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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>10.4 Continuations</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="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" 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"><span style="font-weight: bold">The Racket Reference</span></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="" class="tocviewselflink" 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></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>10.4 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Continuations</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">10.4.1 </td><td><a href="#%28part._.Additional_.Control_.Operators%29" class="tocviewlink" data-pltdoc="x">Additional Control Operators</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="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>continuation-<wbr></wbr>prompt</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">abort-<wbr></wbr>current-<wbr></wbr>continuation</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">make-<wbr></wbr>continuation-<wbr></wbr>prompt-<wbr></wbr>tag</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">default-<wbr></wbr>continuation-<wbr></wbr>prompt-<wbr></wbr>tag</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>current-<wbr></wbr>continuation</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call/<span class="mywbr"> </span>cc</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-composable-continuation%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>composable-<wbr></wbr>continuation</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>escape-<wbr></wbr>continuation</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._call%2Fec%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call/<span class="mywbr"> </span>ec</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._let%2Fcc%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">let/<span class="mywbr"> </span>cc</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2Fec%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">let/<span class="mywbr"> </span>ec</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-barrier%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call-<wbr></wbr>with-<wbr></wbr>continuation-<wbr></wbr>barrier</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-available~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">continuation-<wbr></wbr>prompt-<wbr></wbr>available?</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">continuation?</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">continuation-<wbr></wbr>prompt-<wbr></wbr>tag?</span></span></a></td></tr><tr><td><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">dynamic-<wbr></wbr>wind</span></span></a></td></tr><tr><td><span class="tocsublinknumber">10.4.1<tt> </tt></span><a href="#%28part._.Additional_.Control_.Operators%29" class="tocsubseclink" data-pltdoc="x">Additional Control Operators</a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._call%2Fprompt%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call/<span class="mywbr"> </span>prompt</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%2Fcc%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">abort/<span class="mywbr"> </span>cc</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._call%2Fcomp%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">call/<span class="mywbr"> </span>comp</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">abort</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">%</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._fcontrol%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">fcontrol</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">prompt</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">control</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">prompt-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">control-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">reset</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">shift</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">reset-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">shift-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">prompt0</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">reset0</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">control0</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift0%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">shift0</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">prompt0-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">reset0-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">control0-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift0-at%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">shift0-<wbr></wbr>at</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._spawn%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">spawn</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._splitter%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">splitter</span></span></a></td></tr><tr><td><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._new-prompt%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">new-<wbr></wbr>prompt</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._set%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">set</span></span></a></td></tr><tr><td><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._cupto%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">cupto</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" 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="Delayed_Evaluation.html" title="backward to "10.3 Delayed Evaluation"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="contmarks.html" title="forward to "10.5 Continuation Marks"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""cont"">10.4<tt> </tt><a name="(part._cont)"></a>Continuations</h4><blockquote class="refpara"><blockquote class="refcolumn"><blockquote class="refcontent"><p><span class="imageleft"><img src="finger.png" alt="+" width="24" height="24"/></span><a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=conts.html&version=6.7" class="Sq" data-pltdoc="x">Continuations</a> in <a href="https://download.racket-lang.org/docs/6.7/html/local-redirect/index.html?doc=guide&rel=index.html&version=6.7" class="Sq" data-pltdoc="x"><span style="font-weight: bold">The Racket Guide</span></a> introduces continuations.</p></blockquote></blockquote></blockquote><p>See <a href="eval-model.html#%28part._cont-model%29" data-pltdoc="x">Sub-expression Evaluation and Continuations</a> and <a href="eval-model.html#%28part._prompt-model%29" data-pltdoc="x">Prompts, Delimited Continuations, and Barriers</a> for general
information about continuations. Racket’s support for prompts and
composable continuations most closely resembles Dorai Sitaram’s
<span class="RktSym">%</span> and <span class="RktSym">fcontrol</span> operator [<a href="doc-bibliography.html#%28cite._.Sitaram93%29" data-pltdoc="x">Sitaram93</a>].</p><p>Racket installs a <a href="eval-model.html#%28tech._continuation._barrier%29" class="techoutside" data-pltdoc="x"><span class="techinside">continuation barrier</span></a> around evaluation in the
following contexts, preventing full-continuation jumps into the
evaluation context protected by the barrier:</p><ul><li><p>applying an exception handler, an error escape handler, or an
error display handler (see <a href="exns.html" data-pltdoc="x">Exceptions</a>);</p></li><li><p>applying a macro transformer (see <a href="stxtrans.html" data-pltdoc="x">Syntax Transformers</a>),
evaluating a compile-time expression, or applying a module name
resolver (see <a href="Module_Names_and_Loading.html#%28part._modnameresolver%29" data-pltdoc="x">Resolving Module Names</a>);</p></li><li><p>applying a custom-port procedure (see <a href="customport.html" data-pltdoc="x">Custom Ports</a>), an
event guard procedure (see <a href="sync.html" data-pltdoc="x">Events</a>), or a parameter guard
procedure (see <a href="parameters.html" data-pltdoc="x">Parameters</a>);</p></li><li><p>applying a security-guard procedure (see
<a href="securityguards.html" data-pltdoc="x">Security Guards</a>);</p></li><li><p>applying a will procedure (see <a href="willexecutor.html" data-pltdoc="x">Wills and Executors</a>); or</p></li><li><p>evaluating or loading code from the stand-alone Racket
command line (see <a href="running-sa.html" data-pltdoc="x">Running Racket or GRacket</a>).</p></li></ul><p>In addition, extensions of Racket may install barriers in
additional contexts. Finally,
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-barrier%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-barrier</a></span> applies a thunk barrier
between the application and the current continuation.</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>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-continuation-prompt))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">proc</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span>[</td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">handler</span>]</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">arg</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">=</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">handler</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">arg</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Applies <span class="RktVar">proc</span> to the given <span class="RktVar">arg</span>s with the current
continuation extended by a prompt. The prompt is tagged by
<span class="RktVar">prompt-tag</span>, which must be a result from either
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span> (the default) or
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span>. The result of <span class="RktVar">proc</span> is
the result of the <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span> call.</div></p><p>The <span class="RktVar">handler</span> argument specifies a handler procedure to be
called in tail position with respect to the
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span> call when the installed prompt
is the target of an <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="RktValLink" data-pltdoc="x">abort-current-continuation</a></span> call with
<span class="RktVar">prompt-tag</span>; the remaining arguments of
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="RktValLink" data-pltdoc="x">abort-current-continuation</a></span> are supplied to the handler
procedure. If <span class="RktVar">handler</span> is <span class="RktVal">#f</span>, the default handler
accepts a single <span class="RktVar">abort-thunk</span> argument and calls
<span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span><span class="stt"> </span><span class="RktVar">abort-thunk</span><span class="stt"> </span><span class="RktVar">prompt-tag</span><span class="stt"> </span><span class="RktVal">#f</span><span class="RktPn">)</span>;
that is, the default handler re-installs the prompt and continues with
a given thunk.</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>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._abort-current-continuation))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="RktValDef RktValLink" data-pltdoc="x">abort-current-continuation</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">v</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">prompt-tag</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Resets the current continuation to that of the nearest prompt tagged
by <span class="RktVar">prompt-tag</span> in the current continuation; if no such prompt exists,
the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract~3acontinuation%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract:continuation</a></span> exception is raised. The <span class="RktVar">v</span>s are delivered
as arguments to the target prompt’s handler procedure.</div></p><p>The protocol for <span class="RktVar">v</span>s supplied to an abort is specific to the
<span class="RktVar">prompt-tag</span>. When <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="RktValLink" data-pltdoc="x">abort-current-continuation</a></span> is used with
<span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span>, generally, a single thunk
should be supplied that is suitable for use with the default prompt
handler. Similarly, when <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span> is
used with <span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span>, the associated
handler should generally accept a single thunk argument.</p><p>Each <a href="eval-model.html#%28tech._thread%29" class="techoutside" data-pltdoc="x"><span class="techinside">thread</span></a>’s continuation starts with a prompt for
<span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span> that uses the default
handler, which accepts a single thunk to apply (with the prompt
intact).</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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._make-continuation-prompt-tag))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span></span><span class="RktPn"></span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></p></blockquote></td></tr><tr><td><span class="RktPn">(</span><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span></span><span class="hspace"> </span><span class="RktVar">sym</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">sym</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="symbols.html#%28def._%28%28quote._~23~25kernel%29._symbol~3f%29%29" class="RktValLink" data-pltdoc="x">symbol?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Creates a prompt tag that is not <span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._equal~3f%29%29" class="RktValLink" data-pltdoc="x">equal?</a></span> to the result of any
other value (including prior or future results from
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span>). The optional <span class="RktVar">sym</span>
argument, if supplied, is used when printing the prompt tag.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._default-continuation-prompt-tag))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValDef RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span></span><span class="RktPn"></span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Returns a constant prompt tag for which a prompt is installed at the
start of every thread’s continuation; the handler for each thread’s
initial prompt accepts any number of values and returns. The result of
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span> is the default tag for
any procedure that accepts a prompt tag.</div></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>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-current-continuation))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-current-continuation</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">proc</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span>[</td><td valign="top"><span class="RktVar">prompt-tag</span>]<span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">=</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Captures the current continuation up to the nearest prompt tagged by
<span class="RktVar">prompt-tag</span>; if no such prompt exists, the
<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract~3acontinuation%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract:continuation</a></span> exception is raised. The truncated continuation
includes only continuation marks and <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> frames
installed since the prompt.</div></p><p>The capture continuation is delivered to <span class="RktVar">proc</span>, which is
called in tail position with respect to the
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span> call.</p><p>If the continuation argument to <span class="RktVar">proc</span> is ever applied, then it
removes the portion of the current continuation up to the nearest
prompt tagged by <span class="RktVar">prompt-tag</span> (not including the prompt; if no
such prompt exists, the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract~3acontinuation%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract:continuation</a></span> exception is raised), or
up to the nearest continuation frame (if any) shared by the current
and captured continuations—<wbr></wbr>whichever is first. While removing
continuation frames, <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> <span class="RktVar">post-thunk</span>s are
executed. Finally, the (unshared portion of the) captured continuation
is appended to the remaining continuation, applying
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> <span class="RktVar">pre-thunk</span>s.</p><p>The arguments supplied to an applied procedure become the result
values for the restored continuation. In particular, if multiple
arguments are supplied, then the continuation receives multiple
results.</p><p>If, at application time, a <a href="eval-model.html#%28tech._continuation._barrier%29" class="techoutside" data-pltdoc="x"><span class="techinside">continuation barrier</span></a> would be
introduced by replacing the current continuation with the applied one,
then the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract~3acontinuation%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract:continuation</a></span> exception is raised.</p><p>A continuation can be invoked from the thread (see
<a href="threads.html" data-pltdoc="x">Threads</a>) other than the one where it was captured.</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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/private/more-scheme..rkt)._call/cc))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="RktValDef RktValLink" data-pltdoc="x">call/cc</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>[<span class="RktVar">prompt-tag</span>]<span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">=</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="RktValLink" data-pltdoc="x">call/cc</a></span> binding is an alias for <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span>.</div></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>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-composable-continuation))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-composable-continuation%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-composable-continuation</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">proc</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span>[</td><td valign="top"><span class="RktVar">prompt-tag</span>]<span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">=</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Similar to <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span>, but applying
the resulting continuation procedure does not remove any portion of
the current continuation. Instead, application always extends the
current continuation with the captured continuation (without
installing any prompts other than those captured in the
continuation).</div></p><p>When <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-composable-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-composable-continuation</a></span> is called, if a
continuation barrier appears in the continuation before the closest
prompt tagged by <span class="RktVar">prompt-tag</span>, the
<span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail~3acontract~3acontinuation%29%29" class="RktValLink" data-pltdoc="x">exn:fail:contract:continuation</a></span> exception is raised (because attempting to apply
the continuation would always fail).</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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-escape-continuation))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span>, but <span class="RktVar">proc</span> is not
called in tail position, and the continuation procedure supplied to
<span class="RktVar">proc</span> can only be called during the dynamic extent of the
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span> call.</div></p><p>Due to the limited applicability of its continuation,
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span> can be implemented more efficiently
than <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span>.</p><p>A continuation obtained from <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span> is
actually a kind of prompt. Escape continuations are provided mainly
for backwards compatibility, since they pre-date general prompts in
Racket, and because <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._call%2Fec%29%29" class="RktValLink" data-pltdoc="x">call/ec</a></span> is often an easy replacement
for <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="RktValLink" data-pltdoc="x">call/cc</a></span> to improve performance.</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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/private/letstx-scheme..rkt)._call/ec))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._call%2Fec%29%29" class="RktValDef RktValLink" data-pltdoc="x">call/ec</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._call%2Fec%29%29" class="RktValLink" data-pltdoc="x">call/ec</a></span> binding is an alias for <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span>.</div></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><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/more-scheme..rkt)._let/cc))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._let%2Fcc%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">let/cc</a></span></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Equivalent to <span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="RktValLink" data-pltdoc="x">call/cc</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktVar">body</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span>.</div></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><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/private/letstx-scheme..rkt)._let/ec))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2Fec%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">let/ec</a></span></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Equivalent to <span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._call%2Fec%29%29" class="RktValLink" data-pltdoc="x">call/ec</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="stt"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="stt"> </span><span class="RktVar">body</span><span class="stt"> </span><span class="RktMeta"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span>.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-with-continuation-barrier))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-barrier%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-with-continuation-barrier</a></span></span><span class="hspace"> </span><span class="RktVar">thunk</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Applies <span class="RktVar">thunk</span> with a <a href="eval-model.html#%28tech._continuation._barrier%29" class="techoutside" data-pltdoc="x"><span class="techinside">continuation barrier</span></a> between the
application and the current continuation. The results of
<span class="RktVar">thunk</span> are the results of the
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-barrier%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-barrier</a></span> call.</div></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>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._continuation-prompt-available~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-available~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">continuation-prompt-available?</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span>[</td><td valign="top"><span class="RktVar">cont</span>]<span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">prompt-tag</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">cont</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="RktValLink" data-pltdoc="x">call/cc</a></span><span class="hspace"> </span><span class="RktSym"><a href="values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29" class="RktValLink" data-pltdoc="x">values</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">cont</span>, which must be a continuation,
includes a prompt tagged by <span class="RktVar">prompt-tag</span>, <span class="RktVal">#f</span>
otherwise.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._continuation~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">continuation?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara"> Return <span class="RktVal">#t</span> if
<span class="RktVar">v</span> is a continuation as produced by
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span>,
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-composable-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-composable-continuation</a></span>, or
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span>, <span class="RktVal">#f</span> otherwise.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._continuation-prompt-tag~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="booleans.html#%28def._%28%28quote._~23~25kernel%29._boolean~3f%29%29" class="RktValLink" data-pltdoc="x">boolean?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">v</span> is a continuation prompt tag as produced by
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span> or <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span>.</div></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>procedure</p></div></div><table cellspacing="0" cellpadding="0" class="prototype RForeground"><tr><td valign="top"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._dynamic-wind))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValDef RktValLink" data-pltdoc="x">dynamic-wind</a></span></span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">pre-thunk</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">value-thunk</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">post-thunk</span><span class="RktPn">)</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">→</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></td></tr></table></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">pre-thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">value-thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">post-thunk</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">Applies its three thunk arguments in order. The value of a
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> expression is the value returned by
<span class="RktVar">value-thunk</span>. The <span class="RktVar">pre-thunk</span> procedure is invoked
before calling <span class="RktVar">value-thunk</span> and <span class="RktVar">post-thunk</span> is invoked
after <span class="RktVar">value-thunk</span> returns. The special properties of
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> are manifest when control jumps into or out of
the <span class="RktVar">value-thunk</span> application (either due to a prompt abort or
a continuation invocation): every time control jumps into the
<span class="RktVar">value-thunk</span> application, <span class="RktVar">pre-thunk</span> is invoked, and
every time control jumps out of <span class="RktVar">value-thunk</span>,
<span class="RktVar">post-thunk</span> is invoked. (No special handling is performed for
jumps into or out of the <span class="RktVar">pre-thunk</span> and <span class="RktVar">post-thunk</span>
applications.)</div></p><p>When <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> calls <span class="RktVar">pre-thunk</span> for normal
evaluation of <span class="RktVar">value-thunk</span>, the continuation of the
<span class="RktVar">pre-thunk</span> application calls <span class="RktVar">value-thunk</span> (with
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span>’s special jump handling) and then
<span class="RktVar">post-thunk</span>. Similarly, the continuation of the
<span class="RktVar">post-thunk</span> application returns the value of the preceding
<span class="RktVar">value-thunk</span> application to the continuation of the entire
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> application.</p><p>When <span class="RktVar">pre-thunk</span> is called due to a continuation jump, the
continuation of <span class="RktVar">pre-thunk</span></p><ul><li><p>jumps to a more deeply nested <span class="RktVar">pre-thunk</span>, if any, or jumps
to the destination continuation; then</p></li><li><p>continues with the context of the <span class="RktVar">pre-thunk</span>’s
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> call.</p></li></ul><p>Normally, the second part of this continuation is never reached, due
to a jump in the first part. However, the second part is relevant
because it enables jumps to escape continuations that are contained in
the context of the <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> call. Furthermore, it means
that the continuation marks (see <a href="contmarks.html" data-pltdoc="x">Continuation Marks</a>) and
parameterization (see <a href="parameters.html" data-pltdoc="x">Parameters</a>) for <span class="RktVar">pre-thunk</span>
correspond to those of the <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> call that installed
<span class="RktVar">pre-thunk</span>. The <span class="RktVar">pre-thunk</span> call, however, is
<span class="RktSym"><a href="breakhandler.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxLink" data-pltdoc="x">parameterize-break</a></span>ed to disable breaks (see also
<a href="breakhandler.html" data-pltdoc="x">Breaks</a>).</p><p>Similarly, when <span class="RktVar">post-thunk</span> is called due to a continuation
jump, the continuation of <span class="RktVar">post-thunk</span> jumps to a less deeply
nested <span class="RktVar">post-thunk</span>, if any, or jumps to a <span class="RktVar">pre-thunk</span>
protecting the destination, if any, or jumps to the destination
continuation, then continues from the <span class="RktVar">post-thunk</span>’s
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> application. As for <span class="RktVar">pre-thunk</span>, the
parameterization of the original <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span> call is
restored for the call, and the call is <span class="RktSym"><a href="breakhandler.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize-break%29%29" class="RktStxLink" data-pltdoc="x">parameterize-break</a></span>ed
to disable breaks.</p><p>In both cases, the target for a jump is recomputed after each
<span class="RktVar">pre-thunk</span> or <span class="RktVar">post-thunk</span> completes. When a
prompt-delimited continuation (see <a href="eval-model.html#%28part._prompt-model%29" data-pltdoc="x">Prompts, Delimited Continuations, and Barriers</a>) is
captured in a <span class="RktVar">post-thunk</span>, it might be delimited and
instantiated in such a way that the target of a jump turns out to be
different when the continuation is applied than when the continuation
was captured. There may even be no appropriate target, if a relevant
prompt or escape continuation is not in the continuation after the
restore; in that case, the first step in a <span class="RktVar">pre-thunk</span> or
<span class="RktVar">post-thunk</span>’s continuation can raise an exception.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><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="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2Fec%29%29" class="RktStxLink" data-pltdoc="x">let/ec</a></span><span class="hspace"> </span><span class="RktSym">out</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktVal">"in "</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </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="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktVal">"pre "</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="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._call%2Fcc%29%29" class="RktValLink" data-pltdoc="x">call/cc</a></span><span class="hspace"> </span><span class="RktSym">out</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">#f</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._display%29%29" class="RktValLink" data-pltdoc="x">display</a></span><span class="hspace"> </span><span class="RktVal">"out "</span><span class="RktPn">)</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"><a href="when_unless.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._when%29%29" class="RktStxLink" data-pltdoc="x">when</a></span><span class="hspace"> </span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">v</span><span class="hspace"> </span><span class="RktVal">"post "</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktOut">in pre out in post out </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="#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2Fec%29%29" class="RktStxLink" data-pltdoc="x">let/ec</a></span><span class="hspace"> </span><span class="RktSym">k0</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2Fec%29%29" class="RktStxLink" data-pltdoc="x">let/ec</a></span><span class="hspace"> </span><span class="RktSym">k1</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void%29%29" class="RktValLink" data-pltdoc="x">void</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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">k0</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">cancel</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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">k1</span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">cancel-canceled</span><span class="RktPn">)</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">'cancel-canceled</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="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%2A%29%29" class="RktStxLink" data-pltdoc="x">let*</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="parameters.html#%28def._%28%28quote._~23~25kernel%29._make-parameter%29%29" class="RktValLink" data-pltdoc="x">make-parameter</a></span><span class="hspace"> </span><span class="RktVal">0</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">l</span><span class="hspace"> </span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._null%29%29" class="RktValLink" data-pltdoc="x">null</a></span><span class="RktPn">]</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">[</span><span class="RktSym">add</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</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="set_.html#%28form._%28%28quote._~23~25kernel%29._set%21%29%29" class="RktStxLink" data-pltdoc="x">set!</a></span><span class="hspace"> </span><span class="RktSym">l</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._append%29%29" class="RktValLink" data-pltdoc="x">append</a></span><span class="hspace"> </span><span class="RktSym">l</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._list%29%29" class="RktValLink" data-pltdoc="x">list</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="hspace"> </span><span class="RktSym">b</span><span class="RktPn">)</span><span class="RktPn">)</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"><a href="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</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="#%28def._%28%28quote._~23~25kernel%29._dynamic-wind%29%29" class="RktValLink" data-pltdoc="x">dynamic-wind</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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">add</span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktVal">6</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="let.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._let%29%29" class="RktStxLink" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">k+e</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._let%2Fcc%29%29" class="RktStxLink" data-pltdoc="x">let/cc</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void%29%29" class="RktValLink" data-pltdoc="x">void</a></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">add</span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</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="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cdr%29%29" class="RktValLink" data-pltdoc="x">cdr</a></span><span class="hspace"> </span><span class="RktSym">k+e</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="pairs.html#%28def._%28%28quote._~23~25kernel%29._car%29%29" class="RktValLink" data-pltdoc="x">car</a></span><span class="hspace"> </span><span class="RktSym">k+e</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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">add</span><span class="hspace"> </span><span class="RktVal">3</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">x</span><span class="RktPn">)</span><span class="RktPn">)</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"><a href="parameters.html#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._parameterize%29%29" class="RktStxLink" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">x</span><span class="hspace"> </span><span class="RktVal">7</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="#%28form._%28%28lib._racket%2Fprivate%2Fmore-scheme..rkt%29._let%2Fcc%29%29" class="RktStxLink" data-pltdoc="x">let/cc</a></span><span class="hspace"> </span><span class="RktSym">esc</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="pairs.html#%28def._%28%28quote._~23~25kernel%29._cons%29%29" class="RktValLink" data-pltdoc="x">cons</a></span><span class="hspace"> </span><span class="RktSym"><a href="void.html#%28def._%28%28quote._~23~25kernel%29._void%29%29" class="RktValLink" data-pltdoc="x">void</a></span><span class="hspace"> </span><span class="RktSym">esc</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="RktSym">l</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">'((1 . 5) (2 . 6) (3 . 5) (1 . 5) (2 . 6) (3 . 5))</span></p></td></tr></table></blockquote></div></p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Additional_Control_Operators"">10.4.1<tt> </tt><a name="(part._.Additional_.Control_.Operators)"></a><a name="(mod-path._racket/control)"></a>Additional Control Operators</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="#%28mod-path._racket%2Fcontrol%29" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/control</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller"> package:</span> <span class="stt">base</span></span></td></tr></table></div><div class="SIntrapara">The bindings documented in this section are provided by the <a href="#%28mod-path._racket%2Fcontrol%29" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/control</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>The <span class="RktSym">racket/control</span> library provides various control operators
from the research literature on higher-order control operators, plus a
few extra convenience forms. These control operators are implemented
in terms of <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span>,
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-composable-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-composable-continuation</a></span>, etc<span class="Sendabbrev">.</span>, and they generally
work sensibly together. Many are redundant; for example,
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span> and <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span> are aliases.</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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/control..rkt)._call/prompt))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._call%2Fprompt%29%29" class="RktValDef RktValLink" data-pltdoc="x">call/prompt</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>[<span class="RktVar">prompt-tag</span><span class="hspace"> </span><span class="RktVar">handler</span>]<span class="hspace"> </span><span class="RktVar">arg</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">=</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">handler</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._or%2Fc%29%29" class="RktValLink" data-pltdoc="x">or/c</a></span><span class="hspace"> </span><span class="RktSym"><a href="procedures.html#%28def._%28%28quote._~23~25kernel%29._procedure~3f%29%29" class="RktValLink" data-pltdoc="x">procedure?</a></span><span class="hspace"> </span><span class="RktVal">#f</span><span class="RktPn">)</span><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">arg</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._call%2Fprompt%29%29" class="RktValLink" data-pltdoc="x">call/prompt</a></span> binding is an alias for <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span>.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/control..rkt)._abort/cc))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%2Fcc%29%29" class="RktValDef RktValLink" data-pltdoc="x">abort/cc</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag</span><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">prompt-tag</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%2Fcc%29%29" class="RktValLink" data-pltdoc="x">abort/cc</a></span> binding is an alias for <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="RktValLink" data-pltdoc="x">abort-current-continuation</a></span>.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/control..rkt)._call/comp))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._call%2Fcomp%29%29" class="RktValDef RktValLink" data-pltdoc="x">call/comp</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>[<span class="RktVar">prompt-tag</span>]<span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">prompt-tag</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation-prompt-tag~3f%29%29" class="RktValLink" data-pltdoc="x">continuation-prompt-tag?</a></span></td></tr><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">=</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">The <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._call%2Fcomp%29%29" class="RktValLink" data-pltdoc="x">call/comp</a></span> binding is an alias for <span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._call-with-composable-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-composable-continuation</a></span>.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/control..rkt)._abort))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValDef RktValLink" data-pltdoc="x">abort</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span><span class="RktMeta">...</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the <span class="RktVar">v</span>s to a prompt using the default continuation
prompt tag and the default abort handler.</div></p><p>That is, <span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValLink" data-pltdoc="x">abort</a></span><span class="stt"> </span><span class="RktVar">v</span><span class="stt"> </span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span> is equivalent to</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._abort-current-continuation%29%29" class="RktValLink" data-pltdoc="x">abort-current-continuation</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" 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"><a href="values.html#%28def._%28%28quote._~23~25kernel%29._values%29%29" class="RktValLink" data-pltdoc="x">values</a></span><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span><span class="RktSym"><a href="stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29" class="RktStxLink" data-pltdoc="x">...</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p><div class="SIntrapara">Example:</div><div class="SIntrapara"><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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"start here\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="Writing.html#%28def._%28%28quote._~23~25kernel%29._printf%29%29" class="RktValLink" data-pltdoc="x">printf</a></span><span class="hspace"> </span><span class="RktVal">"answer is ~a\n"</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValLink" data-pltdoc="x">abort</a></span><span class="hspace"> </span><span class="RktVal">3</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="RktOut">start here</span></p></td></tr><tr><td><p><span class="RktRes">3</span></p></td></tr></table></blockquote></div></p><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._~25))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">%</a></span></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">%</a></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktVar">handler-expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">%</a></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktVar">handler-expr</span><span class="hspace"> </span><span class="RktPn">#:tag</span><span class="hspace"> </span><span class="RktVar">tag-expr</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><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._((lib._racket/control..rkt)._fcontrol))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._fcontrol%29%29" class="RktValDef RktValLink" data-pltdoc="x">fcontrol</a></span></span><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span><span class="RktPn">#:tag</span><span class="hspace"> </span><span class="RktVar">prompt-tag</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</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._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">prompt-tag</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._default-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">default-continuation-prompt-tag</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></blockquote><p>Sitaram’s operators [<a href="doc-bibliography.html#%28cite._.Sitaram93%29" data-pltdoc="x">Sitaram93</a>].</p><p>The essential reduction rules are:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span><span class="hspace"> </span><span class="RktVar">val</span><span class="hspace"> </span><span class="RktSym">proc</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">val</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._fcontrol%29%29" class="RktValLink" data-pltdoc="x">fcontrol</a></span><span class="hspace"> </span><span class="RktVar">val</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">proc</span><span class="hspace"> </span><span class="RktVar">val</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">x</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span></td></tr></table></blockquote><p>When <span class="RktVar">handler-expr</span> is omitted, <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span> is the same as
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span>. If <span class="RktVar">prompt-tag</span> is provided, <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span>
uses specific prompt tags like <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span>.</p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._fcontrol%29%29" class="RktValLink" data-pltdoc="x">fcontrol</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="hspace"> </span><span class="RktSym">k</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">7</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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._~25%29%29" class="RktStxLink" data-pltdoc="x">%</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._fcontrol%29%29" class="RktValLink" data-pltdoc="x">fcontrol</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="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="hspace"> </span><span class="RktSym">k</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVar">v</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">5</span></p></td></tr></table></blockquote></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._prompt))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">prompt</a></span></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._control))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">control</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Among the earliest operators for higher-order control
[<a href="doc-bibliography.html#%28cite._.Felleisen88a%29" data-pltdoc="x">Felleisen88a</a>, <a href="doc-bibliography.html#%28cite._.Felleisen88%29" data-pltdoc="x">Felleisen88</a>, <a href="doc-bibliography.html#%28cite._.Sitaram90%29" data-pltdoc="x">Sitaram90</a>].</div></p><p><div class="SIntrapara">The essential reduction rules are:
</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span><span class="hspace"> </span><span class="RktVar">val</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">val</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">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="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr></table></blockquote></div></p><p><div class="SIntrapara">Examples:</div><div class="SIntrapara"><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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktVal">5</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">7</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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktVal">5</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">5</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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">k1</span><span class="hspace"> </span><span class="RktVal">6</span><span class="RktPn">)</span><span class="RktPn">)</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">7</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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktVal">6</span><span class="RktPn">)</span><span class="RktPn">)</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">8</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="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2B%29%29" class="RktValLink" data-pltdoc="x">+</a></span><span class="hspace"> </span><span class="RktVal">2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k1</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktSym">k2</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">k2</span><span class="hspace"> </span><span class="RktVal">6</span><span class="RktPn">)</span><span class="RktPn">)</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">6</span></p></td></tr></table></blockquote></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._prompt-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">prompt-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._control-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">control-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span> and <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span>, but using specific prompt
tags:</div></p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">val</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">val</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control-at%29%29" class="RktStxLink" data-pltdoc="x">control-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">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="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="RktCmt"> for </span><span class="RktVar">tag</span></td></tr></table></blockquote><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._reset))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">reset</a></span></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._shift))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">shift</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Danvy and Filinski’s operators [<a href="doc-bibliography.html#%28cite._.Danvy90%29" data-pltdoc="x">Danvy90</a>].</div></p><p>The essential reduction rules are:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktVar">val</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">val</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift%29%29" class="RktStxLink" data-pltdoc="x">shift</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</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="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span></td></tr></table></blockquote><p>The <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span> and <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span> forms are interchangeable.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._reset-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">reset-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._shift-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">shift-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">identifier</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span> and <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift%29%29" class="RktStxLink" data-pltdoc="x">shift</a></span>, but using the specified prompt
tags.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._prompt0))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">prompt0</a></span></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._reset0))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">reset0</a></span></span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._control0))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">control0</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._shift0))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift0%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">shift0</a></span></span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Generalizations of <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span>, etc<span class="Sendabbrev">.</span> [<a href="doc-bibliography.html#%28cite._.Shan04%29" data-pltdoc="x">Shan04</a>].</div></p><p>The essential reduction rules are:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxLink" data-pltdoc="x">prompt0</a></span><span class="hspace"> </span><span class="RktVar">val</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">val</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxLink" data-pltdoc="x">prompt0</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0%29%29" class="RktStxLink" data-pltdoc="x">control0</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span><span class="hspace"> </span><span class="RktVar">val</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">val</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift0%29%29" class="RktStxLink" data-pltdoc="x">shift0</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span> and <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxLink" data-pltdoc="x">prompt0</a></span> forms are interchangeable.
Furthermore, the following reductions apply:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0%29%29" class="RktStxLink" data-pltdoc="x">control0</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift0%29%29" class="RktStxLink" data-pltdoc="x">shift0</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</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="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxLink" data-pltdoc="x">prompt0</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxLink" data-pltdoc="x">prompt0</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift%29%29" class="RktStxLink" data-pltdoc="x">shift</a></span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0%29%29" class="RktStxLink" data-pltdoc="x">reset0</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">v</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">v</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>That is, both the <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span>/<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span> and
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span>/<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift%29%29" class="RktStxLink" data-pltdoc="x">shift</a></span> sites must agree for <span class="RktVal">0</span>-like
behavior, otherwise the non-<span class="RktVal">0</span> behavior applies.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._prompt0-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">prompt0-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._reset0-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset0-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">reset0-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._control0-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">control0-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._shift0-at))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._shift0-at%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">shift0-at</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-tag-expr</span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Variants of <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0%29%29" class="RktStxLink" data-pltdoc="x">prompt0</a></span>, etc<span class="Sendabbrev">.</span>, that accept a prompt tag.</div></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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/control..rkt)._spawn))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._spawn%29%29" class="RktValDef RktValLink" data-pltdoc="x">spawn</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">proc</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></blockquote></div><div class="SIntrapara">The operators of Hieb and Dybvig [<a href="doc-bibliography.html#%28cite._.Hieb90%29" data-pltdoc="x">Hieb90</a>].</div></p><p>The essential reduction rules are:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">obj</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktVar">obj</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._spawn%29%29" class="RktValLink" data-pltdoc="x">spawn</a></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt%29%29" class="RktStxLink" data-pltdoc="x">prompt</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">proc</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValLink" data-pltdoc="x">abort</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">x</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValLink" data-pltdoc="x">abort</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">proc</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">x</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="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="RktCmt"> for </span><span class="RktVar">tag</span></td></tr></table></blockquote><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._((lib._racket/control..rkt)._splitter))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._splitter%29%29" class="RktValDef RktValLink" data-pltdoc="x">splitter</a></span></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="argcontract"><tr><td valign="top"><span class="hspace"> </span></td><td valign="top"><span class="RktVar">proc</span></td><td valign="top"><span class="hspace"> </span></td><td valign="top">:</td><td valign="top"><span class="hspace"> </span></td><td valign="top"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">. </span><span class="RktSym"><a href="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-pltdoc="x"><span class="nobreak">-></span></a></span><span class="RktPn"> .</span><span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span><span class="RktPn">)</span></td></tr></table></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">The operator of Queinnec and Serpette [<a href="doc-bibliography.html#%28cite._.Queinnec91%29" data-pltdoc="x">Queinnec91</a>].</div></p><p><div class="SIntrapara">The essential reduction rules are:
</div><div class="SIntrapara"><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._splitter%29%29" class="RktValLink" data-pltdoc="x">splitter</a></span><span class="hspace"> </span><span class="RktVar">proc</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">proc</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">thunk</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValLink" data-pltdoc="x">abort</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">thunk</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">proc</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0-at%29%29" class="RktStxLink" data-pltdoc="x">control0-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">proc</span><span class="hspace"> </span><span class="RktVar">k</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="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._abort%29%29" class="RktValLink" data-pltdoc="x">abort</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">thunk</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">thunk</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="RktCmt"> for </span><span class="RktVar">tag</span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktPn">(</span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0-at%29%29" class="RktStxLink" data-pltdoc="x">control0-at</a></span><span class="hspace"> </span><span class="RktVar">tag</span><span class="hspace"> </span><span class="RktVar">k</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktSym"><a href="if.html#%28form._%28%28lib._racket%2Fprivate%2Fletstx-scheme..rkt%29._~3d~3e%29%29" class="RktStxLink" data-pltdoc="x">=></a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">k</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="lambda.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._lambda%29%29" class="RktStxLink" data-pltdoc="x">lambda</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktVar">x</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktVar">E</span><span class="RktPn">[</span><span class="RktVar">x</span><span class="RktPn">]</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">where </span><span class="RktVar">E</span><span class="RktCmt"> has no </span><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt-at%29%29" class="RktStxLink" data-pltdoc="x">prompt-at</a></span><span class="RktCmt"> for </span><span class="RktVar">tag</span></td></tr></table></blockquote></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><table cellspacing="0" cellpadding="0" class="together"><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._((lib._racket/control..rkt)._new-prompt))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._new-prompt%29%29" class="RktValDef RktValLink" data-pltdoc="x">new-prompt</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#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._set))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._set%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">set</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-expr</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="together"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/control..rkt)._cupto))"></a><span title="Provided from: racket/control | Package: base"><span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._cupto%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">cupto</a></span></span><span class="hspace"> </span><span class="RktVar">prompt-expr</span><span class="hspace"> </span><span class="RktVar">id</span><span class="hspace"> </span><span class="RktVar">expr</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">The operators of Gunter et al. [<a href="doc-bibliography.html#%28cite._.Gunter95%29" data-pltdoc="x">Gunter95</a>].</div></p><p>In this library, <span class="RktSym"><a href="#%28def._%28%28lib._racket%2Fcontrol..rkt%29._new-prompt%29%29" class="RktValLink" data-pltdoc="x">new-prompt</a></span> is an alias for
<span class="RktSym"><a href="#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span>, <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._set%29%29" class="RktStxLink" data-pltdoc="x">set</a></span> is an alias for
<span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._prompt0-at%29%29" class="RktStxLink" data-pltdoc="x">prompt0-at</a></span>, and <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._cupto%29%29" class="RktStxLink" data-pltdoc="x">cupto</a></span> is an alias for <span class="RktSym"><a href="#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control0-at%29%29" class="RktStxLink" data-pltdoc="x">control0-at</a></span>.</p><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="Delayed_Evaluation.html" title="backward to "10.3 Delayed Evaluation"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="contmarks.html" title="forward to "10.5 Continuation Marks"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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