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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>10.1 Exceptions</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="../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" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-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">The Racket Guide</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="intro.html" class="tocviewlink" data-pltdoc="x">Welcome to Racket</a></td></tr><tr><td align="right">2 </td><td><a href="to-scheme.html" class="tocviewlink" data-pltdoc="x">Racket Essentials</a></td></tr><tr><td align="right">3 </td><td><a href="datatypes.html" class="tocviewlink" data-pltdoc="x">Built-<wbr></wbr>In Datatypes</a></td></tr><tr><td align="right">4 </td><td><a href="scheme-forms.html" class="tocviewlink" data-pltdoc="x">Expressions and Definitions</a></td></tr><tr><td align="right">5 </td><td><a href="define-struct.html" class="tocviewlink" data-pltdoc="x">Programmer-<wbr></wbr>Defined Datatypes</a></td></tr><tr><td align="right">6 </td><td><a href="modules.html" class="tocviewlink" data-pltdoc="x">Modules</a></td></tr><tr><td align="right">7 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">8 </td><td><a href="i_o.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">9 </td><td><a href="regexp.html" class="tocviewlink" data-pltdoc="x">Regular Expressions</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewselflink" data-pltdoc="x">Exceptions and Control</a></td></tr><tr><td align="right">11 </td><td><a href="for.html" class="tocviewlink" data-pltdoc="x">Iterations and Comprehensions</a></td></tr><tr><td align="right">12 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">13 </td><td><a href="classes.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">14 </td><td><a href="units.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr><tr><td align="right">16 </td><td><a href="macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">17 </td><td><a href="languages.html" class="tocviewlink" data-pltdoc="x">Creating Languages</a></td></tr><tr><td align="right">18 </td><td><a href="concurrency.html" class="tocviewlink" d
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raised. Unless the exception is caught, then it is handled by printing
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a message associated with the exception, and then escaping from the
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computation.</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252F%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">/</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">/: division by zero</span></p></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktVal">17</span><span class="RktPn">)</span></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: 17</span></p></td></tr></table></blockquote><p>To catch an exception, use the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span> form:</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktVar">predicate-expr</span><span class="hspace"> </span><span class="RktVar">handler-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><p>Each <span class="RktVar">predicate-expr</span> in a handler determines a kind of
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exception that is caught by the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span> form, and the
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value representing the exception is passed to the handler procedure
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produced by <span class="RktVar">handler-expr</span>. The result of the
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<span class="RktVar">handler-expr</span> is the result of the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span>
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expression.</p><p>For example, a divide-by-zero error raises an instance of the
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%7E3afail%7E3acontract%7E3adivide-by-zero%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail:contract:divide-by-zero</a></span> structure type:</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3acontract%7E3adivide-by-zero%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail:contract:divide-by-zero?</a></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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">+inf.0</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=generic-numbers.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._%252F%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">/</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktVal">0</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">+inf.0</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3acontract%7E3adivide-by-zero%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail:contract:divide-by-zero?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace">  
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packages an error message and other information into an
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<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%7E3afail%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail</a></span> structure:</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">error</a></span><span class="hspace"> </span><span class="RktVal">"crash!"</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">crash!</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">air-bag</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">error</a></span><span class="hspace"> </span><span class="RktVal">"crash!"</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'air-bag</span></p></td></tr></table></blockquote><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%7E3afail%7E3acontract%7E3adivide-by-zero%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail:contract:divide-by-zero</a></span> and <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%7E3afail%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail</a></span>
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structure types are sub-types of the <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn</a></span> structure
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type. Exceptions raised by core forms and functions always raise an
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instance of <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn</a></span> or one of its sub-types, but an exception
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does not have to be represented by a structure. The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._raise%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">raise</a></span>
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function lets you raise any value as an exception:</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._raise%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">raise</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktErr">uncaught exception: 2</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=Equality.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._equal%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">equal?</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">two</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._raise%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">raise</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'two</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc
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let you handle different kinds of exceptions in different ways. The
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predicates are tried in order, and if none of them match, then the
|
||
|
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exception is propagated to enclosing contexts.</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">always-fail</span><span class="hspace"> </span><span class="RktSym">n</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=number-types.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._even%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">even?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">even</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=number-types.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._positive%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">positive?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">positive</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._raise%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">raise</a></span><span class="hspace"> </span><span class="RktSym">n</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">always-fail</span><span class="hspace">&n
|
||
|
|
types Ctl-C in a terminal window or clicks the <span class="ssansserif">Stop</span> button
|
||
|
|
in DrRacket to interrupt a computation, then normally the
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._exn%7E3abreak%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:break</a></span> exception should not be caught. To catch only
|
||
|
|
exceptions that represent errors, use <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail?</a></span> as the
|
||
|
|
predicate:</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">oops</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktVal">17</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'oops</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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail?</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">oops</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=threads.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._break-thread%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">break-thread</a></span><span c
|
||
|
|
<span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn-message%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn-message</a></span> accessor provides a descriptive message for the
|
||
|
|
exception. The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn-continuation-marks%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn-continuation-marks</a></span> accessor provides
|
||
|
|
information about the point where the exception was raised.
|
||
|
|
</div><div class="SIntrapara"><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p>The <span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=contmarks.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._continuation-mark-set-%7E3econtext%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">continuation-mark-set->context</a></span> procedure provides best-effort structured backtrace information.</p></blockquote></blockquote></blockquote></div></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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._with-handlers%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">with-handlers</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn%7E3afail%7E3f%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn:fail?</a></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/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=8.6" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._error-display-handler%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">error-display-handler</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=exns.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._exn-message%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">exn-message</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym">v</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"><a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=reference&rel=pairs.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._car%2529%2529&version=8.6" class="RktValLink Sq" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktVal">17</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0"><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
|