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101 lines
5.7 KiB
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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
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<!Converted with LaTeX2HTML 0.6.5 (Tue Nov 15 1994) by Nikos Drakos (nikos@cbl.leeds.ac.uk), CBLU, University of Leeds >
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<HEAD>
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<TITLE>11.8. Modules</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Modules">
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<meta name="keywords" value="clm">
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<meta name="resource-type" value="document">
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<meta name="distribution" value="global">
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<P>
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<b>Common Lisp the Language, 2nd Edition</b>
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<BR> <HR><A NAME=tex2html3007 HREF="node120.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3005 HREF="node111.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2999 HREF="node118.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3009 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3010 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3008 HREF="node120.html"> An Example</A>
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<B>Up:</B> <A NAME=tex2html3006 HREF="node111.html"> Packages</A>
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<B> Previous:</B> <A NAME=tex2html3000 HREF="node118.html"> Package System Functions </A>
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<HR> <P>
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<H1><A NAME=SECTION001580000000000000000>11.8. Modules</A></H1>
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<P>
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A <i>module</i> is a Common Lisp subsystem that is loaded from one or more
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files. A module is normally loaded as a single unit, regardless of how
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many files are involved. A module may consist of one package or several
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packages. The file-loading process is necessarily
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implementation-dependent, but Common Lisp provides some very simple
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portable machinery for naming modules, for keeping track of which modules
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have been loaded, and for loading modules as a unit.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in January 1989
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(REQUIRE-PATHNAME-DEFAULTS) <A NAME=11096> </A>
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to eliminate the entire module facility from
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the language; that is, the variable <tt>*modules*</tt> and the functions
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<tt>provide</tt> and <tt>require</tt> are deleted.
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X3J13 commented that the file-loading feature of <tt>require</tt> is not
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portable, and that the remaining functionality is easily implemented
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by user code. (I will add that in any case the specification of
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<tt>require</tt> is so vague that different implementations are likely to
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have differing behavior.)
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<BR><b>[Variable]</b><BR>
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<tt>*modules*</tt><P>The variable <tt>*modules*</tt> is a list of names of the modules
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that have been loaded into the Lisp system so far.
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This list is used by the functions <tt>provide</tt> and <tt>require</tt>.
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<P>
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<BR><b>[Function]</b><BR>
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<tt>provide</tt> <tt><i>module-name</i></tt> <tt><BR></tt><tt>require</tt> <tt><i>module-name</i></tt> <tt>&optional</tt> <tt><i>pathname</i></tt><P>Each module has a unique name (a string). The <tt>provide</tt> and <tt>require</tt>
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functions accept either a string or a symbol as the <i>module-name</i>
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argument. If a symbol is provided, its print name is used as the module
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name. If the module consists of a single package, it is customary for
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the package and module names to be the same.
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<P>
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The <tt>provide</tt>
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function adds a new module name to the list of modules
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maintained in the variable <tt>*modules*</tt>, thereby indicating that
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the module in question has been loaded.
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<P>
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The <tt>require</tt> function tests whether a module is already present
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(using a case-sensitive comparison); if the module is not present,
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<tt>require</tt> proceeds to load the appropriate
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file or set of files. The <i>pathname</i> argument, if present, is a single
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pathname or a list of pathnames whose files are to be loaded in order,
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left to right. If the <i>pathname</i> argument is <tt>nil</tt> or is not provided,
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the system will attempt to determine, in some
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system-dependent manner, which files to load.
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This will typically involve some central
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registry of module names and the associated file lists.
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<P>
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<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
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X3J13 voted in March 1988
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not to permit symbols as pathnames
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(PATHNAME-SYMBOL) <A NAME=11120> </A> and
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to specify exactly which streams may be used as pathnames
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(PATHNAME-STREAM) <A NAME=11121> </A> (see section <A HREF="node214.html#PATHNAMEFUNCTIONS">23.1.6</A>).
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Of course, this is moot if <tt>require</tt> is not in the language.
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<P>
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X3J13 voted in January 1989
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(RETURN-VALUES-UNSPECIFIED) <A NAME=11126> </A>
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to specify that the values returned by <tt>provide</tt> and <tt>require</tt>
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are implementation-dependent. Of course, this is moot
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if <tt>provide</tt> and <tt>require</tt> are not in the language.
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<br><img align=bottom alt="change_end" src="gif/change_end.gif">
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<P>
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<hr>
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<b>Implementation note:</b> One way to implement such a registry on
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many operating systems is simply to use a distinguished ``library''
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directory within the file system, where the name of each file
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is the same as the module it contains.
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<hr>
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<P>
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<BR> <HR><A NAME=tex2html3007 HREF="node120.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3005 HREF="node111.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2999 HREF="node118.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3009 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3010 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3008 HREF="node120.html"> An Example</A>
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<B>Up:</B> <A NAME=tex2html3006 HREF="node111.html"> Packages</A>
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<B> Previous:</B> <A NAME=tex2html3000 HREF="node118.html"> Package System Functions </A>
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<HR> <P>
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<HR>
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<P><ADDRESS>
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AI.Repository@cs.cmu.edu
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</ADDRESS>
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</BODY>
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