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<html><head><meta http-equiv="content-type" content="text/html; charset=utf-8"/><title>15.2 Manipulating Namespaces</title><link rel="stylesheet" type="text/css" href="../scribble.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" 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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>15 </td><td><a href="reflection.html" class="tocviewlink" data-pltdoc="x">Reflection and Dynamic Evaluation</a></td></tr></table><div class="tocviewsublist" style="display: block;" id="tocview_1"><table cellspacing="0" cellpadding="0"><tr><td align="right">15.1 </td><td><a href="eval.html" class="tocviewlink" data-pltdoc="x"><span class="RktSym"><span class="RktValLink">eval</span></span></a></td></tr><tr><td align="right">15.2 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Manipulating Namespaces</a></td></tr><tr><td align="right">15.3 </td><td><a href="load.html" class="tocviewlink" data-pltdoc="x">Scripting Evaluation and Using <span class="RktSym"><span class="RktValLink">load</span></span></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>15.2 </td><td><a href="" class="tocviewselflink" data-pltdoc="x">Manipulating Namespaces</a></td></tr></table><div class="tocviewsublistbottom" style="display: none;" id="tocview_2"><table cellspacing="0" cellpadding="0"><tr><td align="right">15.2.1 </td><td><a href="#%28part._.Creating_and_.Installing_.Namespaces%29" class="tocviewlink" data-pltdoc="x">Creating and Installing Namespaces</a></td></tr><tr><td align="right">15.2.2 </td><td><a href="#%28part._.Sharing_.Data_and_.Code_.Across_.Namespaces%29" class="tocviewlink" data-pltdoc="x">Sharing Data and Code Across Namespaces</a></td></tr></table></div></div></div><div class="tocsub"><div class="tocsubtitle">On this page:</div><table class="tocsublist" cellspacing="0"><tr><td><span class="tocsublinknumber">15.2.1<tt> </tt></span><a href="#%28part._.Creating_and_.Installing_.Namespaces%29" class="tocsubseclink" data-pltdoc="x">Creating and Installing Namespaces</a></td></tr><tr><td><span class="tocsublinknumber">15.2.2<tt> </tt></span><a href="#%28part._.Sharing_.Data_and_.Code_.Across_.Namespaces%29" class="tocsubseclink" data-pltdoc="x">Sharing Data and Code Across Namespaces</a></td></tr></table></div></div><div class="maincolumn"><div class="main"><div class="navsettop"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="eval.html" title="backward to "15.1 eval"" data-pltdoc="x">← prev</a> <a href="reflection.html" title="up to "15 Reflection and Dynamic Evaluation"" data-pltdoc="x">up</a> <a href="load.html" title="forward to "15.3 Scripting Evaluation and Using load"" data-pltdoc="x">next →</a></span> </div><h4 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""mk-namespace"">15.2<tt> </tt><a name="(part._mk-namespace)"></a>Manipulating Namespaces</h4><p>A <a href="eval.html#%28tech._namespace%29" class="techoutside" data-pltdoc="x"><span class="techinside">namespace</span></a> encapsulates two pieces of information:</p><ul><li><p>A mapping from identifiers to bindings. For example, a
namespace might map the identifier <span class="RktSym">lambda</span> to the
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=lambda.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._lambda%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">lambda</a></span> form. An “empty” namespace is one that maps
every identifier to an uninitialized top-level variable.</p></li><li><p>A mapping from module names to module declarations and
instances.</p></li></ul><p>The first mapping is used for evaluating expressions in a top-level
context, as in <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">lambda</span><span class="stt"> </span><span class="RktVal">(</span><span class="RktVal">x</span><span class="RktVal">)</span><span class="stt"> </span><span class="RktVal">(</span><span class="RktVal">+</span><span class="stt"> </span><span class="RktVal">x</span><span class="stt"> </span><span class="RktVal">1</span><span class="RktVal">)</span><span class="RktVal">)</span><span class="RktPn">)</span>. The second
mapping is used, for example, by <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Module_Names_and_Loading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._dynamic-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">dynamic-require</a></span> to locate a
module. The call <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">require</span><span class="stt"> </span><span class="RktVal">racket/base</span><span class="RktVal">)</span><span class="RktPn">)</span> normally uses
both pieces: the identifier mapping determines the binding of
<span class="RktSym">require</span>; if it turns out to mean <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span>, then
the module mapping is used to locate the <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a>
module.</p><p>From the perspective of the core Racket run-time system, all
evaluation is reflective. Execution starts with an initial namespace
that contains a few primitive modules, and that is further populated
by loading files and modules as specified on the command line or as
supplied in the <a href="intro.html#%28tech._repl%29" class="techoutside" data-pltdoc="x"><span class="techinside">REPL</span></a>. Top-level <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span> and
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span> forms adjusts the identifier mapping, and module
declarations (typically loaded on demand for a <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span> form)
adjust the module mapping.</p><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""Creating_and_Installing_Namespaces"">15.2.1<tt> </tt><a name="(part._.Creating_and_.Installing_.Namespaces)"></a>Creating and Installing Namespaces</h5><p>The function <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-empty-namespace</a></span> creates a new, empty
<a href="eval.html#%28tech._namespace%29" class="techoutside" data-pltdoc="x"><span class="techinside">namespace</span></a>. Since the namespace is truly empty, it cannot at
first be used to evaluate any top-level expression—<wbr></wbr>not even
<span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="stt"> </span><span class="RktSym">racket</span><span class="RktPn">)</span>. In particular,</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._make-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-empty-namespace</a></span><span class="RktPn">)</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">racket</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>fails, because the namespace does not include the primitive modules on
which <span class="RktSym">racket</span> is built.</p><p>To make a namespace useful, some modules must be <a name="(tech._attached)"></a><span style="font-style: italic">attached</span>
from an existing namespace. Attaching a module adjusts the mapping of
module names to instances by transitively copying entries (the module
and all its imports) from an existing namespace’s mapping. Normally,
instead of just attaching the primitive modules—<wbr></wbr>whose names and
organization are subject to change—<wbr></wbr>a higher-level module is
attached, such as <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket</span></a> or
<a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a>.</p><p>The <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span> function provides a namespace
that is empty, except that <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a> is
attached. The resulting namespace is still “empty” in the sense that
the identifiers-to-bindings part of the namespace has no mappings;
only the module mapping has been populated. Nevertheless, with an
initial module mapping, further modules can be loaded.</p><p>A namespace created with <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span> is
suitable for many basic dynamic tasks. For example, suppose that a
<span class="RktMod">my-dsl</span> library implements a domain-specific language
in which you want to execute commands from a user-specified file. A
namespace created with <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span> is enough to
get started:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">run-dsl</span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._file%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">file</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span><span class="RktPn">)</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">my-dsl</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._load%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">load</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._file%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">file</a></span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>Note that the <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> of <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span> does
not affect the meaning of identifiers like <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span>
within the <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span> body. Those identifiers obtain their
meaning from the enclosing context (probably a module). Only
expressions that are dynamic with respect to this code, such as the
content of <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._load%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">load</a></span>ed files, are affected by the
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span>.</p><p>Another subtle point in the above example is the use of
<span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">my-dsl</span><span class="RktPn">)</span> instead of <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">require</span><span class="stt"> </span><span class="RktVal">my-dsl</span><span class="RktVal">)</span><span class="RktPn">)</span>. The latter would not work, because <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span>
needs to obtain a meaning for <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span> in the namespace, and
the namespace’s identifier mapping is initially empty. The
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span> function, in contrast, directly imports the
given module into the current namespace. Starting with
<span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">racket/base</span><span class="RktPn">)</span> would introduce a binding
for <span class="RktSym">require</span> and make a subsequent <span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span><span class="stt"> </span><span class="RktVal">'</span><span class="RktVal">(</span><span class="RktVal">require</span><span class="stt"> </span><span class="RktVal">my-dsl</span><span class="RktVal">)</span><span class="RktPn">)</span> work. The above is better, not only because it is
more compact, but also because it avoids introducing bindings that are
not part of the domain-specific languages.</p><h5 x-source-module="(lib "scribblings/guide/guide.scrbl")" x-part-tag=""Sharing_Data_and_Code_Across_Namespaces"">15.2.2<tt> </tt><a name="(part._.Sharing_.Data_and_.Code_.Across_.Namespaces)"></a>Sharing Data and Code Across Namespaces</h5><p>Modules not attached to a new namespace will be loaded and
instantiated afresh if they are demanded by evaluation. For example,
<a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a> does not include
<a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=mzlib_class.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/class</span></a>, and loading <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=mzlib_class.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/class</span></a>
again will create a distinct class datatype:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">racket/class</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></td></tr><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">class?</span><span class="hspace"> </span><span class="RktSym">object%</span><span class="RktPn">)</span></td></tr><tr><td><p><span class="RktRes">#t</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">class?</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">racket/class</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">loads again</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">object%</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></td></tr><tr><td><p><span class="RktRes">#f</span></p></td></tr></table></blockquote><p>For cases when dynamically loaded code needs to share more code and
data with its context, use the <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-attach-module%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-attach-module</a></span>
function. The first argument to <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-attach-module%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-attach-module</a></span> is a
source namespace from which to draw a module instance; in some cases,
the current namespace is known to include the module that needs to be
shared:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">racket/class</span><span class="RktPn">)</span></td></tr><tr><td><table cellspacing="0" cellpadding="0"><tr><td></td></tr></table></td></tr><tr><td><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><span class="stt">> </span><span class="RktPn">(</span><span class="RktSym">class?</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">ns</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span><span class="RktPn">)</span><span class="RktPn">]</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-attach-module%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-attach-module</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">racket/class</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktSym">ns</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span><span class="hspace"> </span><span class="RktSym">ns</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-require</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">racket/class</span><span class="RktPn">)</span><span class="hspace"> </span><span class="RktCmt">;</span><span class="RktCmt"> </span><span class="RktCmt">uses attached</span></td></tr><tr><td><span class="hspace"> </span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=eval.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._eval%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">eval</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">object%</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">#t</span></p></td></tr></table></blockquote><p>Within a module, however, the combination of
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define-namespace-anchor%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-namespace-anchor</a></span> and
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._namespace-anchor-%7E3eempty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-anchor->empty-namespace</a></span> offers a more reliable
method for obtaining a source namespace:</p><blockquote class="SCodeFlow"><table cellspacing="0" cellpadding="0" class="RktBlk"><tr><td><a href="Module_Syntax.html#%28part._hash-lang%29" class="RktModLink" data-pltdoc="x"><span class="RktMod">#lang</span></a><span class="hspace"> </span><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=index.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/base</span></a></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._require%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">require</a></span><span class="hspace"> </span><span class="RktSym">racket/class</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define-namespace-anchor%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define-namespace-anchor</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span></td></tr><tr><td><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=define.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._define%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">define</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym">load-plug-in</span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._file%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">file</a></span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=let.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fletstx-scheme..rkt%2529._let%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">let</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym">ns</span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._make-base-empty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">make-base-empty-namespace</a></span><span class="RktPn">)</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-attach-module%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-attach-module</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._namespace-anchor-%7E3eempty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-anchor->empty-namespace</a></span><span class="hspace"> </span><span class="RktSym">a</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">racket/class</span></td></tr><tr><td><span class="hspace"> </span><span class="RktSym">ns</span><span class="RktPn">)</span></td></tr><tr><td><span class="hspace"> </span><span class="RktPn">(</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=parameters.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fmore-scheme..rkt%2529._parameterize%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">parameterize</a></span><span class="hspace"> </span><span class="RktPn">(</span><span class="RktPn">[</span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._current-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">current-namespace</a></span><span class="hspace"> </span><span class="RktSym">ns</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="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Module_Names_and_Loading.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._dynamic-require%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">dynamic-require</a></span><span class="hspace"> </span><span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=require.html%23%2528form._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._file%2529%2529&version=6.1" class="RktStxLink Sq" data-pltdoc="x">file</a></span><span class="hspace"> </span><span class="RktVal">'</span><span class="RktVal">plug-in%</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span><span class="RktPn">)</span></td></tr></table></blockquote><p>The anchor bound by <span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528quote._%7E23%7E25kernel%2529._namespace-attach-module%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-attach-module</a></span> connects the
run time of a module with the namespace in which a module is loaded
(which might differ from the current namespace). In the above
example, since the enclosing module requires
<a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=mzlib_class.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/class</span></a>, the namespace produced by
<span class="RktSym"><a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=Namespaces.html%23%2528def._%2528%2528lib._racket%252Fprivate%252Fbase..rkt%2529._namespace-anchor-%7E3eempty-namespace%2529%2529&version=6.1" class="RktValLink Sq" data-pltdoc="x">namespace-anchor->empty-namespace</a></span> certainly contains an
instance of <a href="http://download.racket-lang.org/docs/6.1/html/local-redirect/index.html?doc=reference&rel=mzlib_class.html&version=6.1" class="RktModLink Sq" data-pltdoc="x"><span class="RktSym">racket/class</span></a>. Moreover, that instance is
the same as the one imported into the module, so the class datatype is
shared.</p><div class="navsetbottom"><span class="navleft"><form class="searchform"><input class="searchbox" style="color: #888;" type="text" value="...search manuals..." title="Enter a search string to search the manuals" onkeypress="return DoSearchKey(event, this, "6.1", "../");" onfocus="this.style.color="black"; this.style.textAlign="left"; if (this.value == "...search manuals...") this.value="";" onblur="if (this.value.match(/^ *$/)) { this.style.color="#888"; this.style.textAlign="center"; this.value="...search manuals..."; }"/></form> <a href="../index.html" title="up to the documentation top" data-pltdoc="x" onclick="return GotoPLTRoot("6.1");">top</a></span><span class="navright"> <a href="eval.html" title="backward to "15.1 eval"" data-pltdoc="x">← prev</a> <a href="reflection.html" title="up to "15 Reflection and Dynamic Evaluation"" data-pltdoc="x">up</a> <a href="load.html" title="forward to "15.3 Scripting Evaluation and Using load"" data-pltdoc="x">next →</a></span> </div></div></div><div id="contextindicator"> </div></body></html>
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