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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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<TITLE>22.1. Printed Representation of Lisp Objects</TITLE>
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<meta name="description" value=" Printed Representation of Lisp Objects">
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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=tex2html3873 HREF="node188.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3871 HREF="node186.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3865 HREF="node186.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3875 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3876 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3874 HREF="node188.html"> What the Read </A>
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<B>Up:</B> <A NAME=tex2html3872 HREF="node186.html"> Input/Output</A>
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<B> Previous:</B> <A NAME=tex2html3866 HREF="node186.html"> Input/Output</A>
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<HR> <P>
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<H1><A NAME=SECTION002610000000000000000>22.1. Printed Representation of Lisp Objects</A></H1>
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<P>
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Lisp objects in general are not text strings but complex data structures.
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They have very different properties from text strings as a consequence of
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their internal representation. However, to make it possible to get at
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and talk about Lisp objects, Lisp provides a representation of
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most objects in the form of printed text; this is called the <i>printed
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representation</i>, which is used for input/output purposes and in the
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examples throughout this book. Functions such as <tt>print</tt> take a
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Lisp object and send the characters of its printed representation to a
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stream. The collection of routines that does this is known as the
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(Lisp) <i>printer</i>. The <tt>read</tt> function takes characters from a
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stream, interprets them as a printed representation of a Lisp object,
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builds that object, and returns it; the collection of routines
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that does this is called the (Lisp) <i>reader</i>.
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<A NAME=19886> </A>
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<A NAME=19887> </A>
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<A NAME=19888> </A>
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<P>
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Ideally, one could print a Lisp object and then read the printed
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representation back in, and so obtain the same identical object.
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In practice this is difficult and for some purposes not even desirable.
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Instead, reading a printed representation produces an object
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that is (with obscure technical exceptions)
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<tt>equal</tt> to the originally printed object.
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<P>
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Most Lisp objects have more than one possible printed representation.
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For example, the integer twenty-seven can be written in any of these ways:
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<P><pre>
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27 27. #o33 #x1B #b11011 #.(* 3 3 3) 81/3
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</pre><P>
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A list of two symbols <tt>A</tt> and <tt>B</tt> can be printed in many ways:
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<P><pre>
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(A B) (a b) ( a b ) ( A |B|)
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(| A|
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B
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)
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</pre><P>
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The last example, which is spread over three lines, may be ugly, but it
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is legitimate. In general, wherever whitespace is permissible in a printed
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representation, any number of spaces and newlines may appear.
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<P>
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When <tt>print</tt> produces a printed representation, it must choose arbitrarily
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from among many possible printed representations. It attempts to choose
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one that is readable. There are a number of global variables that can
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be used to control the actions of <tt>print</tt>, and a number of different
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printing functions.
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<P>
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This section describes in detail what is the standard printed
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representation for any Lisp object and also describes how <tt>read</tt> operates.
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<P>
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<HR>
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<UL>
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<LI> <A NAME=tex2html3877 HREF="node188.html#SECTION002611000000000000000"> What the Read Function Accepts</A>
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<LI> <A NAME=tex2html3878 HREF="node189.html#SECTION002612000000000000000"> Parsing of Numbers and Symbols</A>
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<LI> <A NAME=tex2html3879 HREF="node190.html#SECTION002613000000000000000"> Macro Characters</A>
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<LI> <A NAME=tex2html3880 HREF="node191.html#SECTION002614000000000000000"> Standard Dispatching Macro Character Syntax</A>
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<LI> <A NAME=tex2html3881 HREF="node192.html#SECTION002615000000000000000"> The Readtable</A>
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<LI> <A NAME=tex2html3882 HREF="node193.html#SECTION002616000000000000000"> What the Print Function Produces</A>
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</UL>
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<BR> <HR><A NAME=tex2html3873 HREF="node188.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html3871 HREF="node186.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html3865 HREF="node186.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html3875 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html3876 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html3874 HREF="node188.html"> What the Read </A>
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<B>Up:</B> <A NAME=tex2html3872 HREF="node186.html"> Input/Output</A>
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<B> Previous:</B> <A NAME=tex2html3866 HREF="node186.html"> Input/Output</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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