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181 lines
9.7 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>23.1.3. Structured Directories</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Structured Directories">
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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=tex2html4131 HREF="node207.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4129 HREF="node203.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4123 HREF="node205.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4133 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4134 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4132 HREF="node207.html"> Extended Wildcards</A>
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<B>Up:</B> <A NAME=tex2html4130 HREF="node203.html"> File Names</A>
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<B> Previous:</B> <A NAME=tex2html4124 HREF="node205.html"> Case Conventions</A>
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<HR> <P>
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<H2><A NAME=SECTION002713000000000000000>23.1.3. Structured Directories</A></H2>
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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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<A NAME=STRUCTUREDDIRECTORYSECTION>X3J13</A>
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voted in June 1989 (PATHNAME-SUBDIRECTORY-LIST) <A NAME=26200> </A>
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to define a specific pathname component format for structured directories.
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<P>
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The value of a pathname's directory component may be a list. The
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<i>car</i> of the list should be a keyword, either <tt>:absolute</tt> or <tt>:relative</tt>.
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Each remaining element of the list should be a string or a symbol (see below).
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Each string names a single level of directory structure and should consist
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of only the directory name without any punctuation characters.
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<P>
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A list whose <i>car</i> is the symbol <tt>:absolute</tt> represents a directory path
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starting from the root directory. For example, the list <tt>(:absolute)</tt> represents
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the root directory itself; the list <tt>(:absolute "foo" "bar" "baz")</tt> represents
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the directory that in a UNIX file system would be called <tt>/foo/bar/baz</tt>.
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<P>
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A list whose <i>car</i> is the symbol <tt>:relative</tt> represents a directory path
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starting from a default directory. The list <tt>(:relative)</tt> has the same
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meaning as <tt>nil</tt> and hence normally is not used. The list <tt>(:relative "foo" "bar")</tt>
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represents the directory named <tt>bar</tt> in the directory named <tt>foo</tt> in the
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default directory.
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<P>
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In place of a string, at any point in the list, a symbol may occur to
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indicate a special file notation. The following symbols have standard
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meanings.
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<DL COMPACT>
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<DT><tt>:wild</tt>
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<DD> Wildcard match of one level of directory structure
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<DT><tt>:wild-inferiors</tt>
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<DD> Wildcard match of any number of directory levels
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<DT><tt>:up</tt>
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<DD> Go upward in directory structure (semantic)
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<DT><tt>:back</tt>
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<DD> Go upward in directory structure (syntactic)
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<P>
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</DL>
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(See section <A HREF="node207.html#WILDPATHNAMESECTION">23.1.4</A> for a discussion of wildcard pathnames.)
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<P>
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Implementations are permitted to add additional objects of any
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non-string type if necessary to represent features of their file systems
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that cannot be represented with the standard strings and symbols.
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Supplying any non-string, including any of the symbols listed below, to a
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file system for which it does not make sense signals an error of type
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<tt>file-error</tt>. For example, most implementations of the UNIX file system
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do not support <tt>:wild-inferiors</tt>. Any directory list in which
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<tt>:absolute</tt> or <tt>:wild-inferiors</tt> is immediately followed by <tt>:up</tt> or <tt>:back</tt>
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is illegal and when processed causes an error to be signaled.
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<P>
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The keyword <tt>:back</tt> has a ``syntactic'' meaning that depends only on the pathname
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and not on the contents of the file system. The keyword <tt>:up</tt> has a ``semantic''
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meaning that depends on the contents of the file system; to resolve
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a pathname containing <tt>:up</tt> to a pathname whose directory component
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contains only <tt>:absolute</tt> and strings requires a search of the file system.
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Note that use of <tt>:up</tt> instead of <tt>:back</tt> can result in designating a different
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actual directory only in file systems that support multiple
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names for directories, perhaps via symbolic links. For example,
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suppose that there is a directory link such that
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<P><pre>
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(:absolute "X" "Y") is linked to (:absolute "A" "B")
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</pre><P>
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and there also exist directories
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<P><pre>
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(:absolute "A" "Q") and (:absolute "X" "Q")
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</pre><P>
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Then
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<P><pre>
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(:absolute "X" "Y" :up "Q") designates (:absolute "A" "Q")
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</pre><P>
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but
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<P><pre>
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(:absolute "X" "Y" :back "Q") designates (:absolute "X" "Q")
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</pre><P>
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<P>
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If a string is used as the value of the <tt>:directory</tt> argument to
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<tt>make-pathname</tt>, it should be the name of a top-level directory and should
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not contain any punctuation characters. Specifying a string <i>s</i> is
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equivalent to specifying the list <tt>(:absolute <i>s</i>)</tt>. Specifying the symbol
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<tt>:wild</tt> is equivalent to specifying the list <tt>(:absolute :wild-inferiors)</tt>
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(or <tt>(:absolute :wild)</tt> in a file system that does not support <tt>:wild-inferiors</tt>).
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<P>
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The function <tt>pathname-directory</tt> always returns <tt>nil</tt>, <tt>:unspecific</tt>, or a
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list-never a string, never <tt>:wild</tt>.
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If a list is returned, it is not guaranteed to be freshly consed; the
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consequences of modifying this list are undefined.
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<P>
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In non-hierarchical file systems, the only valid list values for the
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directory component of a pathname are <tt>(:absolute <i>s</i>)</tt> (where <i>s</i>
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is a string) and
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<tt>(:absolute :wild)</tt>. The keywords <tt>:relative</tt>,
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<tt>:wild-inferiors</tt>, <tt>:up</tt>, and <tt>:back</tt> are not used in non-hierarchical file
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systems.
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<P>
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Pathname merging treats a relative directory specially. Let
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<i>pathname</i> and <i>defaults</i> be the first two arguments to
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<tt>merge-pathnames</tt>. If <tt>(pathname-directory <i>pathname</i>)</tt> is a list whose
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<i>car</i> is <tt>:relative</tt>, and <tt>(pathname-directory <i>defaults</i>)</tt> is a list, then
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the merged directory is the value of
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<P><pre>
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(append (pathname-directory <i>defaults</i>)
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(cdr ;Remove <tt>:relative</tt> from the front
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(pathname-directory <i>pathname</i>)))
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</pre><P>
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except that if the resulting list contains a string or <tt>:wild</tt> immediately
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followed by <tt>:back</tt>, both of them are removed. This removal of redundant
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occurrences of <tt>:back</tt> is repeated as many times as possible.
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If <tt>(pathname-directory <i>defaults</i>)</tt> is not a list or
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<tt>(pathname-directory <i>pathname</i>)</tt> is not a list whose <i>car</i> is <tt>:relative</tt>, the
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merged directory is the value of
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<P><pre>
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(or (pathname-directory <i>pathname</i>)
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(pathname-directory <i>defaults</i>))
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</pre><P>
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<P>
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A relative directory in the pathname argument to a function such as
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<tt>open</tt> is merged with the value of <tt>*default-pathname-defaults*</tt> before the
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file system is accessed.
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<P>
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Here are some examples of the use of structured directories.
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Suppose that host <tt>L</tt> supports a Symbolics Lisp Machine file system,
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host <tt>U</tt> supports a UNIX file system, and
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host <tt>V</tt> supports a VAX/VMS file system.
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<P><pre>
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(pathname-directory (parse-namestring "V:[FOO.BAR]BAZ.LSP"))
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=> (:ABSOLUTE "FOO" "BAR")
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</pre><P>
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<P><pre>
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(pathname-directory (parse-namestring "U:/foo/bar/baz.lisp"))
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=> (:ABSOLUTE "foo" "bar")
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</pre><P>
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<P><pre>
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(pathname-directory (parse-namestring "U:../baz.lisp"))
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=> (:RELATIVE :UP)
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</pre><P>
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<P><pre>
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(pathname-directory (parse-namestring "U:/foo/bar/../mum/baz"))
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=> (:ABSOLUTE "foo" "bar" :UP "mum")
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</pre><P>
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<P><pre>
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(pathname-directory (parse-namestring "U:bar/../../ztesch/zip"))
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=> (:RELATIVE "bar" :UP :UP "ztesch")
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</pre><P>
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<P><pre>
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(pathname-directory (parse-namestring "L:>foo>**>bar>baz.lisp"))
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=> (:ABSOLUTE "FOO" :WILD-INFERIORS "BAR")
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</pre><P>
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<P><pre>
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(pathname-directory (parse-namestring "L:>foo>*>bar>baz.lisp"))
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=> (:ABSOLUTE "FOO" :WILD "BAR")
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</pre><P>
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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> <HR><A NAME=tex2html4131 HREF="node207.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4129 HREF="node203.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4123 HREF="node205.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4133 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4134 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4132 HREF="node207.html"> Extended Wildcards</A>
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<B>Up:</B> <A NAME=tex2html4130 HREF="node203.html"> File Names</A>
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<B> Previous:</B> <A NAME=tex2html4124 HREF="node205.html"> Case Conventions</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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