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73 lines
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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>27.5. Compiling Format Control Strings</TITLE>
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</HEAD>
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<BODY>
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<meta name="description" value=" Compiling Format Control Strings">
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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=tex2html4788 HREF="node259.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4786 HREF="node253.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4780 HREF="node257.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4790 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4791 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4789 HREF="node259.html"> Pretty Printing Dispatch </A>
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<B>Up:</B> <A NAME=tex2html4787 HREF="node253.html"> Pretty Printing</A>
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<B> Previous:</B> <A NAME=tex2html4781 HREF="node257.html"> Format Directive Interface</A>
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<HR> <P>
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<H1><A NAME=SECTION003150000000000000000>27.5. Compiling Format Control Strings</A></H1>
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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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The control strings used by <tt>format</tt> are essentially programs that
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perform printing. The macro <tt>formatter</tt> provides the efficiency of
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using a compiled function for printing without losing the visual compactness of
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<tt>format</tt> strings.
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<P>
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<BR><b>[Macro]</b><BR>
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<tt>formatter <i>control-string</i></tt><P>The <i>control-string</i> must be a literal string. An error is signaled if
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<i>control-string</i> is not a valid <tt>format</tt> control string. The macro
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<tt>formatter</tt> expands into an expression of the form
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<tt>(function (lambda (stream &rest args) ...))</tt> that does the printing
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specified by <i>control-string</i>. The <tt>lambda</tt> created accepts an
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output stream as its first argument and zero or more data values as its
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remaining arguments. The value returned by the <tt>lambda</tt> is the tail (if
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any) of the data values that are not printed out by <i>control-string</i>.
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(For example, if the <i>control-string</i> is <tt>"~A~A"</tt>, the
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<tt>cddr</tt> (if any) of the data values is returned.) The form
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<tt>(formatter "~%~2@{~S, ~}")</tt> is
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equivalent to the following:
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<P><pre>
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#'(lambda (stream &rest args)
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(terpri stream)
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(dotimes (n 2)
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(if (null args) (return nil))
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(prin1 (pop args) stream)
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(write-string ", " stream))
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args)
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</pre><P>
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<P>
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In support of the above mechanism, <tt>format</tt> is extended so that it accepts
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functions as its second argument as well as strings. When a function is
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provided, it must be a function of the form created by <tt>formatter</tt>. The
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function is called with the appropriate output stream as its first argument
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and the data arguments to <tt>format</tt> as its remaining arguments. The
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function should perform whatever output is necessary and return the unused
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tail of the arguments (if any). The directives <tt></tt> and
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<tt>~{~}</tt> with no body are also extended so that they accept
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functions as well as control strings. Every other standard function that
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takes a <tt>format</tt> string as an argument (for example, <tt>error</tt> and <tt>warn</tt>)
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is also extended so that it can accept functions of the form above
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instead.
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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=tex2html4788 HREF="node259.html"><IMG ALIGN=BOTTOM ALT="next" SRC="icons/next_motif.gif"></A> <A NAME=tex2html4786 HREF="node253.html"><IMG ALIGN=BOTTOM ALT="up" SRC="icons/up_motif.gif"></A> <A NAME=tex2html4780 HREF="node257.html"><IMG ALIGN=BOTTOM ALT="previous" SRC="icons/previous_motif.gif"></A> <A NAME=tex2html4790 HREF="node1.html"><IMG ALIGN=BOTTOM ALT="contents" SRC="icons/contents_motif.gif"></A> <A NAME=tex2html4791 HREF="index.html"><IMG ALIGN=BOTTOM ALT="index" SRC="icons/index_motif.gif"></A> <BR>
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<B> Next:</B> <A NAME=tex2html4789 HREF="node259.html"> Pretty Printing Dispatch </A>
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<B>Up:</B> <A NAME=tex2html4787 HREF="node253.html"> Pretty Printing</A>
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<B> Previous:</B> <A NAME=tex2html4781 HREF="node257.html"> Format Directive Interface</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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