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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" 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"><div class="menu__list-item-collapsible"><a class="menu__link menu__link--sublist menu__link--active" aria-expanded="true" tabindex="0" href="../../category/38-evaluation-and-compilation-dictionary.html">3.8 Evaluation and Compilation Dictionary</a><button aria-label="Collapse sidebar category '3.8 Evaluation and Compilation Dictionary'" type="button" class="clean-btn 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href="macroexpand_macroexpand-1_function.html">macroexpand, macroexpand-1</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="optimize_declaration.html">optimize</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="proclaim_function.html">proclaim</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="quote_special-operator.html">quote</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="special-operator-p_function.html">special-operator-p</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="special_declaration.html">special</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="symbol-macrolet_special-operator.html">symbol-macrolet</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="the_special-operator.html">the</a></li><li class="theme-doc-sidebar-item-link theme-doc-sidebar-item-link-level-3 menu__list-item"><a class="menu__link" tabindex="0" href="type_declaration.html">type</a></li></ul></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"><a class="breadcrumbs__link" itemprop="item" href="../../category/38-evaluation-and-compilation-dictionary.html"><span itemprop="name">3.8 Evaluation and Compilation Dictionary</span></a><meta itemprop="position" content="2"></li><li itemscope="" itemprop="itemListElement" itemtype="https://schema.org/ListItem" class="breadcrumbs__item breadcrumbs__item--active"><span class="breadcrumbs__link" itemprop="name">eval-when</span><meta itemprop="position" content="3"></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>eval-when</h1>
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<p><strong>eval-when</strong> <span><i>Special Operator</i></span></p>
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<p><strong>Syntax:</strong></p>
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<p><span><b>eval-when</b></span> (<span><i>{situation}</i></span>*) <span><i>{form}</i></span>* → {result}*</p>
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<p><strong>Arguments and Values:</strong></p>
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<p><span><i>situation</i></span>—One of the <span><i>symbols</i></span> <!-- -->:compile-toplevel<!-- -->, <!-- -->:load-toplevel<!-- -->, <!-- -->:execute<!-- -->, <span><b>compile</b></span>, <span><b>load</b></span>, or <span><b>eval</b></span>. The use of <span><b>eval</b></span>, <span><b>compile</b></span>, and <span><b>load</b></span> is deprecated.</p>
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<p><span><i>forms</i></span>—an <span><i>implicit progn</i></span>.</p>
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<p><em>results</em>—the <span><i>values</i></span> of the <span><i>forms</i></span> if they are executed, or <span><b>nil</b></span> if they are not.</p>
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<p><strong>Description:</strong></p>
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<p>The body of an <span><b>eval-when</b></span> form is processed as an <span><i>implicit progn</i></span>, but only in the <span><i>situations</i></span> listed.</p>
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<p>The use of the <span><i>situations</i></span> <!-- -->:compile-toplevel<!-- --> (or compile) and <!-- -->:load-toplevel<!-- --> (or load) controls whether and when <span><i>evaluation</i></span> occurs when <span><b>eval-when</b></span> appears as a <span><i>top level form</i></span> in code processed by <span><b>compile-file</b></span>. See Section 3.2.3 (File Compilation).</p>
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<p>The use of the <span><i>situation</i></span> <!-- -->:execute<!-- --> (or eval) controls whether evaluation occurs for other <span><b>eval-when</b></span> <span><i>forms</i></span>; that is, those that are not <span><i>top level forms</i></span>, or those in code processed by <span><b>eval</b></span> or <span><b>compile</b></span>. If the <!-- -->:execute<!-- --> situation is specified in such a <span><i>form</i></span>, then the body <span><i>forms</i></span> are processed as an <span><i>implicit progn</i></span>; otherwise, the <span><b>eval-when</b></span> <span><i>form</i></span> returns <span><b>nil</b></span>.</p>
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<p><span><b>eval-when</b></span> normally appears as a <span><i>top level form</i></span>, but it is meaningful for it to appear as a <span><i>non-top-level form</i></span>. However, the compile-time side effects described in Section 3.2 (Compilation) only take place when <span><b>eval-when</b></span> appears as a <span><i>top level form</i></span>.</p>
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<p><strong>Examples:</strong></p>
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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">One example of the use of **eval-when** is that for the compiler to be able to read a file properly when it uses user-defined *reader macros*, it is necessary to write </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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:load-toplevel</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:execute</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"> **eval-when** </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">set-macro-character</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">stream</span><span class="token lambda arguments"> </span><span class="token lambda arguments argument variable" style="color:#36acaa">char</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 declare keyword" style="color:#00009f">declare</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">ignore</span><span class="token plain"> char</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">list</span><span class="token plain"> ’dollar </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">read</span><span class="token plain"> stream</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"> → T </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">This causes the call to **set-macro-character** to be executed in the compiler’s execution environment, thereby modifying its reader syntax table. </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; The EVAL-WHEN in this case is not at toplevel, so only the :EXECUTE </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; keyword is considered. At compile time, this has no effect. </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; At load time (if the LET is at toplevel), or at execution time </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; (if the LET is embedded in some other form which does not execute </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; until later) this sets (SYMBOL-FUNCTION ’FOO1) to a function which </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; returns 1. </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">1</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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:execute</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:load-toplevel</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">setf</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">symbol-function</span><span class="token plain"> ’foo1</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 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 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 heading comment title" style="color:#999988;font-style:italic">;;; If this expression occurs at the toplevel of a file to be compiled, </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; it has BOTH a compile time AND a load-time effect of setting </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; (SYMBOL-FUNCTION ’FOO2) to a function which returns 2. </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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:execute</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:load-toplevel</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">2</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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:execute</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:load-toplevel</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">setf</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">symbol-function</span><span class="token plain"> ’foo2</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 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 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 heading comment title" style="color:#999988;font-style:italic">;;; If this expression occurs at the toplevel of a file to be compiled, </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; it has BOTH a compile time AND a load-time effect of setting the </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; function cell of FOO3 to a function which returns 3. </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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:execute</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:load-toplevel</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">setf</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token car">symbol-function</span><span class="token plain"> ’foo3</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 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"> </span><span class="token number" style="color:#36acaa">3</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 heading comment title" style="color:#999988;font-style:italic">;;; #4: This always does nothing. It simply returns NIL. </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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">print</span><span class="token plain"> ’foo4</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 heading comment title" style="color:#999988;font-style:italic">;;; If this form occurs at toplevel of a file to be compiled, FOO5 is </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; printed at compile time. If this form occurs in a non-top-level </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; position, nothing is printed at compile time. Regardless of context, </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; nothing is ever printed at load time or execution time. </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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:execute</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">print</span><span class="token plain"> ’foo5</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 heading comment title" style="color:#999988;font-style:italic">;;; If this form occurs at toplevel of a file to be compiled, FOO6 is </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; printed at compile time. If this form occurs in a non-top-level </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain">**eval-when** </span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; position, nothing is printed at compile time. Regardless of context, </span><span class="token plain"></span><br></span><span class="token-line" style="color:#393A34"><span class="token plain"></span><span class="token heading comment title" style="color:#999988;font-style:italic">;;; nothing is ever printed at load time or execution time. </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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:execute</span><span class="token plain"> </span><span class="token lisp-property property" style="color:#36acaa">:load-toplevel</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">eval-when</span><span class="token plain"> </span><span class="token punctuation" style="color:#393A34">(</span><span class="token lisp-property property" style="color:#36acaa">:compile-toplevel</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">print</span><span class="token plain"> ’foo6</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></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><strong>See Also:</strong></p>
|
||
<p><span><b>compile-file</b></span>, Section 3.2 (Compilation)</p>
|
||
<p><strong>Notes:</strong></p>
|
||
<p>The following effects are logical consequences of the definition of <span><b>eval-when</b></span>:</p>
|
||
<p><em>•</em> Execution of a single <span><b>eval-when</b></span> expression executes the body code at most once.</p>
|
||
<p><em>• Macros</em> intended for use in <em>top level forms</em> should be written so that side-effects are done by the <span><i>forms</i></span> in the macro expansion. The macro-expander itself should not do the side-effects.</p>
|
||
<p>For example:</p>
|
||
<p>Wrong:</p>
|
||
<p>(defmacro foo ()</p>
|
||
<p>(really-foo)</p>
|
||
<p>‘(really-foo))</p>
|
||
<p>Right:</p>
|
||
<p>(defmacro foo ()</p>
|
||
<p>‘(eval-when (<!-- -->:compile-toplevel<!-- --> <!-- -->:execute<!-- --> <!-- -->:load-toplevel<!-- -->) (really-foo)))</p>
|
||
<p>Adherence to this convention means that such <span><i>macros</i></span> behave intuitively when appearing as <em>non-top-level forms</em>.</p>
|
||
<p><em>•</em> Placing a variable binding around an <span><b>eval-when</b></span> reliably captures the binding because the compile-time-too mode cannot occur (<em>i.e.</em>, introducing a variable binding means that the <span><b>eval-when</b></span> is not a <em>top level form</em>). For example,</p>
|
||
<p>(let ((x 3))</p>
|
||
<p>(eval-when (<!-- -->:execute<!-- --> <!-- -->:load-toplevel<!-- --> <!-- -->:compile-toplevel<!-- -->) (print x)))</p>
|
||
<p>prints 3 at execution (<em>i.e.</em>, load) time, and does not print anything at compile time. This is important so that expansions of <span><b>defun</b></span> and <span><b>defmacro</b></span> can be done in terms of <span><b>eval-when</b></span> and can correctly capture the <em>lexical environment</em>.</p>
|
||
<p>(defun bar (x) (defun foo () (+ x 3)))</p>
|
||
<p>(defun bar (x)</p>
|
||
<p>(progn (eval-when (<!-- -->:compile-toplevel<!-- -->)</p>
|
||
<p>(compiler::notice-function-definition ’foo ’(x)))</p>
|
||
<p>(eval-when (<!-- -->:execute<!-- --> <!-- -->:load-toplevel<!-- -->)</p>
|
||
<p>(setf (symbol-function ’foo) #’(lambda () (+ x 3))))))</p>
|
||
<p>which would be treated by the above rules the same as</p>
|
||
<p>(defun bar (x)</p>
|
||
<p>(setf (symbol-function ’foo) #’(lambda () (+ x 3))))</p>
|
||
<p>when the definition of bar is not a <span><i>top level form</i></span>.</p>
|
||
<h2 class="anchor anchorWithStickyNavbar_LWe7" id="expanded-reference-eval-when">Expanded Reference: eval-when<a href="eval-when_special-operator.html#expanded-reference-eval-when" class="hash-link" aria-label="Direct link to Expanded Reference: eval-when" title="Direct link to Expanded Reference: eval-when"></a></h2>
|
||
<div class="theme-admonition theme-admonition-tip admonition_xJq3 alert alert--success"><div class="admonitionHeading_Gvgb"><span class="admonitionIcon_Rf37"><svg viewBox="0 0 12 16"><path fill-rule="evenodd" d="M6.5 0C3.48 0 1 2.19 1 5c0 .92.55 2.25 1 3 1.34 2.25 1.78 2.78 2 4v1h5v-1c.22-1.22.66-1.75 2-4 .45-.75 1-2.08 1-3 0-2.81-2.48-5-5.5-5zm3.64 7.48c-.25.44-.47.8-.67 1.11-.86 1.41-1.25 2.06-1.45 3.23-.02.05-.02.11-.02.17H5c0-.06 0-.13-.02-.17-.2-1.17-.59-1.83-1.45-3.23-.2-.31-.42-.67-.67-1.11C2.44 6.78 2 5.65 2 5c0-2.2 2.02-4 4.5-4 1.22 0 2.36.42 3.22 1.19C10.55 2.94 11 3.94 11 5c0 .66-.44 1.78-.86 2.48zM4 14h5c-.23 1.14-1.3 2-2.5 2s-2.27-.86-2.5-2z"></path></svg></span>tip</div><div class="admonitionContent_BuS1"><p>TODO: Please contribute to this page by adding explanations and examples</p></div></div>
|
||
<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 car">eval-when</span><span class="token plain"> </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></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-i-dictionary/eval-when_special-operator.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 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