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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>2 Our plan of attack</title><link rel="stylesheet" type="text/css" href="scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="racket.css" title="default"/><link rel="stylesheet" type="text/css" href="manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="gh.css" title="default"/><script type="text/javascript" src="scribble-common.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--><meta name="keywords" content="Racket,macros,Scheme"><meta name="description" content="Practical Racket macros"><meta name="author" content="Greg Hendershott"><meta name="charset" content="utf-8"><link href='http://fonts.googleapis.com/css?family=Fenix' rel='stylesheet' type='text/css'><script type="text/javascript">var _gaq = _gaq || [];_gaq.push(['_setAccount', 'UA-29709446-1']);_gaq.push(['_setDomainName', 'greghendershott.com']);_gaq.push(['_trackPageview']);(function() {var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);})();</script></head><body id="scribble-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">▼</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">Fear of Macros</a></td></tr></table></div><div class="tocviewsublistonly" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="Preface.html" class="tocviewlink" data-pltdoc="x">Preface</a></td></tr><tr><td align="right">2 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Our plan of attack</a></td></tr><tr><td align="right">3 </td><td><a href="Transform_.html" class="tocviewlink" data-pltdoc="x">Transform!</a></td></tr><tr><td align="right">4 </td><td><a href="pattern-matching.html" class="tocviewlink" data-pltdoc="x">Pattern matching:<span class="mywbr"> </span> syntax-<wbr></wbr>case and syntax-<wbr></wbr>rules</a></td></tr><tr><td align="right">5 </td><td><a href="Syntax_parameters.html" class="tocviewlink" data-pltdoc="x">Syntax parameters</a></td></tr><tr><td align="right">6 </td><td><a href="What_s_the_point_of_splicing-let_.html" class="tocviewlink" data-pltdoc="x">What’s the point of <span class="RktSym"><span class="RktStxLink">splicing-<wbr></wbr>let</span></span>?</a></td></tr><tr><td align="right">7 </td><td><a href="Robust_macros__syntax-parse.html" class="tocviewlink" data-pltdoc="x">Robust macros:<span class="mywbr"> </span> syntax-<wbr></wbr>parse</a></td></tr><tr><td align="right">8 </td><td><a href="References_and_Acknowledgments.html" class="tocviewlink" data-pltdoc="x">References and Acknowledgments</a></td></tr><tr><td align="right">9 </td><td><a href="Epilogue.html" class="tocviewlink" data-pltdoc="x">Epilogue</a></td></tr></table></div></div></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"> </span><span class="navright"> <a href="Preface.html" title="backward to "1 Preface"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Fear of Macros"" data-pltdoc="x">up</a> <a href="Transform_.html" title="forward to "3 Transform!"" data-pltdoc="x">next →</a></span> </div><h3>2<tt> </tt><a name="(part._.Our_plan_of_attack)"></a>Our plan of attack</h3><p>The macro system you will mostly want to use for production-quality
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macros is called <span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Parsing_Syntax.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._syntax-parse%29%29" class="RktStxLink" data-pltdoc="x">syntax-parse</a></span>. And don’t worry, we’ll get to
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that soon.</p><p>But if we start there, you’re likely to feel overwhelmed by concepts
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and terminology, and get very confused. I did.</p><p>1. Instead let’s start with the basics: A syntax object and a function
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to change it—<wbr></wbr>a "transformer". We’ll work at that level for awhile to
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get comfortable and to de-mythologize this whole macro business.</p><p>2. Soon we’ll realize that pattern-matching would make life
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easier. We’ll learn about <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._syntax-case%29%29" class="RktStxLink" data-pltdoc="x">syntax-case</a></span> and its shorthand
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cousin, <span class="RktSym"><a href="http://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fmisc..rkt%29._define-syntax-rule%29%29" class="RktStxLink" data-pltdoc="x">define-syntax-rule</a></span>. We’ll discover we can get
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confused if we want to munge pattern variables before sticking them
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back in the template, and learn how to do that.</p><p>3. At this point we’ll be able to write many useful macros. But, what
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if we want to write the ever-popular anaphoric if, with a "magic
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variable"? It turns out we’ve been protected from making certain kind
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of mistakes. When we want to do this kind of thing on purpose, we use
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a syntax parameter. [There are other, older ways to do this. We won’t
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look at them. We also won’t spend a lot of time
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advocating "hygiene"—<wbr></wbr>we’ll just stipulate that it’s good.]</p><p>4. Finally, we’ll realize that our macros could be smarter when
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they’re used in error. Normal Racket functions optionally can have
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contracts and types. These catch usage mistakes and provide clear,
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useful error messages. It would be great if there were something
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similar for macro. There is. One of the more-recent Racket macro
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enhancements is <span class="RktSym"><a href="http://docs.racket-lang.org/syntax/Parsing_Syntax.html#%28form._%28%28lib._syntax%2Fparse..rkt%29._syntax-parse%29%29" class="RktStxLink" data-pltdoc="x">syntax-parse</a></span>.</p><div class="navsetbottom"><span class="navleft"> </span><span class="navright"> <a href="Preface.html" title="backward to "1 Preface"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "Fear of Macros"" data-pltdoc="x">up</a> <a href="Transform_.html" title="forward to "3 Transform!"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |