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144 lines
8 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>11.3. Translating Strings to Symbols</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Translating Strings to Symbols">
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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=tex2html2947 HREF="node115.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2945 HREF="node111.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2939 HREF="node113.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2949 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2950 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2948 HREF="node115.html"> Exporting and Importing </A>
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<B>Up:</B> <A NAME=tex2html2946 HREF="node111.html"> Packages</A>
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<B> Previous:</B> <A NAME=tex2html2940 HREF="node113.html"> Package Names</A>
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<HR> <P>
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<H1><A NAME=SECTION001530000000000000000>11.3. Translating Strings to Symbols</A></H1>
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<P>
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<A NAME=STRINGTOSYMBOLSECTION>The</A>
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value of the special variable <tt>*package*</tt> must always be a package
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object (not a name). Whatever package object is currently the
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value of <tt>*package*</tt> is referred to as the <i>current package</i>.
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<P>
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When the Lisp reader has, by parsing, obtained a string of characters
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thought to name a symbol, that name is looked up in the current package.
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This lookup may involve looking in other packages whose external symbols
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are inherited by the current package. If the name is found,
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the corresponding symbol is returned. If the name is not found
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(that is, there is no symbol of that name accessible in the current package),
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a new symbol is created for it and is placed in the current package as an
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internal symbol. Moreover, the current package becomes the owner
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(home package) of the symbol, and so the symbol becomes interned
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in the current package.
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If the name is later read again while this same package is
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current, the same symbol will then be found and returned.
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<P>
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Often it is desirable to refer to an external symbol in some package
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other than the current one. This is done through the use of a
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<i>qualified name</i>, consisting of a package name, then a colon, then the
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name of the symbol. This causes the symbol's name to be looked up
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in the specified package, rather than in the current one. For example,
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<tt>editor:buffer</tt> refers to the external symbol named <tt>buffer</tt>
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accessible in the package named <tt>editor</tt>, regardless of whether
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there is a symbol named <tt>buffer</tt> in the current package. If there
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is no package named <tt>editor</tt>, or if no symbol named <tt>buffer</tt>
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is accessible in <tt>editor</tt>, or if <tt>buffer</tt> is an internal
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symbol in <tt>editor</tt>, the Lisp reader will signal
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a correctable error to ask the user for instructions.
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<P>
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On rare occasions, a user may need to refer to an <i>internal</i> symbol of
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some package other than the current one. It is illegal to do this with
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the colon qualifier, since accessing an internal symbol of some other
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package is usually a mistake. However, this operation is legal if
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a doubled colon
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<tt>::</tt> is used as the separator in place of the usual single colon. If
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<tt>editor::buffer</tt> is seen, the effect is exactly the same as
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reading <tt>buffer</tt> with <tt>*package*</tt> temporarily rebound to the
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package whose name is <tt>editor</tt>. This special-purpose qualifier
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should be used with caution.
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<P>
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The package named <tt>keyword</tt> contains all keyword symbols used by the
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Lisp system itself and by user-written code. Such symbols must be
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easily accessible from any package, and name conflicts are not an issue
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because these symbols are used only as labels and never to carry
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package-specific values or properties. Because keyword symbols are used
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so frequently, Common Lisp provides a special reader syntax for them.
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Any symbol preceded by a colon but no package name (for example
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<tt>:foo</tt>) is added to (or looked up in) the <tt>keyword</tt> package as
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an <i>external</i> symbol. The <tt>keyword</tt> package is also treated
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specially in that whenever a symbol is added to the <tt>keyword</tt> package
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the symbol is always made external; the symbol
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is also automatically declared to be a constant
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(see <tt>defconstant</tt>) and made to
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have itself as its value. This is why every keyword evaluates to
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itself. As a matter of style, keywords should always be accessed using
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the leading-colon convention; the user should never import or inherit
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keywords into any other package. It is an error to try to apply
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<tt>use-package</tt> to the <tt>keyword</tt> package.
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<P>
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Each symbol contains a package cell that is used to record the home
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package of the symbol, or <tt>nil</tt> if the symbol is uninterned. This cell
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may be accessed by using the function <tt>symbol-package</tt>.
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When an interned
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symbol is printed, if it is a symbol in the keyword package,
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then it is printed with a preceding colon; otherwise, if it is accessible
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(directly or by inheritance) in the current package, it is printed
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without any qualification; otherwise, it is printed with the name of the
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home package as the qualifier, using <tt>:</tt> as the separator if the
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symbol is external and <tt>::</tt> if not.
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<P>
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A symbol whose package slot contains <tt>nil</tt> (that is, has no home
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package)
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is printed preceded by <tt>#:</tt>. It is possible, by the
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use of <tt>import</tt> and <tt>unintern</tt>, to create a symbol that has no
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recorded home package but that in fact is accessible in some package.
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The system does not check for this pathological case, and such symbols
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will always be printed preceded by <tt>#:</tt>.
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<P>
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In summary, the following four uses of symbol qualifier syntax are defined.
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<P>
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<DL COMPACT><DT><tt>foo:bar</tt>
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<DD>
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When read, looks up <tt>BAR</tt> among the external symbols of
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the package named <tt>FOO</tt>. Printed when symbol <tt>bar</tt> is external in its
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home package <tt>foo</tt> and is not accessible in the current package.
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<P>
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<DT><tt>foo::bar</tt>
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<DD>
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When read, interns <tt>BAR</tt> as if <tt>FOO</tt> were the
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current package. Printed when symbol <tt>bar</tt> is internal in its home package
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<tt>foo</tt> and is not accessible in the current package.
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<P>
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<DT><tt>:bar</tt>
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<DD>
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When read, interns <tt>BAR</tt> as an external symbol in the
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<tt>keyword</tt> package and makes it evaluate to itself. Printed when
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the home package of symbol <tt>bar</tt> is <tt>keyword</tt>.
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<P>
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<DT><tt>#:bar</tt>
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<DD>
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When read, creates a new uninterned symbol named <tt>BAR</tt>.
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Printed when the symbol <tt>bar</tt> is uninterned (has no home package),
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even in the pathological case that <tt>bar</tt> is uninterned but
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nevertheless somehow accessible in the current package.
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<P>
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</DL>
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<P>
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All other uses of colons within names of symbols are not defined by
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Common Lisp but are reserved for implementation-dependent use; this
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includes names that end in a colon, contain two or more colons, or
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consist of just a colon.
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<P>
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<BR> <HR><A NAME=tex2html2947 HREF="node115.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html2945 HREF="node111.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html2939 HREF="node113.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html2949 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html2950 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html2948 HREF="node115.html"> Exporting and Importing </A>
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<B>Up:</B> <A NAME=tex2html2946 HREF="node111.html"> Packages</A>
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<B> Previous:</B> <A NAME=tex2html2940 HREF="node113.html"> Package Names</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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