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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>5&nbsp;First tutorial</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="mb.scss" title="default"/><script type="text/javascript" src="scribble-common.js"></script><script type="text/javascript" src="manual-racket.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></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,&quot;tocview_0&quot;);">&#9660;</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">Pollen:<span class="mywbr"> &nbsp;</span> the book is a program</a></td></tr></table></div><div class="tocviewsublisttop" style="display: block;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1&nbsp;</td><td><a href="Installation.html" class="tocviewlink" data-pltdoc="x">Installation</a></td></tr><tr><td align="right">2&nbsp;</td><td><a href="quick-tour.html" class="tocviewlink" data-pltdoc="x">Quick tour</a></td></tr><tr><td align="right">3&nbsp;</td><td><a href="Backstory.html" class="tocviewlink" data-pltdoc="x">Backstory</a></td></tr><tr><td align="right">4&nbsp;</td><td><a href="big-picture.html" class="tocviewlink" data-pltdoc="x">The big picture</a></td></tr><tr><td align="right">5&nbsp;</td><td><a href="first-tutorial.html" class="tocviewselflink" data-pltdoc="x">First tutorial</a></td></tr><tr><td align="right">6&nbsp;</td><td><a href="second-tutorial.html" class="tocviewlink" data-pltdoc="x">Second tutorial</a></td></tr><tr><td align="right">7&nbsp;</td><td><a href="third-tutorial.html" class="tocviewlink" data-pltdoc="x">Third tutorial</a></td></tr><tr><td align="right">8&nbsp;</td><td><a href="raco-pollen.html" class="tocviewlink" data-pltdoc="x">Using <span class="stt">raco pollen</span></a></td></tr><tr><td align="right">9&nbsp;</td><td><a href="File_formats.html" class="tocviewlink" data-pltdoc="x">File formats</a></td></tr><tr><td align="right">10&nbsp;</td><td><a href="reader.html" class="tocviewlink" data-pltdoc="x">&#9674; command overview</a></td></tr><tr><td align="right">11&nbsp;</td><td><a href="Module_reference.html" class="tocviewlink" data-pltdoc="x">Module reference</a></td></tr><tr><td align="right">12&nbsp;</td><td><a href="Acknowledgments.html" class="tocviewlink" data-pltdoc="x">Acknowledgments</a></td></tr><tr><td align="right">13&nbsp;</td><td><a href="License___source_code.html" class="tocviewlink" data-pltdoc="x">License &amp; source code</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,&quot;tocview_1&quot;);">&#9658;</a></td><td>5&nbsp;</td><td><a href="first-tutorial.html" class="tocviewselflink" data-pltdoc="x">First tutorial</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">5.1&nbsp;</td><td><a href="first-tutorial.html#%28part._tutorial-1._.Prerequisites%29" class="tocviewlink" data-pltdoc="x">Prerequisites</a></td></tr><tr><td align="right">5.2&nbsp;</td><td><a href="first-tutorial.html#%28part._.The_relationship_of_.Racket___.Pollen%29" class="tocviewlink" data-pltdoc="x">The relationship of Racket &amp; Pollen</a></td></tr><tr><td
</div><div class="SIntrapara"><ul><li><p>inserting <span class="stt">#lang pollen</span> as the first line,</p></li><li><p>adding the <span class="stt">.pp</span> file extension,</p></li><li><p>running it through Pollen.</p></li></ul></div></p><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>For more about the Pollen processing modes and how to invoke them, see <a href="file-types.html" data-pltdoc="x">File</a>.</p></blockquote></blockquote></blockquote><p>&ldquo;The preprocessor be used with <span style="font-weight: bold">any</span> kind of text-based file?&rdquo; Right. &ldquo;But how?&rdquo; The preprocessor reads the source file, handles any Pollen commands it finds, and lets the rest of the content pass through untouched. To the preprocessor, it&rsquo;s all just text data. It doesn&rsquo;t care whether that text represents HTML, CSS, JavaScript, or even <a href="https://en.wikipedia.org/wiki/TI-BASIC">TI-BASIC</a>.</p><p>One caveat: because the preprocessor only deals with text, the Pollen commands you use in the preprocessor also have to produce text. Moreover, Pollen doesn&rsquo;t enforce the syntax rules of the underlying file &#8212; that&rsquo;s your responsibility. For instance, Pollen won&rsquo;t stop you from doing nonsensical things like this:</p><div class="fileblock"><div class="SIntrapara"><div class="fileblock_filetitle"><span class="fileblock_filename"><span class="stt">"bad-poem.html.pp"</span></span></div></div><div class="SIntrapara"><div class="fileblock_filecontent"><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktMeta">#lang</span><span class="hspace">&nbsp;</span><span class="RktMeta"></span><a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">pollen</span></a><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta">&#160;</span></td></tr><tr><td><span class="RktMeta">The</span><span class="hspace">&nbsp;</span><span class="RktMeta">cave</span><span class="hspace">&nbsp;</span><span class="RktMeta">is</span><span class="hspace">&nbsp;</span><span class="RktMeta">pitch</span><span class="hspace">&nbsp;</span><span class="RktMeta">black.</span></td></tr><tr><td><span class="RktMeta">Look</span><span class="hspace">&nbsp;</span><span class="RktMeta">out</span><span class="hspace">&nbsp;</span><span class="RktMeta">for</span><span class="hspace">&nbsp;</span><span class="RktMeta">the</span><span class="hspace">&nbsp;</span><span class="RktMeta">grue.</span></td></tr><tr><td><span class="RktMeta">&#9674;</span><span class="RktPn">(</span><span class="RktSym">insert-mp3-recording-of-scream</span><span class="RktPn">)</span><span class="RktMeta"></span></td></tr></table></blockquote></div></div></div></div><p>Pollen will fulfill your request, but the result won&rsquo;t be valid HTML, because you can&rsquo;t simply drop binary data in the middle of an HTML file. To paraphrase Mr. Babbage &#8212; garbage in, garbage out.</p><p>I&rsquo;ve encouraged you to mess with the source file, but let&rsquo;s return it to its original state:</p><div class="fileblock"><div class="SIntrapara"><div class="fileblock_filetitle"><span class="fileblock_filename"><span class="stt">"/path/to/tutorial/poem.html.pp"</span></span></div></div><div class="SIntrapara"><div class="fileblock_filecontent"><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktMeta">#lang</span><span class="hspace">&nbsp;</span><span class="RktMeta"></span><a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">pollen</span></a><span class="RktMeta"></span></td></tr><tr><td><span class="RktMeta">&#160;</span></td></tr><tr><td><span class="RktMeta">The</span><span class="hspace">&nbsp;</span><span class="RktMeta">margin</span><span class="hspace">&nbsp;</span><span class="RktMeta">is</span><span class="hspace">&nbsp;</span><span class="RktMeta">42em.</span></td></tr><tr><td><span class="RktMeta"
<br/><span style="font-weight: bold">Mac</span>: option + shift + V
<br/><span style="font-weight: bold">Windows</span>: holding down alt, type 9674 on the num pad
<br/><span style="font-weight: bold">Ubuntu</span>: ctrl + shift + U, then 25CA</p><h5>5.5.4<tt>&nbsp;</tt><a name="(part._.Racket_basics__if_you_re_not_familiar_)"></a>Racket basics (if you&rsquo;re not familiar)</h5><p>&ldquo;But I&rsquo;ve never used Racket.&rdquo; Today, you start. Here are the five basic rules of Racket:</p><ol><li><p>The core building block of Racket is the <span style="font-style: italic">expression</span>. An expression can be a value (like <span class="RktVal">2</span> or <span class="RktVal">"blue"</span>), a variable (like <span class="stt">edge</span>), or a function call (like <span class="RktPn">(</span><span class="RktSym">*</span><span class="stt"> </span><span class="RktSym">inner</span><span class="stt"> </span><span class="RktVal">4</span><span class="RktPn">)</span>).</p></li><li><p>Every expression is <span style="font-style: italic">evaluated</span> to produce a value.</p></li><li><p>A variable evaluates to whatever value it holds (so <span class="stt">inner</span> would become <span class="RktVal">2</span>). A function call evaluates to its return value (so <span class="RktPn">(</span><span class="RktSym">+</span><span class="stt"> </span><span class="RktVal">1</span><span class="stt"> </span><span class="RktVal">1</span><span class="RktPn">)</span> would become <span class="RktVal">2</span>).</p></li><li><p>Function calls go between parentheses. Unlike most languages, the function name comes <span style="font-style: italic">first</span>, followed by its arguments (so it&rsquo;s <span class="RktPn">(</span><span class="RktSym">*</span><span class="stt"> </span><span class="RktSym">inner</span><span class="stt"> </span><span class="RktVal">4</span><span class="RktPn">)</span>, not <span class="RktPn">(</span><span class="RktSym">inner</span><span class="stt"> </span><span class="RktSym">*</span><span class="stt"> </span><span class="RktVal">4</span><span class="RktPn">)</span>). This is called <span style="font-style: italic">prefix notation</span>.</p></li><li><p>Expressions can contain recursively nested expressions. Thus, <span class="RktPn">(</span><span class="RktSym">*</span><span class="stt"> </span><span class="RktSym">inner</span><span class="stt"> </span><span class="RktVal">4</span><span class="RktPn">)</span> could be written <span class="RktPn">(</span><span class="RktSym">*</span><span class="stt"> </span><span class="RktSym">inner</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">+</span><span class="stt"> </span><span class="RktVal">2</span><span class="stt"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span> or <span class="RktPn">(</span><span class="RktSym">*</span><span class="stt"> </span><span class="RktSym">inner</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">+</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">+</span><span class="stt"> </span><span class="RktVal">1</span><span class="stt"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym">+</span><span class="stt"> </span><span class="RktVal">1</span><span class="stt"> </span><span class="RktVal">1</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span>.</p></li></ol><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>Newcomers to Racket often gripe about prefix notation and the parentheses. If you need to get it out of your system, go ahead. Keep in mind, however, that it&rsquo;s not some peculiar affectation, but rather a necessary consequence of rule #1. As you&rsquo;ll come to learn, rule #1 is where the magic happens.</p></blockquote></blockquote></blockquote><p>That should tell you enough to infer what&rsquo;s going on in the Pollen commands above:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktMeta">#lang</span><span class="hspace">&nbsp;</sp