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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>16.2 Ephemerons</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="tocviewlink" 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="tocviewselflink" 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>16 </td><td><a href="memory.html" class="tocviewlink" data-pltdoc="x">Memory Management</a></td></tr></table><div class="tocviewsublistbottom" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">16.1 </td><td><a href="weakbox.html" class="tocviewlink" data-pltdoc="x">Weak Boxes</a></td></tr><tr><td align="right">16.2 </td><td><a href="ephemerons.html" class="tocviewselflink" data-pltdoc="x">Ephemerons</a></td></tr><tr><td align="right">16.3 </td><td><a href="willexecutor.html" class="tocviewlink" data-pltdoc="x">Wills and Executors</a></td></tr><tr><td align="right">16.4 </td><td><a href="garbagecollection.html" class="tocviewlink" data-pltdoc="x">Garbage Collection</a></td></tr><tr><td align="right">16.5 </td><td><a href="phantom-bytes.html" class="tocviewlink" data-pltdoc="x">Phantom Byte Strings</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="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._make-ephemeron%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">make-<wbr></wbr>ephemeron</span></span></a></td></tr><tr><td><a href="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._ephemeron-value%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">ephemeron-<wbr></wbr>value</span></span></a></td></tr><tr><td><a href="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="tocsublink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">ephemeron?</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="weakbox.html" title="backward to "16.1 Weak Boxes"" data-pltdoc="x">← prev</a> <a href="memory.html" title="up to "16 Memory Management"" data-pltdoc="x">up</a> <a href="willexecutor.html" title="forward to "16.3 Wills and Executors"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/reference/reference.scrbl")" x-source-pkg="racket-doc" x-part-tag=""ephemerons"">16.2<tt> </tt><a name="(part._ephemerons)"></a>Ephemerons</h4><p>An <a name="(tech._ephemeron)"></a><span style="font-style: italic">ephemeron</span> [<a href="doc-bibliography.html#%28cite._.Hayes97%29" data-pltdoc="x">Hayes97</a>] is a generalization of a
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<a href="weakbox.html#%28tech._weak._box%29" class="techoutside" data-pltdoc="x"><span class="techinside">weak box</span></a> (see <a href="weakbox.html" data-pltdoc="x">Weak Boxes</a>). Instead of just containing
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one value, an ephemeron holds two values: one that is considered the
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value of the ephemeron and another that is the ephemeron’s key. Like
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the value in a weak box, the value in an ephemeron may be replaced by
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<span class="RktVal">#f</span>, but when the <span class="emph">key</span> is no longer reachable (except
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possibly via weak references) instead of when the value is no longer
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reachable.</p><p>As long as an ephemeron’s value is retained, the reference is
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considered a non-weak reference. References to the key via the value
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are treated specially, however, in that the reference does not
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necessarily count toward the key’s reachability. A <a href="weakbox.html#%28tech._weak._box%29" class="techoutside" data-pltdoc="x"><span class="techinside">weak box</span></a> can
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be seen as a specialization of an ephemeron where the key and value
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are the same.</p><p>One particularly common use of ephemerons is to combine them with a
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weak hash table (see <a href="hashtables.html" data-pltdoc="x">Hash Tables</a>) to produce a mapping where
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the memory manager can reclaim key–value pairs even when the value
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refers to the key; see <span class="RktSym"><a href="hashtables.html#%28def._%28%28quote._~23~25kernel%29._make-ephemeron-hash%29%29" class="RktValLink" data-pltdoc="x">make-ephemeron-hash</a></span>.
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A related use is to retain a reference to a value
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as long as any value for which it is an <a href="chaperones.html#%28tech._impersonator%29" class="techoutside" data-pltdoc="x"><span class="techinside">impersonator</span></a> is
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reachable; see <span class="RktSym"><a href="chaperones.html#%28def._%28%28quote._~23~25kernel%29._impersonator-ephemeron%29%29" class="RktValLink" data-pltdoc="x">impersonator-ephemeron</a></span>.</p><p><div class="SIntrapara">More precisely,
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</div><div class="SIntrapara"><ul><li><p>the value in an ephemeron is replaced
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by <span class="RktVal">#f</span> when the automatic memory manager can prove that
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either the ephemeron or the key is reachable only through weak
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references (see <a href="weakbox.html" data-pltdoc="x">Weak Boxes</a>); and</p></li><li><p>nothing reachable from the value in an ephemeron counts toward
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the reachability of an ephemeron key (whether for the same ephemeron
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or another), unless the same value is reachable through a non-weak
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reference, or unless the value’s ephemeron key is reachable through a
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non-weak reference (see <a href="weakbox.html" data-pltdoc="x">Weak Boxes</a> for information on weak
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references).</p></li></ul></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)._make-ephemeron))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._make-ephemeron%29%29" class="RktValDef RktValLink" data-pltdoc="x">make-ephemeron</a></span></span><span class="hspace"> </span><span class="RktVar">key</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="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="RktValLink" data-pltdoc="x">ephemeron?</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">key</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">Returns a new <a href="ephemerons.html#%28tech._ephemeron%29" class="techoutside" data-pltdoc="x"><span class="techinside">ephemeron</span></a> whose key is <span class="RktVar">key</span> and whose
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value is initially <span class="RktVar">v</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)._ephemeron-value))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._ephemeron-value%29%29" class="RktValDef RktValLink" data-pltdoc="x">ephemeron-value</a></span></span><span class="hspace"> </span><span class="RktVar">ephemeron</span><span class="hspace"> </span><span class="RktOpt">[</span><span class="RktVar">gced-v</span><span class="hspace"> </span><span class="RktVar">retain-v</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#%28def._%28%28lib._racket%2Fcontract%2Fprivate%2Fmisc..rkt%29._any%2Fc%29%29" class="RktValLink" data-pltdoc="x">any/c</a></span></p></blockquote></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">ephemeron</span><span class="hspace"> </span>:<span class="hspace"> </span><span class="RktSym"><a href="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="RktValLink" data-pltdoc="x">ephemeron?</a></span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">gced-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><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVar">retain-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><span class="hspace"> </span>=<span class="hspace"> </span><span class="RktVal">#f</span></td></tr></table></blockquote></div><div class="SIntrapara">Returns the value contained in <span class="RktVar">ephemeron</span>. If the garbage
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collector has proven that the key for <span class="RktVar">ephemeron</span> is only
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weakly reachable, then the result is <span class="RktVar">gced-v</span> (which defaults to <span class="RktVal">#f</span>).</div></p><p>The <span class="RktVar">retain-v</span> argument is retained as reachable until the
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ephemeron’s value is extracted. It is useful, for example, when
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<span class="RktVar">ephemeron</span> was obtained through a weak, <span class="RktSym"><a href="Equality.html#%28def._%28%28quote._~23~25kernel%29._eq~3f%29%29" class="RktValLink" data-pltdoc="x">eq?</a></span>-based
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mapping from <span class="RktVar">key</span> and <span class="RktVar">ephemeron</span> was created with
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<span class="RktVar">key</span> as the key; in that case, supplying <span class="RktVar">key</span> as
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<span class="RktVar">retain-v</span> ensures that <span class="RktVar">ephemeron</span> retains its value
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long enough for it to be extracted, even if <span class="RktVar">key</span> is otherwise
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unreachable.</p><p class="SHistory">Changed in version 7.1.0.10 of package <span class="stt">base</span>: Added the <span class="RktVar">retain-v</span> argument.</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)._ephemeron~3f))"></a><span title="Provided from: racket/base, racket | Package: base"><span class="RktSym"><a href="ephemerons.html#%28def._%28%28quote._~23~25kernel%29._ephemeron~3f%29%29" class="RktValDef RktValLink" data-pltdoc="x">ephemeron?</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 an <a href="ephemerons.html#%28tech._ephemeron%29" class="techoutside" data-pltdoc="x"><span class="techinside">ephemeron</span></a>, <span class="RktVal">#f</span>
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otherwise.</div></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="weakbox.html" title="backward to "16.1 Weak Boxes"" data-pltdoc="x">← prev</a> <a href="memory.html" title="up to "16 Memory Management"" data-pltdoc="x">up</a> <a href="willexecutor.html" title="forward to "16.3 Wills and Executors"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html> |