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<!DOCTYPE HTML PUBLIC "-//W3O//DTD W3 HTML 2.0//EN">
<!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>27.5. Compiling Format Control Strings</TITLE>
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<b>Common Lisp the Language, 2nd Edition</b>
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<H1><A NAME=SECTION003150000000000000000>27.5. Compiling Format Control Strings</A></H1>
<P>
<img align=bottom alt="change_begin" src="gif/change_begin.gif"><br>
The control strings used by <tt>format</tt> are essentially programs that
perform printing. The macro <tt>formatter</tt> provides the efficiency of
using a compiled function for printing without losing the visual compactness of
<tt>format</tt> strings.
<P>
<BR><b>[Macro]</b><BR>
<tt>formatter <i>control-string</i></tt><P>The <i>control-string</i> must be a literal string. An error is signaled if
<i>control-string</i> is not a valid <tt>format</tt> control string. The macro
<tt>formatter</tt> expands into an expression of the form
<tt>(function (lambda (stream &rest args) ...))</tt> that does the printing
specified by <i>control-string</i>. The <tt>lambda</tt> created accepts an
output stream as its first argument and zero or more data values as its
remaining arguments. The value returned by the <tt>lambda</tt> is the tail (if
any) of the data values that are not printed out by <i>control-string</i>.
(For example, if the <i>control-string</i> is <tt>"~A~A"</tt>, the
<tt>cddr</tt> (if any) of the data values is returned.) The form
<tt>(formatter "~%~2@{~S, ~}")</tt> is
equivalent to the following:
<P><pre>
#'(lambda (stream &rest args)
(terpri stream)
(dotimes (n 2)
(if (null args) (return nil))
(prin1 (pop args) stream)
(write-string ", " stream))
args)
</pre><P>
<P>
In support of the above mechanism, <tt>format</tt> is extended so that it accepts
functions as its second argument as well as strings. When a function is
provided, it must be a function of the form created by <tt>formatter</tt>. The
function is called with the appropriate output stream as its first argument
and the data arguments to <tt>format</tt> as its remaining arguments. The
function should perform whatever output is necessary and return the unused
tail of the arguments (if any). The directives <tt></tt> and
<tt>~{~}</tt> with no body are also extended so that they accept
functions as well as control strings. Every other standard function that
takes a <tt>format</tt> string as an argument (for example, <tt>error</tt> and <tt>warn</tt>)
is also extended so that it can accept functions of the form above
instead.
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