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<H2><A NAME="SECTION00092000000000000000"> </A>
<A NAME="cook-unp"> </A>
<BR>
Unparsing
</H2>
<P>
Unparsing is in some ways a special case of structural induction.
An example of an unparsing definition has been given in Section <A HREF="node13.html#unp-def">1.7</A>.
In this subsection we show how a printer function can look that implements indentation.
The idea is to define special control sequences to indicate the increment or decrement of
indentation.
We have chosen these control sequences in accordance with the SSL style.
For example, the string
<PRE><HR><!--lgrindfile: unparse_control_sequences.k 16:19 Sep 28 1992-->
<code>"start%t%nlevel1%nlevel1%b%nend"</code>
<HR></PRE>
should be printed as follows.
<PRE><HR><!--lgrindfile: unparse_output.k 16:19 Sep 28 1992-->
start
level1
level1
end
<HR></PRE>
<P>
An example of a printer function to do that is the following:
<A NAME="1088"> </A><A NAME="1089"> </A>
<PRE><HR><!--lgrindfile: printer_function_with_indentation.k 14:04 Feb 19 1997-->
<B>#include</B> <stdio.h>
<B>#include</B> <code>"unpk.h"</code>
<B>static</B> indent=0;
<BR>
<B>void</B> printer(<B>char</B> *s, uview v) {
<B>char</B> c; <B>int</B> j;
<BR>
<B>while</B>(c=*s++) {
<B>if</B> (c!=<code>'%'</code>) putchar(c);
<B>else</B> <B>switch</B>(c=*s++) {
<B>case</B> <code>'b'</code>: indent-; <B>break</B>;
<B>case</B> <code>'t'</code>: indent++; <B>break</B>;
<B>case</B> <code>'n'</code>: putchar(<code>'\n'</code>);
<B>for</B> (j=indent; j>0; j-) putchar(<code>'\t'</code>);
<B>break</B>;
<B>case</B> <code>'\0'</code>: <B>return</B>;
<B>default</B>: putchar(c);
}}}
<HR></PRE>
<P>
The approach above has an advantage that is at the same time a disadvantage.
The <I>printer</I> function also prints <I>casestring</I> and <I>nocasestring</I> terms,
which has the advantage that it is possible to dynamically adapt the indentation, simply by assigning an
unparsing control sequence to a <I>casestring</I> and <I>nocasestring</I> variable before it will be unparsed.
The disadvantage is that one has to be careful that <I>casestring</I> and <I>nocasestring</I> terms do not contain
unexpected unparsing control sequences - it may be necessary to quote the `escape character' (the character % in the example above)
in <I>casestring</I> and <I>nocasestring</I> terms, unless the `escape character' does
not normally appear in the <I>casestring</I> and <I>nocasestring</I> terms.
<P>
The following alternative does not have this problem - but neither has it the advantage of `dynamic' control sequences.
The idea is to use unparse <I>views</I> instead of control sequences - remember that the <I>printer</I> function was passed both
a string and a view argument.
Instead of a single string, a number of strings are unparsed, some of which have <EM>:view</EM> postfix - these strings are
only meant to invoke the printer function with a `control view'.
To reduce the number of printer calls, the `control views' can be combined with the `normal' strings.
This is left as an exercise for the reader.
In the following example the printer function recognizes newline characters ('
<!-- MATH: $\backslash$ -->
<IMG
WIDTH="11" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img42.gif"
ALT="$\backslash$">n'):
<PRE><HR><!--lgrindfile: unparse_control_view_sequences.k 13:12 Oct 26 1992-->
<code>"start"</code> <code>""</code>:v_right <code>"\nlevel1\nlevel1"</code> <code>""</code>:v_left <code>"\nend"</code>
<HR></PRE>
The printer function:
<PRE><HR><!--lgrindfile: printer_function_with_control_view_indentation.k 14:04 Feb 19 1997-->
<B>#include</B> <stdio.h>
<B>#include</B> <code>"unpk.h"</code>
<BR>
<B>void</B> printer(<B>char</B> *s, uview v) {
<B>char</B> c; <B>int</B> j;
<B>static</B> indent=0; <I>/* static here, or static at file level */</I>
<BR>
<B>switch</B>(v) {
<B>case</B> v_left: indent-; <B>break</B>;
<B>case</B> v_right: indent++; <B>break</B>;
<B>default</B>: <B>while</B>(c=*s++) {
<B>switch</B>(c){
<B>case</B> <code>'\n'</code>:putchar(c);
<B>for</B> (j=indent; j>0; j-) putchar(<code>'\t'</code>);
<B>break</B>;
<B>default</B>: putchar(c); <B>break</B>;
}}}}
<HR></PRE>
If instead of newline characters a newline view would be used (eg. <I>"":v_nl</I>) the printer function
could be simplified even more: the <I>case '</I>
<!-- MATH: $\backslash$ -->
<IMG
WIDTH="11" HEIGHT="31" ALIGN="MIDDLE" BORDER="0"
SRC="img42.gif"
ALT="$\backslash$"><I>n':</I> code in the <I>switch(c)</I> can then be
placed as <I>case v_nl:</I> in the <I>switch(v)</I>, and the code for the <I>default:</I> case in that <I>switch(v)</I>
can be replaced by a simple <I>printf("%s",s); </I>.
<P>
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