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class="vector-toc-text"> <span class="vector-toc-numb">1.3</span> <span>Program-defined function redefinition</span> </div> </a> <ul id="toc-Program-defined_function_redefinition-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Strategies_used_in_languages_that_lack_hygienic_macros" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Strategies_used_in_languages_that_lack_hygienic_macros"> <div class="vector-toc-text"> <span class="vector-toc-numb">2</span> <span>Strategies used in languages that lack hygienic macros</span> </div> </a> <button aria-controls="toc-Strategies_used_in_languages_that_lack_hygienic_macros-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Strategies used in languages that lack hygienic macros subsection</span> </button> <ul id="toc-Strategies_used_in_languages_that_lack_hygienic_macros-sublist" class="vector-toc-list"> <li id="toc-Obfuscation" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Obfuscation"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.1</span> <span>Obfuscation</span> </div> </a> <ul id="toc-Obfuscation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Temporary_symbol_creation" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Temporary_symbol_creation"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.2</span> <span>Temporary symbol creation</span> </div> </a> <ul id="toc-Temporary_symbol_creation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Read-time_uninterned_symbol" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Read-time_uninterned_symbol"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.3</span> <span>Read-time uninterned symbol</span> </div> </a> <ul id="toc-Read-time_uninterned_symbol-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Packages" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Packages"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.4</span> <span>Packages</span> </div> </a> <ul id="toc-Packages-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Literal_objects" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Literal_objects"> <div class="vector-toc-text"> <span class="vector-toc-numb">2.5</span> <span>Literal objects</span> </div> </a> <ul id="toc-Literal_objects-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Hygienic_transformation" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Hygienic_transformation"> <div class="vector-toc-text"> <span class="vector-toc-numb">3</span> <span>Hygienic transformation</span> </div> </a> <ul id="toc-Hygienic_transformation-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Implementations" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Implementations"> <div class="vector-toc-text"> <span class="vector-toc-numb">4</span> <span>Implementations</span> </div> </a> <button aria-controls="toc-Implementations-sublist" class="cdx-button cdx-button--weight-quiet cdx-button--icon-only vector-toc-toggle"> <span class="vector-icon mw-ui-icon-wikimedia-expand"></span> <span>Toggle Implementations subsection</span> </button> <ul id="toc-Implementations-sublist" class="vector-toc-list"> <li id="toc-Syntax-rules" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Syntax-rules"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.1</span> <span>Syntax-rules</span> </div> </a> <ul id="toc-Syntax-rules-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Syntax-case" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Syntax-case"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.2</span> <span>Syntax-case</span> </div> </a> <ul id="toc-Syntax-case-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Other_systems" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Other_systems"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.3</span> <span>Other systems</span> </div> </a> <ul id="toc-Other_systems-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Languages_with_hygienic_macro_systems" class="vector-toc-list-item vector-toc-level-2"> <a class="vector-toc-link" href="#Languages_with_hygienic_macro_systems"> <div class="vector-toc-text"> <span class="vector-toc-numb">4.4</span> <span>Languages with hygienic macro systems</span> </div> </a> <ul id="toc-Languages_with_hygienic_macro_systems-sublist" class="vector-toc-list"> </ul> </li> </ul> </li> <li id="toc-Criticism" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Criticism"> <div class="vector-toc-text"> <span class="vector-toc-numb">5</span> <span>Criticism</span> </div> </a> <ul id="toc-Criticism-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-See_also" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#See_also"> <div class="vector-toc-text"> <span class="vector-toc-numb">6</span> <span>See also</span> </div> </a> <ul id="toc-See_also-sublist" class="vector-toc-list"> </ul> </li> <li id="toc-Notes" class="vector-toc-list-item vector-toc-level-1 vector-toc-list-item-expanded"> <a class="vector-toc-link" href="#Notes"> <div class="vector-toc-text"> <span class="vector-toc-numb">7</span> <span>Notes</span> </div> </a> <ul id="toc-Notes-sublist" class="vector-toc-list"> </ul> </li> <li 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0.5em;text-align:center}.mw-parser-output .ambox .mbox-imageright{border:none;padding:2px 0.5em 2px 0;text-align:center}.mw-parser-output .ambox .mbox-empty-cell{border:none;padding:0;width:1px}.mw-parser-output .ambox .mbox-image-div{width:52px}@media(min-width:720px){.mw-parser-output .ambox{margin:0 10%}}@media print{body.ns-0 .mw-parser-output .ambox{display:none!important}}</style><table class="box-Technical plainlinks metadata ambox ambox-style ambox-technical" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/en/thumb/f/f2/Edit-clear.svg/40px-Edit-clear.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/en/thumb/f/f2/Edit-clear.svg/60px-Edit-clear.svg.png 1.5x, //upload.wikimedia.org/wikipedia/en/thumb/f/f2/Edit-clear.svg/80px-Edit-clear.svg.png 2x" data-file-width="48" data-file-height="48" /></span></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article <b>may be too technical for most readers to understand</b>.<span class="hide-when-compact"> Please <a class="external text" href="https://en.wikipedia.org/w/index.php?title=Hygienic_macro&action=edit">help improve it</a> to <a href="/wiki/Wikipedia:Make_technical_articles_understandable" title="Wikipedia:Make technical articles understandable">make it understandable to non-experts</a>, without removing the technical details.</span> <span class="date-container"><i>(<span class="date">November 2016</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <p>In <a href="/wiki/Computer_science" title="Computer science">computer science</a>, <b>hygienic macros</b> are <a href="/wiki/Macro_(computer_science)" title="Macro (computer science)">macros</a> whose expansion is guaranteed not to cause the accidental <a href="#The_hygiene_problem">capture</a> of <a href="/wiki/Identifier" title="Identifier">identifiers</a>. They are a feature of <a href="/wiki/Programming_language" title="Programming language">programming languages</a> such as <a href="/wiki/Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a>,<sup id="cite_ref-r5rs_1-0" class="reference"><a href="#cite_note-r5rs-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Dylan_(programming_language)" title="Dylan (programming language)">Dylan</a>,<sup id="cite_ref-dylan_2-0" class="reference"><a href="#cite_note-dylan-2"><span class="cite-bracket">[</span>2<span class="cite-bracket">]</span></a></sup> <a href="/wiki/Rust_(programming_language)" title="Rust (programming language)">Rust</a>, <a href="/wiki/Nim_(programming_language)" title="Nim (programming language)">Nim</a>, and <a href="/wiki/Julia_(programming_language)" title="Julia (programming language)">Julia</a>. The general problem of accidental capture was well known in the <a href="/wiki/Lisp_(programming_language)" title="Lisp (programming language)">Lisp</a> community before the introduction of hygienic macros. Macro writers would use language features that would generate unique identifiers (e.g., gensym) or use obfuscated identifiers to avoid the problem. Hygienic macros are a programmatic solution to the capture problem that is integrated into the macro expander. The term "hygiene" was coined in Kohlbecker et al.'s 1986 paper that introduced hygienic macro expansion, inspired by terminology used in mathematics.<sup id="cite_ref-hygiene_3-0" class="reference"><a href="#cite_note-hygiene-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> </p> <meta property="mw:PageProp/toc" /> <div class="mw-heading mw-heading2"><h2 id="The_hygiene_problem">The hygiene problem</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=1" title="Edit section: The hygiene problem"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <div class="mw-heading mw-heading3"><h3 id="Variable_shadowing">Variable shadowing</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=2" title="Edit section: Variable shadowing"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In programming languages that have non-hygienic macro systems, it is possible for existing variable bindings to be hidden from a macro by variable bindings that are created during its expansion. In <a href="/wiki/C_(programming_language)" title="C (programming language)">C</a>, this problem can be illustrated by the following fragment: </p> <div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre><span></span><span class="cp">#define INCI(i) { int a=0; ++i; }</span> <span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">8</span><span class="p">;</span> <span class="w"> </span><span class="n">INCI</span><span class="p">(</span><span class="n">a</span><span class="p">);</span> <span class="w"> </span><span class="n">INCI</span><span class="p">(</span><span class="n">b</span><span class="p">);</span> <span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"a is now %d, b is now %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">);</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> <p>Running the above through the <a href="/wiki/C_preprocessor" title="C preprocessor">C preprocessor</a> produces: </p> <div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre><span></span><span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">8</span><span class="p">;</span> <span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">a</span><span class="p">;</span><span class="w"> </span><span class="p">};</span> <span class="w"> </span><span class="p">{</span><span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span><span class="w"> </span><span class="o">++</span><span class="n">b</span><span class="p">;</span><span class="w"> </span><span class="p">};</span> <span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"a is now %d, b is now %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">);</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> <p>The variable <code>a</code> declared in the top scope is <a href="/wiki/Variable_shadowing" title="Variable shadowing">shadowed</a> by the <code>a</code> variable in the macro, which introduces a new <a href="/wiki/Scope_(computer_science)#Lexical_scoping" title="Scope (computer science)">scope</a>. As a result, <code>a</code> is never altered by the execution of the program, as the output of the compiled program shows: </p> <pre>a is now 4, b is now 9 </pre> <div class="mw-heading mw-heading3"><h3 id="Standard_library_function_redefinition">Standard library function redefinition</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=3" title="Edit section: Standard library function redefinition"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The hygiene problem can extend beyond variable bindings. Consider this <a href="/wiki/Common_Lisp" title="Common Lisp">Common Lisp</a> macro: </p> <div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre><span></span><span class="p">(</span><span class="nb">defmacro</span><span class="w"> </span><span class="nv">my-unless</span><span class="w"> </span><span class="p">(</span><span class="kt">condition</span><span class="w"> </span><span class="k">&body</span><span class="w"> </span><span class="nv">body</span><span class="p">)</span> <span class="w"> </span><span class="o">`</span><span class="p">(</span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nb">not</span><span class="w"> </span><span class="o">,</span><span class="kt">condition</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">progn</span> <span class="w"> </span><span class="o">,@</span><span class="nv">body</span><span class="p">)))</span> </pre></div> <p>While there are no references to variables in this macro, it assumes the symbols "if", "not", and "progn" are all bound to their usual definitions in the standard library. If, however the above macro is used in the following code: </p> <div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre><span></span><span class="p">(</span><span class="k">flet</span><span class="w"> </span><span class="p">((</span><span class="nb">not</span><span class="w"> </span><span class="p">(</span><span class="nv">x</span><span class="p">)</span><span class="w"> </span><span class="nv">x</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="nv">my-unless</span><span class="w"> </span><span class="no">t</span> <span class="w"> </span><span class="p">(</span><span class="nb">format</span><span class="w"> </span><span class="no">t</span><span class="w"> </span><span class="s">"This should not be printed!"</span><span class="p">)))</span> </pre></div> <p>The definition of "not" has been locally altered and so the expansion of <code>my-unless</code> changes. </p><p>Note however that for Common Lisp this behavior is forbidden, as per <a rel="nofollow" class="external text" href="https://www.lispworks.com/documentation/lw70/CLHS/Body/11_abab.htm">11.1.2.1.2 Constraints on the COMMON-LISP Package for Conforming Programs</a>. It is also possible to completely redefine functions anyway. Some implementations of Common Lisp provide <a rel="nofollow" class="external text" href="https://www.sbcl.org/manual/#Package-Locks">Package Locks</a> to prevent the user to change definitions in packages by mistake. </p> <div class="mw-heading mw-heading3"><h3 id="Program-defined_function_redefinition">Program-defined function redefinition</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=4" title="Edit section: Program-defined function redefinition"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Of course, the problem can occur for program-defined functions in a similar way: </p> <div class="mw-highlight mw-highlight-lang-lisp mw-content-ltr" dir="ltr"><pre><span></span><span class="p">(</span><span class="nb">defun</span><span class="w"> </span><span class="nv">user-defined-operator</span><span class="w"> </span><span class="p">(</span><span class="nb">cond</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="nb">not</span><span class="w"> </span><span class="nb">cond</span><span class="p">))</span> <span class="p">(</span><span class="nb">defmacro</span><span class="w"> </span><span class="nv">my-unless</span><span class="w"> </span><span class="p">(</span><span class="kt">condition</span><span class="w"> </span><span class="k">&body</span><span class="w"> </span><span class="nv">body</span><span class="p">)</span> <span class="w"> </span><span class="o">`</span><span class="p">(</span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nv">user-defined-operator</span><span class="w"> </span><span class="o">,</span><span class="kt">condition</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">progn</span> <span class="w"> </span><span class="o">,@</span><span class="nv">body</span><span class="p">)))</span> <span class="c1">; ... later ...</span> <span class="p">(</span><span class="k">flet</span><span class="w"> </span><span class="p">((</span><span class="nv">user-defined-operator</span><span class="w"> </span><span class="p">(</span><span class="nv">x</span><span class="p">)</span><span class="w"> </span><span class="nv">x</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="nv">my-unless</span><span class="w"> </span><span class="no">t</span> <span class="w"> </span><span class="p">(</span><span class="nb">format</span><span class="w"> </span><span class="no">t</span><span class="w"> </span><span class="s">"This should not be printed!"</span><span class="p">)))</span> </pre></div> <p>The use site redefines <code>user-defined-operator</code> and hence changes the behavior of the macro. </p> <div class="mw-heading mw-heading2"><h2 id="Strategies_used_in_languages_that_lack_hygienic_macros">Strategies used in languages that lack hygienic macros</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=5" title="Edit section: Strategies used in languages that lack hygienic macros"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The hygiene problem can be resolved with conventional macros using several alternative solutions. </p> <div class="mw-heading mw-heading3"><h3 id="Obfuscation">Obfuscation</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=6" title="Edit section: Obfuscation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>The simplest solution, if temporary storage is needed during macro expansion, is to use unusual variables names in the macro in hope that the same names will never be used by the rest of the program. </p> <div class="mw-highlight mw-highlight-lang-c mw-content-ltr" dir="ltr"><pre><span></span><span class="cp">#define INCI(i) { int INCIa = 0; ++i; }</span> <span class="kt">int</span><span class="w"> </span><span class="nf">main</span><span class="p">(</span><span class="kt">void</span><span class="p">)</span> <span class="p">{</span> <span class="w"> </span><span class="kt">int</span><span class="w"> </span><span class="n">a</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">4</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="w"> </span><span class="o">=</span><span class="w"> </span><span class="mi">8</span><span class="p">;</span> <span class="w"> </span><span class="n">INCI</span><span class="p">(</span><span class="n">a</span><span class="p">);</span> <span class="w"> </span><span class="n">INCI</span><span class="p">(</span><span class="n">b</span><span class="p">);</span> <span class="w"> </span><span class="n">printf</span><span class="p">(</span><span class="s">"a is now %d, b is now %d</span><span class="se">\n</span><span class="s">"</span><span class="p">,</span><span class="w"> </span><span class="n">a</span><span class="p">,</span><span class="w"> </span><span class="n">b</span><span class="p">);</span> <span class="w"> </span><span class="k">return</span><span class="w"> </span><span class="mi">0</span><span class="p">;</span> <span class="p">}</span> </pre></div> <p>Until a variable named <code>INCIa</code> is created, this solution produces the correct output: </p> <pre>a is now 5, b is now 9 </pre> <p>The problem is solved for the current program, but this solution is not robust. The variables used inside the macro and those in the rest of the program have to be kept in sync by the programmer. Specifically, using the macro <code>INCI</code> on a variable <code>INCIa</code> is going to fail in the same way that the original macro failed on a variable <code>a</code>. </p> <div class="mw-heading mw-heading3"><h3 id="Temporary_symbol_creation"><span class="anchor" id="Gensym"></span> Temporary symbol creation</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=7" title="Edit section: Temporary symbol creation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In some programming languages, it is possible for a new variable name, or symbol, to be generated and bound to a temporary location. The language processing system ensures that this never clashes with another name or location in the execution environment. The responsibility for choosing to use this feature within the body of a macro definition is left to the programmer. This method was used in <a href="/wiki/MacLisp" class="mw-redirect" title="MacLisp">MacLisp</a>, where a function named <code>gensym</code> could be used to generate a new symbol name. Similar functions (usually named <code>gensym</code> as well) exist in many Lisp-like languages, including the widely implemented <a href="/wiki/Common_Lisp" title="Common Lisp">Common Lisp</a> standard<sup id="cite_ref-4" class="reference"><a href="#cite_note-4"><span class="cite-bracket">[</span>4<span class="cite-bracket">]</span></a></sup> and <a href="/wiki/Elisp" class="mw-redirect" title="Elisp">Elisp</a>. </p><p>Although symbol creation solves the variable shadowing issue, it does not directly solve the issue of function redefinition.<sup id="cite_ref-5" class="reference"><a href="#cite_note-5"><span class="cite-bracket">[</span>5<span class="cite-bracket">]</span></a></sup> However, <code>gensym</code>, macro facilities, and standard library functions are sufficient to embed hygienic macros in an unhygienic language.<sup id="cite_ref-6" class="reference"><a href="#cite_note-6"><span class="cite-bracket">[</span>6<span class="cite-bracket">]</span></a></sup> </p> <div class="mw-heading mw-heading3"><h3 id="Read-time_uninterned_symbol">Read-time uninterned symbol</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=8" title="Edit section: Read-time uninterned symbol"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>This is similar to obfuscation in that a single name is shared by multiple expansions of the same macro. Unlike an unusual name, however, a read time uninterned symbol is used (denoted by the <code>#:</code> notation), for which it is impossible to occur outside of the macro, similar to <code>gensym</code>. </p> <div class="mw-heading mw-heading3"><h3 id="Packages">Packages</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=9" title="Edit section: Packages"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Using packages such as in Common Lisp, the macro simply uses a private symbol from the package in which the macro is defined. The symbol will not accidentally occur in user code. User code would have to reach inside the package using the double colon (<code>::</code>) notation to give itself permission to use the private symbol, for instance <code>cool-macros::secret-sym</code>. At that point, the issue of accidental lack of hygiene is moot. Furthermore the ANSI Common Lisp standard categorizes redefining standard functions and operators, globally or locally, as invoking <a href="/wiki/Undefined_behavior" title="Undefined behavior">undefined behavior</a>. Such usage can be thus diagnosed by the implementation as erroneous. Thus the Lisp package system provide a viable, complete solution to the macro hygiene problem, which can be regarded as an instance of name clashing. </p><p>For example, in the <a href="#Program-defined_function_redefinition">program-defined function redefinition example</a>, the <code>my-unless</code> macro can reside in its own package, where <code>user-defined-operator</code> is a private symbol in that package. The symbol <code>user-defined-operator</code> occurring in the user code will then be a different symbol, unrelated to the one used in the definition of the <code>my-unless</code> macro. </p> <div class="mw-heading mw-heading3"><h3 id="Literal_objects">Literal objects</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=10" title="Edit section: Literal objects"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>In some languages the expansion of a macro does not need to correspond to textual code; rather than expanding to an expression containing the symbol <code>f</code>, a macro may produce an expansion containing the actual object referred to by <code>f</code>. Similarly if the macro needs to use local variables or objects defined in the macro's package, it can expand to an invocation of a closure object whose enclosing lexical environment is that of the macro definition. </p> <div class="mw-heading mw-heading2"><h2 id="Hygienic_transformation">Hygienic transformation</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=11" title="Edit section: Hygienic transformation"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Hygienic macro systems in languages such as <a href="/wiki/Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a> use a macro expansion process that preserves the lexical scoping of all identifiers and prevents accidental capture. This property is called <a href="/wiki/Referential_transparency" title="Referential transparency">referential transparency</a>. In cases where capture is desired, some systems allow the programmer to explicitly violate the hygiene mechanisms of the macro system. </p><p>For example, Scheme's <code>let-syntax</code> and <code>define-syntax</code> macro creation systems are hygienic, so the following Scheme implementation of <code>my-unless</code> will have the desired behavior: </p> <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre><span></span><span class="p">(</span><span class="k">define-syntax</span><span class="w"> </span><span class="nv">my-unless</span> <span class="w"> </span><span class="p">(</span><span class="k">syntax-rules</span><span class="w"> </span><span class="p">()</span> <span class="w"> </span><span class="p">((</span><span class="k">_</span><span class="w"> </span><span class="nv">condition</span><span class="w"> </span><span class="nv">body</span><span class="w"> </span><span class="o">...</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">if</span><span class="w"> </span><span class="p">(</span><span class="nb">not</span><span class="w"> </span><span class="nv">condition</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">begin</span><span class="w"> </span><span class="nv">body</span><span class="w"> </span><span class="o">...</span><span class="p">)))))</span> <span class="p">(</span><span class="k">let</span><span class="w"> </span><span class="p">((</span><span class="nb">not</span><span class="w"> </span><span class="p">(</span><span class="k">lambda</span><span class="w"> </span><span class="p">(</span><span class="nf">x</span><span class="p">)</span><span class="w"> </span><span class="nv">x</span><span class="p">)))</span> <span class="w"> </span><span class="p">(</span><span class="nf">my-unless</span><span class="w"> </span><span class="no">#t</span> <span class="w"> </span><span class="p">(</span><span class="nb">display</span><span class="w"> </span><span class="s">"This should not be printed!"</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="nb">newline</span><span class="p">)))</span> </pre></div> <p>The hygienic macro processor responsible for transforming the patterns of the input form into an output form detects symbol clashes and resolves them by temporarily changing the names of symbols. The basic strategy is to identify <i>bindings</i> in the macro definition and replace those names with gensyms, and to identify <i>free variables</i> in the macro definition and make sure those names are looked up in the scope of the macro definition instead of the scope where the macro was used. </p> <div class="mw-heading mw-heading2"><h2 id="Implementations">Implementations</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=12" title="Edit section: Implementations"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Macro systems that automatically enforce hygiene originated with Scheme. The original KFFD algorithm for a hygienic macro system was presented by Kohlbecker in '86.<sup id="cite_ref-hygiene_3-1" class="reference"><a href="#cite_note-hygiene-3"><span class="cite-bracket">[</span>3<span class="cite-bracket">]</span></a></sup> At the time, no standard macro system was adopted by Scheme implementations. Shortly thereafter in '87, Kohlbecker and <a href="/wiki/Mitchell_Wand" title="Mitchell Wand">Wand</a> proposed a declarative pattern-based language for writing macros, which was the predecessor to the <code>syntax-rules</code> macro facility adopted by the R5RS standard.<sup id="cite_ref-r5rs_1-1" class="reference"><a href="#cite_note-r5rs-1"><span class="cite-bracket">[</span>1<span class="cite-bracket">]</span></a></sup><sup id="cite_ref-7" class="reference"><a href="#cite_note-7"><span class="cite-bracket">[</span>7<span class="cite-bracket">]</span></a></sup> Syntactic closures, an alternative hygiene mechanism, was proposed as an alternative to Kohlbecker et al.'s system by Bawden and Rees in '88.<sup id="cite_ref-syntactic-closures_8-0" class="reference"><a href="#cite_note-syntactic-closures-8"><span class="cite-bracket">[</span>8<span class="cite-bracket">]</span></a></sup> Unlike the KFFD algorithm, syntactic closures require the programmer to explicitly specify the resolution of the scope of an identifier. In 1993, Dybvig et al. introduced the <code>syntax-case</code> macro system, which uses an alternative representation of syntax and maintains hygiene automatically.<sup id="cite_ref-syntax-case_9-0" class="reference"><a href="#cite_note-syntax-case-9"><span class="cite-bracket">[</span>9<span class="cite-bracket">]</span></a></sup> The <code>syntax-case</code> system can express the <code>syntax-rules</code> pattern language as a derived macro. The term <i>macro system</i> can be ambiguous because, in the context of Scheme, it can refer to both a pattern-matching construct (e.g., syntax-rules) and a framework for representing and manipulating syntax (e.g., syntax-case, syntactic closures). </p> <div class="mw-heading mw-heading3"><h3 id="Syntax-rules">Syntax-rules</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=13" title="Edit section: Syntax-rules"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Syntax-rules is a high-level <a href="/wiki/Pattern_matching" title="Pattern matching">pattern matching</a> facility that attempts to make macros easier to write. However, <code>syntax-rules</code> is not able to succinctly describe certain classes of macros and is insufficient to express other macro systems. Syntax-rules was described in the R4RS document in an appendix but not mandated. Later, R5RS adopted it as a standard macro facility. Here is an example <code>syntax-rules</code> macro that swaps the value of two variables: </p> <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre><span></span><span class="p">(</span><span class="k">define-syntax</span><span class="w"> </span><span class="nv">swap!</span> <span class="w"> </span><span class="p">(</span><span class="k">syntax-rules</span><span class="w"> </span><span class="p">()</span> <span class="w"> </span><span class="p">((</span><span class="k">_</span><span class="w"> </span><span class="nv">a</span><span class="w"> </span><span class="nv">b</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">let</span><span class="w"> </span><span class="p">((</span><span class="nf">temp</span><span class="w"> </span><span class="nv">a</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="k">set!</span><span class="w"> </span><span class="nv">a</span><span class="w"> </span><span class="nv">b</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">set!</span><span class="w"> </span><span class="nv">b</span><span class="w"> </span><span class="nv">temp</span><span class="p">)))))</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Syntax-case">Syntax-case</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=14" title="Edit section: Syntax-case"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Due to the deficiencies of a purely <code>syntax-rules</code> based macro system, the <a href="/wiki/R6RS" class="mw-redirect" title="R6RS">R6RS</a> Scheme standard adopted the syntax-case macro system.<sup id="cite_ref-r6rs_10-0" class="reference"><a href="#cite_note-r6rs-10"><span class="cite-bracket">[</span>10<span class="cite-bracket">]</span></a></sup> Unlike <code>syntax-rules</code>, <code>syntax-case</code> contains both a pattern matching language and a low-level facility for writing macros. The former allows macros to be written declaratively, while the latter allows the implementation of alternative frontends for writing macros. The swap example from before is nearly identical in <code>syntax-case</code> because the pattern matching language is similar: </p> <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre><span></span><span class="p">(</span><span class="k">define-syntax</span><span class="w"> </span><span class="nv">swap!</span> <span class="w"> </span><span class="p">(</span><span class="k">lambda</span><span class="w"> </span><span class="p">(</span><span class="nf">stx</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">syntax-case</span><span class="w"> </span><span class="nv">stx</span><span class="w"> </span><span class="p">()</span> <span class="w"> </span><span class="p">((</span><span class="k">_</span><span class="w"> </span><span class="nv">a</span><span class="w"> </span><span class="nv">b</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">syntax</span> <span class="w"> </span><span class="p">(</span><span class="k">let</span><span class="w"> </span><span class="p">((</span><span class="nf">temp</span><span class="w"> </span><span class="nv">a</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="k">set!</span><span class="w"> </span><span class="nv">a</span><span class="w"> </span><span class="nv">b</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">set!</span><span class="w"> </span><span class="nv">b</span><span class="w"> </span><span class="nv">temp</span><span class="p">)))))))</span> </pre></div> <p>However, <code>syntax-case</code> is more powerful than syntax-rules. For example, <code>syntax-case</code> macros can specify side-conditions on its pattern matching rules via arbitrary Scheme functions. Alternatively, a macro writer can choose not to use the pattern matching frontend and manipulate the syntax directly. Using the <code>datum->syntax</code> function, syntax-case macros can also intentionally capture identifiers, thus breaking hygiene. </p> <div class="mw-heading mw-heading3"><h3 id="Other_systems">Other systems</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=15" title="Edit section: Other systems"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Other macro systems have also been proposed and implemented for Scheme. Syntactic closures and explicit renaming<sup id="cite_ref-11" class="reference"><a href="#cite_note-11"><span class="cite-bracket">[</span>11<span class="cite-bracket">]</span></a></sup> are two alternative macro systems. Both systems are lower-level than syntax-rules and leave the enforcement of hygiene to the macro writer. This differs from both syntax-rules and syntax-case, which automatically enforce hygiene by default. The swap examples from above are shown here using a syntactic closure and explicit renaming implementation respectively: </p> <div class="mw-highlight mw-highlight-lang-scheme mw-content-ltr" dir="ltr"><pre><span></span><span class="c1">;; syntactic closures</span> <span class="p">(</span><span class="k">define-syntax</span><span class="w"> </span><span class="nv">swap!</span> <span class="w"> </span><span class="p">(</span><span class="nf">sc-macro-transformer</span> <span class="w"> </span><span class="p">(</span><span class="k">lambda</span><span class="w"> </span><span class="p">(</span><span class="nf">form</span><span class="w"> </span><span class="nv">environment</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">let</span><span class="w"> </span><span class="p">((</span><span class="nf">a</span><span class="w"> </span><span class="p">(</span><span class="nf">close-syntax</span><span class="w"> </span><span class="p">(</span><span class="nb">cadr</span><span class="w"> </span><span class="nv">form</span><span class="p">)</span><span class="w"> </span><span class="nv">environment</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="nf">b</span><span class="w"> </span><span class="p">(</span><span class="nf">close-syntax</span><span class="w"> </span><span class="p">(</span><span class="nb">caddr</span><span class="w"> </span><span class="nv">form</span><span class="p">)</span><span class="w"> </span><span class="nv">environment</span><span class="p">)))</span> <span class="w"> </span><span class="o">`</span><span class="p">(</span><span class="k">let</span><span class="w"> </span><span class="p">((</span><span class="nf">temp</span><span class="w"> </span><span class="o">,</span><span class="nv">a</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="k">set!</span><span class="w"> </span><span class="o">,</span><span class="nv">a</span><span class="w"> </span><span class="o">,</span><span class="nv">b</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">set!</span><span class="w"> </span><span class="o">,</span><span class="nv">b</span><span class="w"> </span><span class="nv">temp</span><span class="p">))))))</span> <span class="c1">;; explicit renaming</span> <span class="p">(</span><span class="k">define-syntax</span><span class="w"> </span><span class="nv">swap!</span> <span class="w"> </span><span class="p">(</span><span class="nf">er-macro-transformer</span> <span class="w"> </span><span class="p">(</span><span class="k">lambda</span><span class="w"> </span><span class="p">(</span><span class="nf">form</span><span class="w"> </span><span class="nv">rename</span><span class="w"> </span><span class="nv">compare</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="k">let</span><span class="w"> </span><span class="p">((</span><span class="nf">a</span><span class="w"> </span><span class="p">(</span><span class="nb">cadr</span><span class="w"> </span><span class="nv">form</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="nf">b</span><span class="w"> </span><span class="p">(</span><span class="nb">caddr</span><span class="w"> </span><span class="nv">form</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="nf">temp</span><span class="w"> </span><span class="p">(</span><span class="nf">rename</span><span class="w"> </span><span class="ss">'temp</span><span class="p">)))</span> <span class="w"> </span><span class="o">`</span><span class="p">(</span><span class="o">,</span><span class="p">(</span><span class="nf">rename</span><span class="w"> </span><span class="ss">'let</span><span class="p">)</span><span class="w"> </span><span class="p">((</span><span class="o">,</span><span class="nv">temp</span><span class="w"> </span><span class="o">,</span><span class="nv">a</span><span class="p">))</span> <span class="w"> </span><span class="p">(</span><span class="o">,</span><span class="p">(</span><span class="nf">rename</span><span class="w"> </span><span class="ss">'set!</span><span class="p">)</span><span class="w"> </span><span class="o">,</span><span class="nv">a</span><span class="w"> </span><span class="o">,</span><span class="nv">b</span><span class="p">)</span> <span class="w"> </span><span class="p">(</span><span class="o">,</span><span class="p">(</span><span class="nf">rename</span><span class="w"> </span><span class="ss">'set!</span><span class="p">)</span><span class="w"> </span><span class="o">,</span><span class="nv">b</span><span class="w"> </span><span class="o">,</span><span class="nv">temp</span><span class="p">))))))</span> </pre></div> <div class="mw-heading mw-heading3"><h3 id="Languages_with_hygienic_macro_systems">Languages with hygienic macro systems</h3><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=16" title="Edit section: Languages with hygienic macro systems"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Scheme_(programming_language)" title="Scheme (programming language)">Scheme</a> – syntax-rules, syntax-case, syntactic closures, and others.</li> <li><a href="/wiki/Racket_(programming_language)" title="Racket (programming language)">Racket</a> – a Scheme variant, its macro system was originally based on syntax-case, but now has more features.</li> <li><a href="/wiki/Nemerle" title="Nemerle">Nemerle</a><sup id="cite_ref-nemerle_12-0" class="reference"><a href="#cite_note-nemerle-12"><span class="cite-bracket">[</span>12<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Dylan_(programming_language)" title="Dylan (programming language)">Dylan</a></li> <li><a href="/wiki/Elixir_(programming_language)" title="Elixir (programming language)">Elixir</a><sup id="cite_ref-13" class="reference"><a href="#cite_note-13"><span class="cite-bracket">[</span>13<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Nim_(programming_language)" title="Nim (programming language)">Nim</a></li> <li><a href="/wiki/Rust_(programming_language)" title="Rust (programming language)">Rust</a></li> <li><a href="/wiki/Haxe" title="Haxe">Haxe</a></li> <li><a href="/wiki/Mary_(programming_language)" title="Mary (programming language)">Mary2</a> – scoped macro bodies in an <a href="/wiki/ALGOL_68" title="ALGOL 68">ALGOL 68</a>-derivative language circa 1978</li> <li><a href="/wiki/Julia_(programming_language)" title="Julia (programming language)">Julia</a><sup id="cite_ref-14" class="reference"><a href="#cite_note-14"><span class="cite-bracket">[</span>14<span class="cite-bracket">]</span></a></sup></li> <li><a href="/wiki/Raku_(programming_language)" title="Raku (programming language)">Raku</a> – supports both hygienic and unhygienic macros<sup id="cite_ref-15" class="reference"><a href="#cite_note-15"><span class="cite-bracket">[</span>15<span class="cite-bracket">]</span></a></sup></li></ul> <div class="mw-heading mw-heading2"><h2 id="Criticism">Criticism</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=17" title="Edit section: Criticism"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <p>Hygienic macros offer safety and referential transparency at the expense of making intentional variable capture less straight-forward. Doug Hoyte, author of <i>Let Over Lambda</i>, writes:<sup id="cite_ref-16" class="reference"><a href="#cite_note-16"><span class="cite-bracket">[</span>16<span class="cite-bracket">]</span></a></sup> </p> <style data-mw-deduplicate="TemplateStyles:r1244412712">.mw-parser-output .templatequote{overflow:hidden;margin:1em 0;padding:0 32px}.mw-parser-output .templatequotecite{line-height:1.5em;text-align:left;margin-top:0}@media(min-width:500px){.mw-parser-output .templatequotecite{padding-left:1.6em}}</style><blockquote class="templatequote"><p>Almost all approaches taken to reducing the impact of variable capture serve only to reduce what you can do with defmacro. Hygienic macros are, in the best of situations, a beginner's safety guard-rail; in the worst of situations they form an electric fence, trapping their victims in a sanitised, capture-safe prison.</p><div class="templatequotecite">— <cite>Doug Hoyte</cite></div></blockquote> <p>Many hygienic macro systems do offer escape hatches without compromising on the guarantees that hygiene provides; for instance, Racket allows you to define <a rel="nofollow" class="external text" href="https://docs.racket-lang.org/reference/stxparam.html#%28tech._syntax._parameter%29">syntax parameters</a>, which allow you to selectively introduce bound variables. Gregg Hendershott gives an example at Fear of Macros<sup id="cite_ref-17" class="reference"><a href="#cite_note-17"><span class="cite-bracket">[</span>17<span class="cite-bracket">]</span></a></sup> of implementing an anaphoric if operator in this way. </p> <div class="mw-heading mw-heading2"><h2 id="See_also">See also</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=18" title="Edit section: See also"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <ul><li><a href="/wiki/Anaphoric_macro" title="Anaphoric macro">Anaphoric macro</a></li> <li><a href="/wiki/Partial_evaluation" title="Partial evaluation">Partial evaluation</a></li> <li><a href="/wiki/Preprocessor" title="Preprocessor">Preprocessor</a></li> <li><a href="/wiki/Syntactic_closure" title="Syntactic closure">Syntactic closure</a></li></ul> <div class="mw-heading mw-heading2"><h2 id="Notes">Notes</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=19" title="Edit section: Notes"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <style data-mw-deduplicate="TemplateStyles:r1239543626">.mw-parser-output .reflist{margin-bottom:0.5em;list-style-type:decimal}@media screen{.mw-parser-output .reflist{font-size:90%}}.mw-parser-output .reflist .references{font-size:100%;margin-bottom:0;list-style-type:inherit}.mw-parser-output .reflist-columns-2{column-width:30em}.mw-parser-output .reflist-columns-3{column-width:25em}.mw-parser-output .reflist-columns{margin-top:0.3em}.mw-parser-output .reflist-columns ol{margin-top:0}.mw-parser-output .reflist-columns li{page-break-inside:avoid;break-inside:avoid-column}.mw-parser-output .reflist-upper-alpha{list-style-type:upper-alpha}.mw-parser-output .reflist-upper-roman{list-style-type:upper-roman}.mw-parser-output .reflist-lower-alpha{list-style-type:lower-alpha}.mw-parser-output .reflist-lower-greek{list-style-type:lower-greek}.mw-parser-output .reflist-lower-roman{list-style-type:lower-roman}</style><div class="reflist reflist-columns references-column-width" style="column-width: 30em;"> <ol class="references"> <li id="cite_note-r5rs-1"><span class="mw-cite-backlink">^ <a href="#cite_ref-r5rs_1-0"><sup><i><b>a</b></i></sup></a> <a href="#cite_ref-r5rs_1-1"><sup><i><b>b</b></i></sup></a></span> <span class="reference-text"><style data-mw-deduplicate="TemplateStyles:r1238218222">.mw-parser-output cite.citation{font-style:inherit;word-wrap:break-word}.mw-parser-output .citation q{quotes:"\"""\"""'""'"}.mw-parser-output .citation:target{background-color:rgba(0,127,255,0.133)}.mw-parser-output .id-lock-free.id-lock-free a{background:url("//upload.wikimedia.org/wikipedia/commons/6/65/Lock-green.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-limited.id-lock-limited a,.mw-parser-output .id-lock-registration.id-lock-registration a{background:url("//upload.wikimedia.org/wikipedia/commons/d/d6/Lock-gray-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .id-lock-subscription.id-lock-subscription a{background:url("//upload.wikimedia.org/wikipedia/commons/a/aa/Lock-red-alt-2.svg")right 0.1em center/9px no-repeat}.mw-parser-output .cs1-ws-icon a{background:url("//upload.wikimedia.org/wikipedia/commons/4/4c/Wikisource-logo.svg")right 0.1em center/12px no-repeat}body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-free a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-limited a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-registration a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .id-lock-subscription a,body:not(.skin-timeless):not(.skin-minerva) .mw-parser-output .cs1-ws-icon a{background-size:contain;padding:0 1em 0 0}.mw-parser-output .cs1-code{color:inherit;background:inherit;border:none;padding:inherit}.mw-parser-output .cs1-hidden-error{display:none;color:var(--color-error,#d33)}.mw-parser-output .cs1-visible-error{color:var(--color-error,#d33)}.mw-parser-output .cs1-maint{display:none;color:#085;margin-left:0.3em}.mw-parser-output .cs1-kern-left{padding-left:0.2em}.mw-parser-output .cs1-kern-right{padding-right:0.2em}.mw-parser-output .citation .mw-selflink{font-weight:inherit}@media screen{.mw-parser-output .cs1-format{font-size:95%}html.skin-theme-clientpref-night .mw-parser-output .cs1-maint{color:#18911f}}@media screen and (prefers-color-scheme:dark){html.skin-theme-clientpref-os .mw-parser-output .cs1-maint{color:#18911f}}</style><cite id="CITEREFKelseyClingerReesRozas1998" class="citation journal cs1">Kelsey, Richard; Clinger, William; Rees, Jonathan; et al. 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Retrieved <span class="nowrap">2014-06-03</span></span>.</cite><span title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Abook&rft.genre=unknown&rft.btitle=Synopsis+6%3A+Subroutines&rft_id=http%3A%2F%2Fperlcabal.org%2Fsyn%2FS06.html%23Macros&rfr_id=info%3Asid%2Fen.wikipedia.org%3AHygienic+macro" class="Z3988"></span></span> </li> <li id="cite_note-16"><span class="mw-cite-backlink"><b><a href="#cite_ref-16">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external autonumber" href="https://letoverlambda.com/index.cl/guest/chap3.html#sec_4">[1]</a>, Let Over Lambda—50 Years of Lisp by Doug Hoyte</span> </li> <li id="cite_note-17"><span class="mw-cite-backlink"><b><a href="#cite_ref-17">^</a></b></span> <span class="reference-text"><a rel="nofollow" class="external autonumber" href="https://www.greghendershott.com/fear-of-macros/Syntax_parameters.html">[2]</a>, Fear of Macros</span> </li> </ol></div> <div class="mw-heading mw-heading2"><h2 id="References">References</h2><span class="mw-editsection"><span class="mw-editsection-bracket">[</span><a href="/w/index.php?title=Hygienic_macro&action=edit&section=20" title="Edit section: References"><span>edit</span></a><span class="mw-editsection-bracket">]</span></span></div> <link rel="mw-deduplicated-inline-style" href="mw-data:TemplateStyles:r1251242444"><table class="box-More_footnotes_needed plainlinks metadata ambox ambox-style ambox-More_footnotes_needed" role="presentation"><tbody><tr><td class="mbox-image"><div class="mbox-image-div"><span typeof="mw:File"><span><img alt="" src="//upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/40px-Text_document_with_red_question_mark.svg.png" decoding="async" width="40" height="40" class="mw-file-element" srcset="//upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/60px-Text_document_with_red_question_mark.svg.png 1.5x, //upload.wikimedia.org/wikipedia/commons/thumb/a/a4/Text_document_with_red_question_mark.svg/80px-Text_document_with_red_question_mark.svg.png 2x" data-file-width="48" data-file-height="48" /></span></span></div></td><td class="mbox-text"><div class="mbox-text-span">This article includes a list of <a href="/wiki/Wikipedia:Citing_sources#General_references" title="Wikipedia:Citing sources">general references</a>, but <b>it lacks sufficient corresponding <a href="/wiki/Wikipedia:Citing_sources#Inline_citations" title="Wikipedia:Citing sources">inline citations</a></b>.<span class="hide-when-compact"> Please help to <a href="/wiki/Wikipedia:WikiProject_Reliability" title="Wikipedia:WikiProject Reliability">improve</a> this article by <a href="/wiki/Wikipedia:When_to_cite" title="Wikipedia:When to cite">introducing</a> more precise citations.</span> <span class="date-container"><i>(<span class="date">April 2012</span>)</i></span><span class="hide-when-compact"><i> (<small><a href="/wiki/Help:Maintenance_template_removal" title="Help:Maintenance template removal">Learn how and when to remove this message</a></small>)</i></span></div></td></tr></tbody></table> <ul><li><i><a href="/wiki/On_Lisp" title="On Lisp">On Lisp</a></i>, <a href="/wiki/Paul_Graham_(computer_programmer)" class="mw-redirect" title="Paul Graham (computer programmer)">Paul Graham</a></li> <li><a rel="nofollow" class="external text" href="http://community.schemewiki.org/?syntax-rules">syntax-rules on schemewiki</a></li> <li><a rel="nofollow" class="external text" href="http://community.schemewiki.org/?syntax-case">syntax-case on schemewiki</a></li> <li><a rel="nofollow" class="external text" href="http://community.schemewiki.org/?syntax-case-examples">examples of syntax-case on schemewiki</a></li> <li><a rel="nofollow" class="external text" href="http://community.schemewiki.org/?syntactic-closures">syntactic closures on schemewiki</a></li> <li><a rel="nofollow" class="external text" href="http://community.schemewiki.org/?simpler-macros">simpler-macros on schemewiki</a></li> <li><a rel="nofollow" class="external text" href="http://community.schemewiki.org/?simpler-macros-examples">examples of simpler-macros on schemewiki</a></li> <li><a rel="nofollow" class="external text" href="http://www.cs.indiana.edu/~dyb/pubs/tr356.pdf">Writing Hygienic Macros in Scheme with Syntax-Case</a></li></ul>    </div><noscript><img src="https://login.wikimedia.org/wiki/Special:CentralAutoLogin/start?type=1x1" alt="" width="1" height="1" style="border: none; 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