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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>13.1.8 Structures as Ports</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="tocviewselflink" 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" cl
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properties identify structure types whose instances can serve as input
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and output ports, respectively.</p><p>Each property value can be either of the following:</p><ul><li><p>An input port (for <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3ainput-port%29%29" class="RktValLink" data-pltdoc="x">prop:input-port</a></span>) or output port
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(for <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3aoutput-port%29%29" class="RktValLink" data-pltdoc="x">prop:output-port</a></span>): In this case, using the structure
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as port is equivalent to using the given input or output port.</p></li><li><p>An exact, non-negative integer between <span class="RktVal">0</span> (inclusive) and
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the number of non-automatic fields in the structure type (exclusive, not
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counting supertype fields): The integer identifies a field in
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the structure, and the field must be designated as immutable. If the
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field contains an input port (for <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3ainput-port%29%29" class="RktValLink" data-pltdoc="x">prop:input-port</a></span>) or
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output port (for <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3aoutput-port%29%29" class="RktValLink" data-pltdoc="x">prop:output-port</a></span>), the port is used.
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Otherwise, an empty string input port is used for <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3ainput-port%29%29" class="RktValLink" data-pltdoc="x">prop:input-port</a></span>,
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and a port that discards all data is used for <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3aoutput-port%29%29" class="RktValLink" data-pltdoc="x">prop:output-port</a></span>.</p></li></ul><p>Some procedures, such as <span class="RktSym"><a href="port-buffers.html#%28def._%28%28quote._~23~25kernel%29._file-position%29%29" class="RktValLink" data-pltdoc="x">file-position</a></span>, work on both input
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and output ports. When given an instance of a structure type with both
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the <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3ainput-port%29%29" class="RktValLink" data-pltdoc="x">prop:input-port</a></span> and <span class="RktSym"><a href="portstructs.html#%28def._%28%28quote._~23~25kernel%29._prop~3aoutput-port%29%29" class="RktValLink" data-pltdoc="x">prop:output-port</a></span> properties,
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the instance is used as an input port.</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="pipeports.html" title="backward to "13.1.7 Pipes"" data-pltdoc="x">← prev</a> <a href="ports.html" title="up to "13.1 Ports"" data-pltdoc="x">up</a> <a href="customport.html" title="forward to "13.1.9 Custom Ports"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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