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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>4.13 Dynamic Binding: parameterize</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="icons.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="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" 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class="RktSym"><span class="RktStxLink">set!</span></span></a></td></tr><tr><td align="right">4.10 </td><td><a href="quote.html" class="tocviewlink" data-pltdoc="x">Quoting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quote</span></span> and <span class="RktVal">'</span></a></td></tr><tr><td align="right">4.11 </td><td><a href="qq.html" class="tocviewlink" data-pltdoc="x">Quasiquoting:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">quasiquote</span></span> and <span class="RktVal">‘</span></a></td></tr><tr><td align="right">4.12 </td><td><a href="case.html" class="tocviewlink" data-pltdoc="x">Simple Dispatch:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">case</span></span></a></td></tr><tr><td align="right">4.13 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Dynamic Binding:<span class="mywbr"> </span> <span class="RktSym"><span class="RktStxLink">parameterize</span></span></a></td></tr></table></div></div></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="case.html" title="backward to "4.12 Simple Dispatch: case"" data-pltdoc="x">← prev</a> <a href="scheme-forms.html" title="up to "4 Expressions and Definitions"" data-pltdoc="x">up</a> <a href="define-struct.html" title="forward to "5 Programmer-Defined Datatypes"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-source-pkg="racket-doc" x-part-tag=""parameterize"">4.13<tt> </tt><a name="(part._parameterize)"></a>Dynamic Binding: <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span></h4><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="magnify.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html&version=6.7" class="Sq" data-pltdoc="x">Parameters</a> in <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.7" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Reference</span></a> also documents <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span>.</p></blockquote></blockquote></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form associates a new value with a
<a name="(tech._parameter)"></a><span style="font-style: italic">parameter</span> during the evaluation of <span class="RktVar">body</span>
expressions:</p><blockquote class="leftindent"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktVar">parameter-expr</span><span class="hspace"> </span><span class="RktVar">value-expr</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></blockquote><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>The term “parameter” is sometimes used to refer to the
arguments of a function, but “parameter” in Racket
has the more specific meaning described here.</p></blockquote></blockquote></blockquote><p>For example, the <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error-print-width%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">error-print-width</a></span> parameter controls how
many characters of a value are printed in an error message:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error-print-width%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">error-print-width</a></span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._expt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">expt</a></span><span class="hspace"> </span><span class="RktVal">10</span><span class="hspace"> </span><span class="RktVal">1024</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">car: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: pair?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: 10...</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error-print-width%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">error-print-width</a></span><span class="hspace"> </span><span class="RktVal">10</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._expt%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">expt</a></span><span class="hspace"> </span><span class="RktVal">10</span><span class="hspace"> </span><span class="RktVal">1024</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">car: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: pair?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: 1000000...</span></p></td></tr></table></blockquote><p>More generally, parameters implement a kind of dynamic binding. The
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-parameter%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-parameter</a></span> function takes any value and returns a new
parameter that is initialized to the given value. Applying the
parameter as a function returns its current value:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-parameter%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">make-parameter</a></span><span class="hspace"> </span><span class="RktVal">"here"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"here"</span></p></td></tr></table></blockquote><p>In a <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form, each <span class="RktVar">parameter-expr</span> must
produce a parameter. During the evaluation of the <span class="RktSym">body</span>s, each
specified parameter is given the result of the corresponding
<span class="RktVar">value-expr</span>. When control leaves the <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span>
form—<wbr></wbr>either through a normal return, an exception, or some other
escape—<wbr></wbr>the parameter reverts to its earlier value:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"there"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"there"</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"here"</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"in a house"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"with a mouse"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("in a house" "with a mouse" "in a house")</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"in a box"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktErr">car: contract violation</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">expected: pair?</span></p></td></tr><tr><td><p><span class="RktErr"></span><span class="hspace"> </span><span class="RktErr">given: "in a box"</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"here"</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form is not a binding form like
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span>; each use of <span class="RktSym">location</span> above refers directly to
the original definition. A <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form adjusts the
value of a parameter during the whole time that the
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> body is evaluated, even for uses of the
parameter that are textually outside of the <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span>
body:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">would-you-could-you?</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">"here"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">"there"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">would-you-could-you?</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"on a bus"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">would-you-could-you?</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote><p>If a use of a parameter is textually inside the body of a
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> but not evaluated before the
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form produces a value, then the use does not see
the value installed by the <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">get</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"with a fox"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">get</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">"here"</span></p></td></tr></table></blockquote><p>The current binding of a parameter can be adjusted imperatively by
calling the parameter as a function with a value. If a
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> has adjusted the value of the parameter, then
directly applying the parameter procedure affects only the value
associated with the active <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span>:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">try-again!</span><span class="hspace"> </span><span class="RktSym">where</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktSym">where</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"here"</span></p></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">location</span><span class="hspace"> </span><span class="RktVal">"on a train"</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._list%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=begin.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._begin%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">begin</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">try-again!</span><span class="hspace"> </span><span class="RktVal">"in a boat"</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'("on a train" "in a boat")</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">location</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">"here"</span></p></td></tr></table></blockquote><p>Using <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> is generally preferable to updating a
parameter value imperatively—<wbr></wbr>for much the same reasons that binding
a fresh variable with <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">let</a></span> is preferable to using
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">set!</a></span> (see <a href="set_.html" data-pltdoc="x">Assignment: <span class="RktSym"><span class="RktStxLink">set!</span></span></a>).</p><p>It may seem that variables and <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">set!</a></span> can solve many of the
same problems that parameters solve. For example, <span class="RktSym">lokation</span>
could be defined as a string, and <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">set!</a></span> could be used
to adjust its value:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktSym">lokation</span><span class="hspace"> </span><span class="RktVal">"here"</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">would-ya-could-ya?</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=if.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._and%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">and</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">lokation</span><span class="hspace"> </span><span class="RktVal">"here"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._not%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">not</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=booleans.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=6.7" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">lokation</span><span class="hspace"> </span><span class="RktVal">"there"</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">lokation</span><span class="hspace"> </span><span class="RktVal">"on a bus"</span><span class="RktPn">)</span></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">would-ya-could-ya?</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</span></p></td></tr></table></blockquote><p>Parameters, however, offer several crucial advantages over
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">set!</a></span>:</p><ul><li><p>The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form helps automatically reset the
value of a parameter when control escapes due to an exception.
Adding exception handlers and other forms to rewind a
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=set_.html%23%2528form._%2528%2528quote._%7E23%7E25kernel%2529._set%2521%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">set!</a></span> is relatively tedious.</p></li><li><p>Parameters work nicely with tail calls (see
<a href="Lists__Iteration__and_Recursion.html#%28part._tail-recursion%29" data-pltdoc="x">Tail Recursion</a>). The last <span class="RktVar">body</span> in a
<span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form is in <a href="Lists__Iteration__and_Recursion.html#%28tech._tail._position%29" class="techoutside" data-pltdoc="x"><span class="techinside">tail position</span></a> with
respect to the <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form.</p></li><li><p>Parameters work properly with threads (see
<a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=threads.html&version=6.7" class="Sq" data-pltdoc="x">Threads</a>). The <span class="RktSym"><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.7" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> form adjusts
the value of a parameter only for evaluation in the current
thread, which avoids race conditions with other threads.</p></li></ul><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.7", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.7");">top</a></span><span class="navright"> <a href="case.html" title="backward to "4.12 Simple Dispatch: case"" data-pltdoc="x">← prev</a> <a href="scheme-forms.html" title="up to "4 Expressions and Definitions"" data-pltdoc="x">up</a> <a href="define-struct.html" title="forward to "5 Programmer-Defined Datatypes"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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