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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>10.8 Unreachable Expressions</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.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 Reference</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="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewselflink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</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>10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr></table><div class="tocviewsublist" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">10.1 </td><td><a href="values.html" class="tocviewlink" data-pltdoc="x">Multiple Values</a></td></tr><tr><td align="right">10.2 </td><td><a href="exns.html" class="tocviewlink" data-pltdoc="x">Exceptions</a></td></tr><tr><td align="right">10.3 </td><td><a href="Delayed_Evaluation.html" class="tocviewlink" data-pltdoc="x">Delayed Evaluation</a></td></tr><tr><td align="right">10.4 </td><td><a href="cont.html" class="tocviewlink" data-pltdoc="x">Continuations</a></td></tr><tr><td align="right">10.5 </td><td><a href="contmarks.html" class="tocviewlink" data-pltdoc="x">Continuation Marks</a></td></tr><tr><td align="right">10.6 </td><td><a href="breakhandler.html" class="tocviewlink" data-pltdoc="x">Breaks</a></td></tr><tr><td align="right">10.7 </td><td><a href="Exiting.html" class="tocviewlink" data-pltdoc="x">Exiting</a></td></tr><tr><td align="right">10.8 </td><td><a href="unreachable.html" class="tocviewselflink" data-pltdoc="x">Unreachable Expressions</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_2");">►</a></td><td>10.8 </td><td><a href="unreachable.html" class="tocviewselflink" data-pltdoc="x">Unreachable Expressions</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">10.8.1 </td><td><a href="unreachable.html#%28part._with-unreachable%29" class="tocviewlink" data-pltdoc="x">Customized Unreachable Reporting</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="unreachable.html#%28def._%28%28quote._~23~25kernel%29._assert-unreachable%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">assert-<wbr></wbr>unreachable</span></span></a></td></tr><tr><td><span class="tocsublinknumber">10.8.1<tt> </tt></span><a href="unreachable.html#%28part._with-unreachable%29" class="tocsubseclink" data-pltdoc="x">Customized Unreachable Reporting</a></td></tr><tr><td><a href="unreachable.html#%28form._%28%28lib._racket%2Funreachable..rkt%29._with-assert-unreachable%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">with-<wbr></wbr>assert-<wbr></wbr>unreachable</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="Exiting.html" title="backward to "10.7 Exiting"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="concurrency.html" title="forward to "11 Concurrency and Parallelism"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""unreachable"">10.8<tt> </tt><a name="(part._unreachable)"></a>Unreachable Expressions</h4><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._assert-unreachable))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="unreachable.html#%28def._%28%28quote._~23~25kernel%29._assert-unreachable%29%29" class="RktValDef RktValLink" data-pltdoc="x">assert-unreachable</a></span></span><span class="RktPn"></span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._none%2Fc%29%29" class="RktValLink" data-pltdoc="x">none/c</a></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Reports an assertion failure by raising <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract</a></span>,
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which is useful as a safe counterpart to
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<span class="RktSym"><a href="unsafe.html#%28def._%28%28lib._racket%2Funsafe%2Fops..rkt%29._unsafe-assert-unreachable%29%29" class="RktValLink" data-pltdoc="x">unsafe-assert-unreachable</a></span>.</div></p><p class="SHistory">Added in version 8.0.0.11 of package <span class="stt">base</span>.</p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""with-unreachable"">10.8.1<tt> </tt><a name="(part._with-unreachable)"></a><a name="(mod-path._racket/unreachable)"></a>Customized Unreachable Reporting</h5><p><div class="SIntrapara"><table cellspacing="0" cellpadding="0" class="defmodule"><tr><td align="left"><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="stt"> </span><a href="unreachable.html#%28mod-path._racket%2Funreachable%29" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/unreachable</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller"> package:</span> <a href="https://pkgs.racket-lang.org/package/base" title="Install this package using `raco pkg install base`"><span class="stt">base</span></a></span></td></tr></table></div><div class="SIntrapara">The bindings documented in this section are provided by the <a href="unreachable.html#%28mod-path._racket%2Funreachable%29" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/unreachable</span></a> library, not <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/base</span></a> or <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket</span></a>.</div></p><p class="SHistory">Added in version 8.0.0.11 of package <span class="stt">base</span>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><table cellspacing="0" cellpadding="0" class="RktBlk RForeground"><tr><td><span class="RktPn">(</span><a name="(form._((lib._racket/unreachable..rkt)._with-assert-unreachable))"></a><span title="Provided from: racket/unreachable | Package: base"><span class="RktSym"><a href="unreachable.html#%28form._%28%28lib._racket%2Funreachable..rkt%29._with-assert-unreachable%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">with-assert-unreachable</a></span></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></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Similar to <span class="RktPn">(</span><span class="RktSym"><a href="unreachable.html#%28def._%28%28quote._~23~25kernel%29._assert-unreachable%29%29" class="RktValLink" data-pltdoc="x">assert-unreachable</a></span><span class="RktPn">)</span>, asserts that the
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<span class="RktVar">body</span> forms should not be reached.</div></p><p>Unless the expression is part of a module that includes
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<span class="RktPn">(</span><span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._~23~25declare%29%29" class="RktStxLink" data-pltdoc="x">#%declare</a></span><span class="stt"> </span><span class="RktPn">#:unsafe</span><span class="RktPn">)</span>, then it is equivalent to
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<span class="RktPn">(</span><span class="RktSym"><a href="let.html#%28form._%28%28quote._~23~25kernel%29._let-values%29%29" class="RktStxLink" data-pltdoc="x">let-values</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktVar">body</span><span class="stt"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span>. The intent is that the
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<span class="RktVar">body</span> forms will raise <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract</a></span>.</p><p>When a <span class="RktSym"><a href="unreachable.html#%28form._%28%28lib._racket%2Funreachable..rkt%29._with-assert-unreachable%29%29" class="RktStxLink" data-pltdoc="x">with-assert-unreachable</a></span> expression is part of a module
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with <span class="RktPn">(</span><span class="RktSym"><a href="module.html#%28form._%28%28quote._~23~25kernel%29._~23~25declare%29%29" class="RktStxLink" data-pltdoc="x">#%declare</a></span><span class="stt"> </span><span class="RktPn">#:unsafe</span><span class="RktPn">)</span>, then it is equivalent to
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<span class="RktPn">(</span><span class="RktSym"><a href="unsafe.html#%28def._%28%28lib._racket%2Funsafe%2Fops..rkt%29._unsafe-assert-unreachable%29%29" class="RktValLink" data-pltdoc="x">unsafe-assert-unreachable</a></span><span class="RktPn">)</span>.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="Exiting.html" title="backward to "10.7 Exiting"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="concurrency.html" title="forward to "11 Concurrency and Parallelism"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |