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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>7 Contracts</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="icons.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="../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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Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Synchronization</a></td></tr><tr><td align="right">19 </td><td><a href="performance.html" class="tocviewlink" data-pltdoc="x">Performance</a></td></tr><tr><td align="right">20 </td><td><a href="parallelism.html" class="tocviewlink" data-pltdoc="x">Parallelism</a></td></tr><tr><td align="right">21 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running and Creating Executables</a></td></tr><tr><td align="right">22 </td><td><a href="More_Libraries.html" class="tocviewlink" data-pltdoc="x">More Libraries</a></td></tr><tr><td align="right">23 </td><td><a href="dialects.html" class="tocviewlink" data-pltdoc="x">Dialects of Racket and Scheme</a></td></tr><tr><td align="right">24 </td><td><a href="other-editors.html" class="tocviewlink" data-pltdoc="x">Command-<wbr></wbr>Line Tools and Your Editor of Choice</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</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,"tocview_1");">▼</a></td><td>7 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Contracts</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">7.1 </td><td><a href="contract-boundaries.html" class="tocviewlink" data-pltdoc="x">Contracts and Boundaries</a></td></tr><tr><td align="right">7.2 </td><td><a href="contract-func.html" class="tocviewlink" data-pltdoc="x">Simple Contracts on Functions</a></td></tr><tr><td align="right">7.3 </td><td><a href="contracts-general-functions.html" class="tocviewlink" data-pltdoc="x">Contracts on Functions in General</a></td></tr><tr><td align="right">7.4 </td><td><a href="contracts-first.html" class="tocviewlink" data-pltdoc="x">Contracts:<span class="mywbr"> </span> A Thorough Example</a></td></tr><tr><td align="right">7.5 </td><td><a href="contracts-struct.html" class="tocviewlink" data-pltdoc="x">Contracts on Structures</a></td></tr><tr><td align="right">7.6 </td><td><a href="contracts-exists.html" class="tocviewlink" data-pltdoc="x">Abstract Contracts using <span class="RktPn">#:<span class="mywbr"> </span>exists</span> and <span class="RktPn">#:<span class="mywbr"> </span>∃</span></a></td></tr><tr><td align="right">7.7 </td><td><a href="contracts-examples.html" class="tocviewlink" data-pltdoc="x">Additional Examples</a></td></tr><tr><td align="right">7.8 </td><td><a href="contracts-gotchas.html" class="tocviewlink" data-pltdoc="x">Gotchas</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, "6.1", "../");" 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("6.1");">top</a></span><span class="navright"> <a href="module-set.html" title="backward to "6.6 Assignment and Redefinition"" data-pltdoc="x">← prev</a> <a href="index.html" title="up to "The Racket Guide"" data-pltdoc="x">up</a> <a href="contract-boundaries.html" title="forward to "7.1 Contracts and Boundaries"" data-pltdoc="x">next →</a></span> </div><h3 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""contracts"">7<tt> </tt><a name="(part._contracts)"></a>Contracts</h3><p>This chapter provides a gentle introduction to Racket’s
contract system.</p><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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=contracts.html&version=6.1" class="Sq" data-pltdoc="x">Contracts</a> in <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Reference</span></a> provides more on contracts.</p></blockquote></blockquote></blockquote><table cellspacing="0" cellpadding="0"><tr><td><p><span class="hspace"> </span><a href="contract-boundaries.html" class="toclink" data-pltdoc="x">7.1<span class="hspace"> </span>Contracts and Boundaries</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-boundaries.html#%28part._contracts-amount0%29" class="toclink" data-pltdoc="x">7.1.1<span class="hspace"> </span>Contract Violations</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-boundaries.html#%28part._.Experimenting_with_.Contracts_and_.Modules%29" class="toclink" data-pltdoc="x">7.1.2<span class="hspace"> </span>Experimenting with Contracts and Modules</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-boundaries.html#%28part._contracts-intro-nested%29" class="toclink" data-pltdoc="x">7.1.3<span class="hspace"> </span>Experimenting with Nested Contract Boundaries</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html" class="toclink" data-pltdoc="x">7.2<span class="hspace"> </span>Simple Contracts on Functions</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._.Styles_of_-_%29" class="toclink" data-pltdoc="x">7.2.1<span class="hspace"> </span>Styles of <span class="RktSym"><span class="RktStxLink"><span class="nobreak">-></span></span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._simple-nested%29" class="toclink" data-pltdoc="x">7.2.2<span class="hspace"> </span>Using <span class="RktSym"><span class="RktStxLink">define/contract</span></span> and <span class="RktSym"><span class="RktStxLink"><span class="nobreak">-></span></span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._any_and_any_c%29" class="toclink" data-pltdoc="x">7.2.3<span class="hspace"> </span><span class="RktSym"><span class="RktStxLink">any</span></span> and <span class="RktSym"><span class="RktValLink">any/c</span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._contracts-own%29" class="toclink" data-pltdoc="x">7.2.4<span class="hspace"> </span>Rolling Your Own Contracts</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._.Contracts_on_.Higher-order_.Functions%29" class="toclink" data-pltdoc="x">7.2.5<span class="hspace"> </span>Contracts on Higher-order Functions</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._contracts-flat-named-contracts%29" class="toclink" data-pltdoc="x">7.2.6<span class="hspace"> </span>Contract Messages with “???”</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contract-func.html#%28part._contracts-dissecting-contract-errors%29" class="toclink" data-pltdoc="x">7.2.7<span class="hspace"> </span>Dissecting a contract error message</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html" class="toclink" data-pltdoc="x">7.3<span class="hspace"> </span>Contracts on Functions in General</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-optional%29" class="toclink" data-pltdoc="x">7.3.1<span class="hspace"> </span>Optional Arguments</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-rest-args%29" class="toclink" data-pltdoc="x">7.3.2<span class="hspace"> </span>Rest Arguments</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-keywords%29" class="toclink" data-pltdoc="x">7.3.3<span class="hspace"> </span>Keyword Arguments</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-optional-keywords%29" class="toclink" data-pltdoc="x">7.3.4<span class="hspace"> </span>Optional Keyword Arguments</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-case-lambda%29" class="toclink" data-pltdoc="x">7.3.5<span class="hspace"> </span>Contracts for <span class="RktSym"><span class="RktStxLink">case-lambda</span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-arrow-d%29" class="toclink" data-pltdoc="x">7.3.6<span class="hspace"> </span>Argument and Result Dependencies</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-arrow-d-eval-order%29" class="toclink" data-pltdoc="x">7.3.7<span class="hspace"> </span>Checking State Changes</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-multiple%29" class="toclink" data-pltdoc="x">7.3.8<span class="hspace"> </span>Multiple Result Values</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-general-functions.html#%28part._contracts-no-domain%29" class="toclink" data-pltdoc="x">7.3.9<span class="hspace"> </span>Fixed but Statically Unknown Arities</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-first.html" class="toclink" data-pltdoc="x">7.4<span class="hspace"> </span>Contracts: A Thorough Example</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-struct.html" class="toclink" data-pltdoc="x">7.5<span class="hspace"> </span>Contracts on Structures</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-struct.html#%28part._contracts-single-struct%29" class="toclink" data-pltdoc="x">7.5.1<span class="hspace"> </span>Guarantees for a Specific Value</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-struct.html#%28part._contracts-define-struct%29" class="toclink" data-pltdoc="x">7.5.2<span class="hspace"> </span>Guarantees for All Values</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-struct.html#%28part._contracts-lazy-contracts%29" class="toclink" data-pltdoc="x">7.5.3<span class="hspace"> </span>Checking Properties of Data Structures</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-exists.html" class="toclink" data-pltdoc="x">7.6<span class="hspace"> </span>Abstract Contracts using <span class="RktPn">#:exists</span> and <span class="RktPn">#:∃</span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-examples.html" class="toclink" data-pltdoc="x">7.7<span class="hspace"> </span>Additional Examples</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-examples.html#%28part._.A_.Customer-.Manager_.Component%29" class="toclink" data-pltdoc="x">7.7.1<span class="hspace"> </span>A Customer-Manager Component</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-examples.html#%28part._.A_.Parameteric__.Simple__.Stack%29" class="toclink" data-pltdoc="x">7.7.2<span class="hspace"> </span>A Parameteric (Simple) Stack</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-examples.html#%28part._.A_.Dictionary%29" class="toclink" data-pltdoc="x">7.7.3<span class="hspace"> </span>A Dictionary</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-examples.html#%28part._.A_.Queue%29" class="toclink" data-pltdoc="x">7.7.4<span class="hspace"> </span>A Queue</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-gotchas.html" class="toclink" data-pltdoc="x">7.8<span class="hspace"> </span>Gotchas</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-gotchas.html#%28part._.Contracts_and_eq_%29" class="toclink" data-pltdoc="x">7.8.1<span class="hspace"> </span>Contracts and <span class="RktSym"><span class="RktValLink">eq?</span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-gotchas.html#%28part._contracts-gotcha-nested%29" class="toclink" data-pltdoc="x">7.8.2<span class="hspace"> </span>Contract boundaries and <span class="RktSym"><span class="RktStxLink">define/contract</span></span></a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-gotchas.html#%28part._contracts-exists-gotcha%29" class="toclink" data-pltdoc="x">7.8.3<span class="hspace"> </span>Exists Contracts and Predicates</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-gotchas.html#%28part._.Defining_.Recursive_.Contracts%29" class="toclink" data-pltdoc="x">7.8.4<span class="hspace"> </span>Defining Recursive Contracts</a></p></td></tr><tr><td><p><span class="hspace"> </span><a href="contracts-gotchas.html#%28part._.Mixing_set__and_contract-out%29" class="toclink" data-pltdoc="x">7.8.5<span class="hspace"> </span>Mixing <span class="RktSym"><span class="RktStxLink">set!</span></span> and <span class="RktSym"><span class="RktStxLink">contract-out</span></span></a></p></td></tr></table><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" 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