other than exactly one value from a <span><i>form</i></span>, one of several <span><i>special forms</i></span> or <span><i>macros</i></span> must be used to request those values. If a <span><i>form</i></span> produces <span><i>multiple values</i></span> which were not requested in this way, then the first value is given to the caller and all others are discarded; if the <span><i>form</i></span> produces zero values, then the caller receives <span><b>nil</b></span> as a value.</p>
<p>Figure 3–5 lists some <span><i>operators</i></span> for receiving <span><i>multiple values</i></span><sub>2</sub>. These <span><i>operators</i></span> can be used to specify one or more <span><i>forms</i></span> to <span><i>evaluate</i></span> and where to put the <span><i>values</i></span> returned by those <span><i>forms</i></span>.</p>
<p>|</p><p><strong>multiple-value-bind multiple-value-prog1 return-from multiple-value-call multiple-value-setq throw</strong> </p><p><strong>multiple-value-list return</strong></p>|<p></p>
<p>| :- |</p>
<p><strong>Figure 3–5. Some operators applicable to receiving multiple values</strong></p>
<p>The <span><i>function</i></span> <span><b>values</b></span> can produce <span><i>multiple values</i></span><sub>2</sub>. (values) returns zero values; (values <span><i>form</i></span>) returns the <span><i>primary value</i></span> returned by <span><i>form</i></span>; (values <em>form1 form2</em>) returns two values, the <span><i>primary value</i></span> of <em>form1</em> and the <span><i>primary value</i></span> of <em>form2</em>; and so on.</p>
<p>See <span><b>multiple-values-limit</b></span> and <span><b>values-list</b></span>.</p></div><footer class="theme-doc-footer docusaurus-mt-lg"><div class="theme-doc-footer-edit-meta-row row"><div class="col"><a href="https://github.com/lisp-docs/cl-language-reference/tree/main/docs/chap-3/d-b-evaluation.md" target="_blank" rel="noopener noreferrer" class="theme-edit-this-page"><svg fill="currentColor" height="20" width="20" viewBox="0 0 40 40" class="iconEdit_Z9Sw" aria-hidden="true"><g><path d="m34.5 11.7l-3 3.1-6.3-6.3 3.1-3q0.5-0.5 1.2-0.5t1.1 0.5l3.9 3.9q0.5 0.4 0.5 1.1t-0.5 1.2z m-29.5 17.1l18.4-18.5 6.3 6.3-18.4 18.4h-6.3v-6.2z"></path></g></svg>Edit this page</a></div><div class="col lastUpdated_vwxv"><span class="theme-last-updated">Last updated<!-- --> by <b>daninus14</b></span></div></div></footer></article><nav class="pagination-nav docusaurus-mt-lg" aria-label="Docs pages"><a class="pagination-nav__link pagination-nav__link--prev" href="../chap-2/c-e-standard-macro-characters.html"><div class="pagination-nav__sublabel">Previous</div><div class="pagination-nav__label">2.4 Standard Macro Characters</div></a><a class="pagination-nav__link pagination-nav__link--next" href="d-c-compilation.html"><div class="pagination-nav__sublabel">Next</div><div class="pagination-nav__label">3.2 Compilation</div></a></nav></div></div><div class="col col--3"><div class="tableOfContents_bqdL thin-scrollbar theme-doc-toc-desktop"><ul class="table-of-contents table-of-contents__left-border"><li><a href="d-b-evaluation.html#311-introduction-to-environments" class="table-of-contents__link toc-highlight">3.1.1 Introduction to Environments</a><ul><li><a href="d-b-evaluation.html#3111-the-global-environment" class="table-of-contents__link toc-highlight">3.1.1.1 The Global Environment</a></li><li><a href="d-b-evaluation.html#3112-dynamic-environments" class="table-of-contents__link toc-highlight">3.1.1.2 Dynamic Environments</a></li><li><a href="d-b-evaluation.html#3113-lexical-environments" class="table-of-contents__link toc-highlight">3.1.1.3 Lexical Environments</a><ul><li><a href="d-b-evaluation.html#31131-the-null-lexical-environment" class="table-of-contents__link toc-highlight">3.1.1.3.1 The Null Lexical Environment</a></li></ul></li><li><a href="d-b-evaluation.html#3114-environment-objects" class="table-of-contents__link toc-highlight">3.1.1.4 Environment Objects</a></li></ul></li><li><a href="d-b-evaluation.html#312-the-evaluation-model" class="table-of-contents__link toc-highlight">3.1.2 The Evaluation Model</a><ul><li><a href="d-b-evaluation.html#3121-form-evaluation" class="table-of-contents__link toc-highlight">3.1.2.1 Form Evaluation</a><ul><li><a href="d-b-evaluation.html#31211-symbols-as-forms" class="table-of-contents__link toc-highlight">3.1.2.1.1 Symbols as Forms</a><ul><li><a href="d-b-evaluation.html#312111-lexical-variables" class="table-of-contents__link toc-highlight">3.1.2.1.1.1 Lexical Variables</a></li><li><a href="d-b-evaluation.html#312112-dynamic-variables" class="table-of-contents__link toc-highlight">3.1.2.1.1.2 Dynamic Variables</a></li><li><a href="d-b-evaluation.html#312113-constant-variables" class="table-of-contents__link toc-highlight">3.1.2.1.1.3 Constant Variables</a></li><li><a href="d-b-evaluation.html#312114-symbols-naming-both-lexical-and-dynamic-variables" class="table-of-contents__link toc-highlight">3.1.2.1.1.4 Symbols Naming Both Lexical and Dynamic Variables</a></li></ul></li><li><a href="d-b-evaluation.html#31212-conses-as-forms" class="table-of-contents__link toc-highlight">3.1.2.1.2 Conses as Forms</a><ul><li><a href="d-b-evaluation.html#312121-special-forms" class="table-of-contents__link toc-highlight">3.1.2.1.2.1 Special Forms</a></li><li><a href="d-b-evaluation.html#312122-macro-forms" class="table-of-contents__link toc-highlight">3.1.2.1.2.2 Macro Forms</a></li><li><a href="d-b-evaluation.html#312123-function-forms" class="table-of-contents__link toc-highlight">3.1.2.1.2.3 Function Forms</a></li><li><a href="d-b-evaluation.html#312124-lambda-forms" class="table-of-contents__link toc-highlight">3.1.2.1.2.4 Lambda Forms</a></li></ul></li><li><a href="d-b-evaluation.html#31213-self" class="table-of-contents__link toc-highlight">3.1.2.1.3 Self</a><ul><li><a href="d-b-evaluation.html#312131-examples-of-self" class="table-of-contents__link toc-highlight">3.1.2.1.3.1 Examples of Self</a></li></ul></li></ul></li></ul></li><li><a href="d-b-evaluation.html#313-lambda-expressions" class="table-of-contents__link toc-highlight">3.1.3 Lambda Expressions</a></li><li><a href="d-b-evaluation.html#314-closures-and-lexical-binding" class="table-of-contents__link toc-highlight">3.1.4 Closures and Lexical Binding</a></li><li><a href="d-b-evaluation.html#315-shadowing" class="table-of-contents__link toc-highlight">3.1.5 Shadowing</a></li><li><a href="d-b-evaluation.html#316-extent" class="table-of-contents__link toc-highlight">3.1.6 Extent</a></li><li><a href="d-b-evaluation.html#317-return-values" class="table-of-contents__link toc-highlight">3.1.7 Return Values</a></li></ul></div></div></div></div></main></div></div></div><footer class="footer footer--dark"><div class="container container-fluid"><div class="row footer__links"><div class="col footer__col"><div class="footer__title">Docs</div><ul class="footer__items clean-list"><li class="footer__item"><a href="../../docs/tutorial.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Common Lisp Tutorial</a></li><li class="footer__item"><a href="../index.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Common Lisp Technical Reference</a></li><li class="footer__item"><a href="../../docs/help.html" target="_blank" rel="noopener noreferrer" class="footer__link-item">Help</a></li></ul></div><div class="col footer__col"><div class="footer__title">Community</div><ul class="footer__items clean-list"><li 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