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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><meta name="viewport" content="width=device-width, initial-scale=0.8"/><title>10.3 Delayed Evaluation</title><link rel="stylesheet" type="text/css" href="../scribble.css" title="default"/><link rel="stylesheet" type="text/css" href="extras.css" title="default"/><link rel="stylesheet" type="text/css" href="../racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-style.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../manual-racket.css" title="default"/><link rel="stylesheet" type="text/css" href="../doc-site.css" title="default"/><script type="text/javascript" src="../scribble-common.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../manual-racket.js"></script><script type="text/javascript" src="../doc-site.js"></script><script type="text/javascript" src="../local-redirect/local-redirect.js"></script><script type="text/javascript" src="../local-redirect/local-user-redirect.js"></script><!--[if IE 6]><style type="text/css">.SIEHidden { overflow: hidden; }</style><![endif]--></head><body id="doc-racket-lang-org"><div class="tocset"><div class="tocview"><div class="tocviewlist tocviewlisttopspace"><div class="tocviewtitle"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_0");">►</a></td><td></td><td><a href="index.html" class="tocviewlink" data-pltdoc="x">The Racket Reference</a></td></tr></table></div><div class="tocviewsublisttop" style="display: none;" id="tocview_0"><table cellspacing="0" cellpadding="0"><tr><td align="right">1 </td><td><a href="model.html" class="tocviewlink" data-pltdoc="x">Language Model</a></td></tr><tr><td align="right">2 </td><td><a href="notation.html" class="tocviewlink" data-pltdoc="x">Notation for Documentation</a></td></tr><tr><td align="right">3 </td><td><a href="syntax.html" class="tocviewlink" data-pltdoc="x">Syntactic Forms</a></td></tr><tr><td align="right">4 </td><td><a href="data.html" class="tocviewlink" data-pltdoc="x">Datatypes</a></td></tr><tr><td align="right">5 </td><td><a href="structures.html" class="tocviewlink" data-pltdoc="x">Structures</a></td></tr><tr><td align="right">6 </td><td><a href="mzlib_class.html" class="tocviewlink" data-pltdoc="x">Classes and Objects</a></td></tr><tr><td align="right">7 </td><td><a href="mzlib_unit.html" class="tocviewlink" data-pltdoc="x">Units</a></td></tr><tr><td align="right">8 </td><td><a href="contracts.html" class="tocviewlink" data-pltdoc="x">Contracts</a></td></tr><tr><td align="right">9 </td><td><a href="match.html" class="tocviewlink" data-pltdoc="x">Pattern Matching</a></td></tr><tr><td align="right">10 </td><td><a href="control.html" class="tocviewselflink" data-pltdoc="x">Control Flow</a></td></tr><tr><td align="right">11 </td><td><a href="concurrency.html" class="tocviewlink" data-pltdoc="x">Concurrency and Parallelism</a></td></tr><tr><td align="right">12 </td><td><a href="Macros.html" class="tocviewlink" data-pltdoc="x">Macros</a></td></tr><tr><td align="right">13 </td><td><a href="input-and-output.html" class="tocviewlink" data-pltdoc="x">Input and Output</a></td></tr><tr><td align="right">14 </td><td><a href="security.html" class="tocviewlink" data-pltdoc="x">Reflection and Security</a></td></tr><tr><td align="right">15 </td><td><a href="os.html" class="tocviewlink" data-pltdoc="x">Operating System</a></td></tr><tr><td align="right">16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr><tr><td align="right">17 </td><td><a href="unsafe.html" class="tocviewlink" data-pltdoc="x">Unsafe Operations</a></td></tr><tr><td align="right">18 </td><td><a href="running.html" class="tocviewlink" data-pltdoc="x">Running Racket</a></td></tr><tr><td align="right"></td><td><a href="doc-bibliography.html" class="tocviewlink" data-pltdoc="x">Bibliography</a></td></tr><tr><td align="right"></td><td><a href="doc-index.html" class="tocviewlink" data-pltdoc="x">Index</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_1");">▼</a></td><td>10 </td><td><a href="control.html" class="tocviewlink" data-pltdoc="x">Control Flow</a></td></tr></table><div class="tocviewsublist" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">10.1 </td><td><a href="values.html" class="tocviewlink" data-pltdoc="x">Multiple Values</a></td></tr><tr><td align="right">10.2 </td><td><a href="exns.html" class="tocviewlink" data-pltdoc="x">Exceptions</a></td></tr><tr><td align="right">10.3 </td><td><a href="Delayed_Evaluation.html" class="tocviewselflink" data-pltdoc="x">Delayed Evaluation</a></td></tr><tr><td align="right">10.4 </td><td><a href="cont.html" class="tocviewlink" data-pltdoc="x">Continuations</a></td></tr><tr><td align="right">10.5 </td><td><a href="contmarks.html" class="tocviewlink" data-pltdoc="x">Continuation Marks</a></td></tr><tr><td align="right">10.6 </td><td><a href="breakhandler.html" class="tocviewlink" data-pltdoc="x">Breaks</a></td></tr><tr><td align="right">10.7 </td><td><a href="Exiting.html" class="tocviewlink" data-pltdoc="x">Exiting</a></td></tr><tr><td align="right">10.8 </td><td><a href="unreachable.html" class="tocviewlink" data-pltdoc="x">Unreachable Expressions</a></td></tr></table></div></div><div class="tocviewlist"><table cellspacing="0" cellpadding="0"><tr><td style="width: 1em;"><a href="javascript:void(0);" title="Expand/Collapse" class="tocviewtoggle" onclick="TocviewToggle(this,"tocview_2");">►</a></td><td>10.3 </td><td><a href="Delayed_Evaluation.html" class="tocviewselflink" data-pltdoc="x">Delayed Evaluation</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">10.3.1 </td><td><a href="Delayed_Evaluation.html#%28part._.Additional_.Promise_.Kinds%29" class="tocviewlink" data-pltdoc="x">Additional Promise Kinds</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">promise?</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">delay</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._lazy%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">lazy</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">force</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise-forced~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">promise-<wbr></wbr>forced?</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise-running~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">promise-<wbr></wbr>running?</span></span></a></td></tr><tr><td><span class="tocsublinknumber">10.3.1<tt> </tt></span><a href="Delayed_Evaluation.html#%28part._.Additional_.Promise_.Kinds%29" class="tocsubseclink" data-pltdoc="x">Additional Promise Kinds</a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fname%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">delay/<span class="mywbr"> </span>name</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise%2Fname~3f%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">promise/<span class="mywbr"> </span>name?</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fstrict%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">delay/<span class="mywbr"> </span>strict</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fsync%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">delay/<span class="mywbr"> </span>sync</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fthread%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">delay/<span class="mywbr"> </span>thread</span></span></a></td></tr><tr><td><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fidle%29%29" class="tocsubnonseclink" data-pltdoc="x"><span class="RktSym"><span class="RktStxLink">delay/<span class="mywbr"> </span>idle</span></span></a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" id="searchbox" type="text" tabindex="1" placeholder="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "8.6", "../");"/></form> <a href="https://docs.racket-lang.org/index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("8.6");">top</a><span class="tocsettoggle"> <a href="javascript:void(0);" title="show/hide table of contents" onclick="TocsetToggle();">contents</a></span></span><span class="navright"> <a href="exns.html" title="backward to "10.2 Exceptions"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="cont.html" title="forward to "10.4 Continuations"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Delayed_Evaluation"">10.3<tt> </tt><a name="(part._.Delayed_.Evaluation)"></a><a name="(mod-path._racket/promise)"></a>Delayed Evaluation</h4><p><div class="SIntrapara"><table cellspacing="0" cellpadding="0" class="defmodule"><tr><td align="left"><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="require.html#%28form._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._require%29%29" class="RktStxLink" data-pltdoc="x">require</a></span><span class="stt"> </span><a href="Delayed_Evaluation.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/promise</span></a><span class="RktPn">)</span></td><td align="right"><span class="RpackageSpec"><span class="Smaller"> package:</span> <a href="https://pkgs.racket-lang.org/package/base" title="Install this package using `raco pkg install base`"><span class="stt">base</span></a></span></td></tr></table></div><div class="SIntrapara">The bindings documented in this section are provided by the <a href="Delayed_Evaluation.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/promise</span></a> and <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket</span></a> libraries, but not <a href="index.html" class="RktModLink" data-pltdoc="x"><span class="RktSym">racket/base</span></a>.</div></p><p>A <a name="(tech._promise)"></a><span style="font-style: italic">promise</span> encapsulates an expression to be evaluated on
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demand via <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>. After a promise has been <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>d,
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every later <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> of the promise produces the same result.</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/promise..rkt)._promise~3f))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">promise?</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 promise, <span class="RktVal">#f</span>
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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>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._delay))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">delay</a></span></span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Creates a promise that, when <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>d, evaluates the
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<span class="RktVar">body</span>s to produce its value. The result is then cached, so
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further uses of <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> produce the cached value immediately.
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This includes multiple values and exceptions.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._lazy))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._lazy%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">lazy</a></span></span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%29%29" class="RktStxLink" data-pltdoc="x">delay</a></span>, if the last <span class="RktVar">body</span> produces a promise when
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forced, then this promise is <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>d, too, to obtain a value.
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In other words, this form creates a composable promise, where the
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computation of its body is “attached” to the computation of the
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following promise, and a single <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> iterates through the
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whole chain, tail-calling each step.</div></p><p>Note that the last <span class="RktVar">body</span> of this form must produce a single
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value, but the value can itself be a <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%29%29" class="RktStxLink" data-pltdoc="x">delay</a></span> promise that
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returns multiple values.</p><p>The <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._lazy%29%29" class="RktStxLink" data-pltdoc="x">lazy</a></span> form is useful for implementing lazy libraries and
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languages, where tail calls can be wrapped in a promise.</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/promise..rkt)._force))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValDef RktValLink" data-pltdoc="x">force</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="data-structure-contracts.html#%28form._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%29%29" class="RktStxLink" data-pltdoc="x">any</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">v</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="data-structure-contracts.html#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></td></tr></table></blockquote></div><div class="SIntrapara">If <span class="RktVar">v</span> is a promise, then the promise is forced to obtain a
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value. If the promise has not been forced before, then the result is
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recorded in the promise so that future <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>s on the promise
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produce the same value (or values). If forcing the promise raises an
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exception, then the exception is similarly recorded so that forcing
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the promise will raise the same exception every time.</div></p><p>If <span class="RktVar">v</span> is <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>d again before the original call to
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<span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> returns, then the <span class="RktSym"><a href="exns.html#%28def._%28%28lib._racket%2Fprivate%2Fbase..rkt%29._exn~3afail%29%29" class="RktValLink" data-pltdoc="x">exn:fail</a></span> exception is raised.</p><p>If <span class="RktVar">v</span> is not a promise, then it is returned as the result.</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/promise..rkt)._promise-forced~3f))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise-forced~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">promise-forced?</a></span></span><span class="hspace"> </span><span class="RktVar">promise</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">promise</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise~3f%29%29" class="RktValLink" data-pltdoc="x">promise?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">promise</span> has been forced.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/promise..rkt)._promise-running~3f))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise-running~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">promise-running?</a></span></span><span class="hspace"> </span><span class="RktVar">promise</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">promise</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise~3f%29%29" class="RktValLink" data-pltdoc="x">promise?</a></span></td></tr></table></blockquote></div><div class="SIntrapara">Returns <span class="RktVal">#t</span> if <span class="RktVar">promise</span> is currently being forced.
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(Note that a promise can be either running or forced but not both.)</div></p><h5 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""Additional_Promise_Kinds"">10.3.1<tt> </tt><a name="(part._.Additional_.Promise_.Kinds)"></a>Additional Promise Kinds</h5><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._delay/name))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fname%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">delay/name</a></span></span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Creates a “call-by-name” promise that is similar to
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<span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%29%29" class="RktStxLink" data-pltdoc="x">delay</a></span>-promises, except that the resulting value is not
|
|
cached. This kind of promise is essentially a thunk that is wrapped
|
|
in a way that <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> recognizes.</div></p><p>If a <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fname%29%29" class="RktStxLink" data-pltdoc="x">delay/name</a></span> promise forces itself, no exception is
|
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raised, the promise is never considered “running” or “forced” in
|
|
the sense of <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise-running~3f%29%29" class="RktValLink" data-pltdoc="x">promise-running?</a></span> and <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise-forced~3f%29%29" class="RktValLink" data-pltdoc="x">promise-forced?</a></span>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>procedure</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(def._((lib._racket/promise..rkt)._promise/name~3f))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._promise%2Fname~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">promise/name?</a></span></span><span class="hspace"> </span><span class="RktVar">promise</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">promise</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">promise</span> is a promise created with <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fname%29%29" class="RktStxLink" data-pltdoc="x">delay/name</a></span>.
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</div><div class="SIntrapara"><p class="SHistory">Added in version 6.3 of package <span class="stt">base</span>.</p></div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._delay/strict))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fstrict%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">delay/strict</a></span></span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Creates a “strict” promise: it is evaluated immediately, and the
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|
result is wrapped in a promise value. Note that the body can evaluate
|
|
to multiple values, and forcing the resulting promise will return these
|
|
values.</div></p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._delay/sync))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fsync%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">delay/sync</a></span></span><span class="hspace"> </span><span class="RktVar">body</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr></table></blockquote></div><div class="SIntrapara">Produces a promise where an attempt to <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> the promise by a
|
|
thread other than one currently running the promise causes the
|
|
<span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> to block until a result is available. This kind of
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promise is also a <a href="sync.html#%28tech._synchronizable._event%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronizable event</span></a> for use with
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|
<span class="RktSym"><a href="sync.html#%28def._%28%28quote._~23~25kernel%29._sync%29%29" class="RktValLink" data-pltdoc="x">sync</a></span>; <span class="RktSym"><a href="sync.html#%28def._%28%28quote._~23~25kernel%29._sync%29%29" class="RktValLink" data-pltdoc="x">sync</a></span>ing on the promise does not <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>
|
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it, but merely waits until a value is forced by another thread.
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|
The <a href="sync.html#%28tech._synchronization._result%29" class="techoutside" data-pltdoc="x"><span class="techinside">synchronization result</span></a> is <a href="https://download.racket-lang.org/releases/8.6/doc/local-redirect/index.html?doc=guide&rel=void_undefined.html&version=8.6" class="RktModLink Sq" data-pltdoc="x"><span class="nobreak"><span class="RktRes">#<void></span></span></a>.</div></p><p>If a promise created by <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fsync%29%29" class="RktStxLink" data-pltdoc="x">delay/sync</a></span> is forced on a thread that
|
|
is already running the promise, an exception is raised in the same way
|
|
as for promises created with <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%29%29" class="RktStxLink" data-pltdoc="x">delay</a></span>.</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._delay/thread))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fthread%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">delay/thread</a></span></span><span class="hspace"> </span><span class="RktVar">body/option</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">body/option</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">body</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:group</span><span class="hspace"> </span><span class="RktVar">thread-group-expr</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fsync%29%29" class="RktStxLink" data-pltdoc="x">delay/sync</a></span>, but begins the computation immediately on a
|
|
newly created thread. The thread is created under the <a href="threadgroups.html#%28tech._thread._group%29" class="techoutside" data-pltdoc="x"><span class="techinside">thread
|
|
group</span></a> specified by <span class="RktVar">thread-group-expr</span>, which defaults to
|
|
<span class="RktPn">(</span><span class="RktSym"><a href="threadgroups.html#%28def._%28%28quote._~23~25kernel%29._make-thread-group%29%29" class="RktValLink" data-pltdoc="x">make-thread-group</a></span><span class="RktPn">)</span>. A <span class="RktPn">#:group</span> specification can
|
|
appear at most once.</div></p><p>Exceptions raised by the <span class="RktVar">body</span>s are caught as usual and raised
|
|
only when the promise is <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span>d. Unlike <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fsync%29%29" class="RktStxLink" data-pltdoc="x">delay/sync</a></span>,
|
|
if the thread running <span class="RktVar">body</span> terminates without producing a result
|
|
or exception, <span class="RktSym"><a href="Delayed_Evaluation.html#%28def._%28%28lib._racket%2Fpromise..rkt%29._force%29%29" class="RktValLink" data-pltdoc="x">force</a></span> of the promise raises an exception (instead
|
|
of blocking).</p><p><div class="SIntrapara"><blockquote class="SVInsetFlow"><table cellspacing="0" cellpadding="0" class="boxed RBoxed"><tr><td><blockquote class="SubFlow"><div class="RBackgroundLabel SIEHidden"><div class="RBackgroundLabelInner"><p>syntax</p></div></div><p class="RForeground"><span class="RktPn">(</span><a name="(form._((lib._racket/promise..rkt)._delay/idle))"></a><span title="Provided from: racket/promise, racket | Package: base"><span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fidle%29%29" class="RktStxDef RktStxLink" data-pltdoc="x">delay/idle</a></span></span><span class="hspace"> </span><span class="RktVar">body/option</span><span class="hspace"> </span><span class="RktMeta">...+</span><span class="RktPn">)</span></p></blockquote></td></tr><tr><td><span class="stt"> </span></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="specgrammar"><tr><td align="right" valign="baseline"><span class="RktVar">body/option</span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">=</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktVar">body</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:wait-for</span><span class="hspace"> </span><span class="RktVar">wait-evt-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:work-while</span><span class="hspace"> </span><span class="RktVar">while-evt-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:tick</span><span class="hspace"> </span><span class="RktVar">tick-secs-expr</span></td></tr><tr><td align="right" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="center" valign="baseline">|</td><td align="left" valign="baseline"><span class="stt"> </span></td><td align="left" valign="baseline"><span class="RktPn">#:use</span><span class="hspace"> </span><span class="RktVar">use-ratio-expr</span></td></tr></table></td></tr></table></blockquote></div><div class="SIntrapara">Like <span class="RktSym"><a href="Delayed_Evaluation.html#%28form._%28%28lib._racket%2Fpromise..rkt%29._delay%2Fthread%29%29" class="RktStxLink" data-pltdoc="x">delay/thread</a></span>, but with the following differences:</div></p><ul><li><p>the computation does not start until the event produced by
|
|
<span class="RktVar">wait-evt-expr</span> is ready, where the default is
|
|
<span class="RktPn">(</span><span class="RktSym"><a href="sync.html#%28def._%28%28quote._~23~25kernel%29._system-idle-evt%29%29" class="RktValLink" data-pltdoc="x">system-idle-evt</a></span><span class="RktPn">)</span>;</p></li><li><p>the computation thread gets to work only when the process is
|
|
otherwise idle as determined by <span class="RktVar">while-evt-expr</span>, which
|
|
also defaults to <span class="RktPn">(</span><span class="RktSym"><a href="sync.html#%28def._%28%28quote._~23~25kernel%29._system-idle-evt%29%29" class="RktValLink" data-pltdoc="x">system-idle-evt</a></span><span class="RktPn">)</span>;</p></li><li><p>the thread is allowed to run only periodically: out of every
|
|
<span class="RktVar">tick-secs-expr</span> (defaults to <span class="RktVal">0.2</span>) seconds, the
|
|
thread is allowed to run <span class="RktVar">use-ratio-expr</span> (defaults to
|
|
<span class="RktVal">0.12</span>) of the time proportionally; i.e., the thread
|
|
runs for <span class="RktPn">(</span><span class="RktSym"><a href="generic-numbers.html#%28def._%28%28quote._~23~25kernel%29._%2A%29%29" class="RktValLink" data-pltdoc="x">*</a></span><span class="stt"> </span><span class="RktVar">tick-secs-expr</span><span class="stt"> </span><span class="RktVar">use-ratio-expr</span><span class="RktPn">)</span> seconds.</p></li></ul><p>If the promise is <span class="RktSym">forced</span> before the computation is done, it
|
|
runs the rest of the computation immediately without waiting on events
|
|
or periodically restricting evaluation.</p><p>A <span class="RktPn">#:wait-for</span>, <span class="RktPn">#:work-while</span>, <span class="RktPn">#:tick</span>, or
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|
<span class="RktPn">#:use</span> specification can appear at most once.</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="exns.html" title="backward to "10.2 Exceptions"" data-pltdoc="x">← prev</a> <a href="control.html" title="up to "10 Control Flow"" data-pltdoc="x">up</a> <a href="cont.html" title="forward to "10.4 Continuations"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |