177 lines
219 KiB
HTML
177 lines
219 KiB
HTML
|
<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
|
||
|
|
<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>10.4 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">The Racket Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewselflink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr
|
||
|
|
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="cont.html#%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="cont.html#%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><span class="RktOpt">[</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><span class="RktOpt">]</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="cont.html#%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="cont.html#%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 clas
|
||
|
|
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="cont.html#%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="cont.html#%28def._%28%28quote._~23~25kernel%29._make-continuation-prompt-tag%29%29" class="RktValLink" data-pltdoc="x">make-continuation-prompt-tag</a></span>. The call to
|
||
|
|
<span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-with-continuation-prompt%29%29" class="RktValLink" data-pltdoc="x">call-with-continuation-prompt</a></span> returns the result of
|
||
|
|
<span class="RktVar">proc</span>.</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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">name</span><span class="RktPn">)</span><span class="hspace"> </span>→<span class="hspace"> </span><span class="RktSym"><a href="cont.html#%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">name</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="Equality.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="cont.html#%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">name</span>
|
||
|
|
argument, if supplied, specifies the name of the prompt tag
|
||
|
|
for printing or <span class="RktSym"><a href="inspectors.html#%28def._%28%28quote._~23~25kernel%29._object-name%29%29" class="RktValLink" data-pltdoc="x">object-name</a></span>.</div></p><p class="SHistory">Changed in version 7.9.0.13 of package <span class="stt">base</span>: The <span class="RktVar">name</span> argument
|
||
|
|
gives the name of the prompt tag.</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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><span class="RktOpt">[</span></td><td valign="top"><span class="RktVar">prompt-tag</span><span class="RktOpt">]</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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 captured continuation is delivered to <span class="RktVar">proc</span>, which is
|
||
|
|
called in tail position with respect to the
|
||
|
|
<span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="RktOpt">[</span><span class="RktVar">prompt-tag</span><span class="RktOpt">]</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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-co
|
||
|
|
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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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>A continuation obtained from <span class="RktSym"><a href="cont.html#%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. In the <a href="implementations.html#%28tech._bc%29" class="techoutside" data-pltdoc="x"><span class="techinside">BC</span></a> implementation of Racket,
|
||
|
|
<span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span> is implemented more efficiently
|
||
|
|
than <span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span>, so
|
||
|
|
<span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-with-escape-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-escape-continuation</a></span> can sometimes replace
|
||
|
|
<span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-with-current-continuation%29%29" class="RktValLink" data-pltdoc="x">call-with-current-continuation</a></span> to improve performance in
|
||
|
|
those older Racket variants.</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((quote._~23~25kernel)._call-in-continuation))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-in-continuation%29%29" class="RktValDef RktValLink" data-pltdoc="x">call-in-continuation</a></span></span><span class="hspace"> </span><span class="RktVar">k</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">k</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._continuation~3f%29%29" class="RktValLink" data-pltdoc="x">continuation?</a></span></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="function-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fbase..rkt%29._-~3e%29%29" class="RktStxLink" data-plt
|
||
|
|
delivering values to the continuation, <span class="RktVar">proc</span> is called with
|
||
|
|
<span class="RktVar">k</span> as the continuation of the call (so the result of
|
||
|
|
<span class="RktVar">proc</span> is returned to the continuation). If <span class="RktVar">k</span>
|
||
|
|
is a composable continuation, the continuation of the call to
|
||
|
|
<span class="RktVar">proc</span> is the current continuation extended with <span class="RktVar">k</span>.</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="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></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="cont.html#%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="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></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="cont.html#%28def._%28%28quote._~23~25kernel%29._call-in-continuation%29%29" class="RktValLink" data-pltdoc="x">call-in-continuation</a></span><span class="hspace"> </span><span class="RktVar">k</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="RktVal">4</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">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="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></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="cont.html#%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="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></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">n</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"><a href="
|
||
|
|
application and the current continuation. The results of
|
||
|
|
<span class="RktVar">thunk</span> are the results of the
|
||
|
|
<span class="RktSym"><a href="cont.html#%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="cont.html#%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><span class="RktOpt">[</span></td><td valign="top"><span class="RktVar">cont</span><span class="RktOpt">]</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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">&n
|
||
|
|
<span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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._%2
|
||
|
|
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="cont.html#%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="cont.html#%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="cont.html#%28form._%28%28lib._racket%2Fcontrol..rkt%29._reset%29%29" class="RktStxLink" data-pltdoc="x">reset</a></span> and <span class="RktSym"><a href="cont.html#%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="cont.html#%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="RktOpt">[</span><span class="RktVar">prompt-tag</span><span class="hspace"> </span><span class="RktVar">handler</span><span class="RktOpt">]</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="cont.html#%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="cont.html#%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="Rkt
|
||
|
|
prompt tag and the default abort handler.</div></p><p>That is, <span class="RktPn">(</span><span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="SVInsetFlo
|
||
|
|
<span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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 cla
|
||
|
|
[<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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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._~2
|
||
|
|
tags:</div></p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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</s
|
||
|
|
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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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
|
||
|
|
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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="RktSym"><a href="cont.html#%28form._%28%28lib._racket%2Fcontrol..rkt%29._control%29%29" class="RktStxLink" data-pltdoc="x">control</a></span>/<span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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
|
||
|
|
</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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="R
|
||
|
|
<span class="RktSym"><a href="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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="cont.html#%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" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="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>
|