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<H2><A NAME="SECTION00084000000000000000"> </A>
<A NAME="yl-io"> </A>
<BR>
Interfacing with Yacc and Lex
</H2>
The first piece of advice is: try to avoid it, because the SG (SSL) structure
file format is much more convenient.
Notwithstanding that, it is fairly simple, and the term processor handles
a lot of the machinery involved, such as defining a data type for the
<I>yacc</I> stack.
The two important considerations in using <I>yacc</I> are the difference between abstract
syntax and concrete syntax, and handling terminal symbols with values,
such as identifiers.
Abstract syntax should be the input of the term processor and concrete syntax
goes into <I>yacc</I>.
First a simple abstract syntax.
<PRE><HR><!--lgrindfile: yacc_abstract_syntax_example.k 14:54 Jul 22 1996-->
<I>/* Abstract syntax */</I>
funnytree: Str(<B>casestring</B>)
| Cons(funnytree funnytree)
;
<HR></PRE>
<P>
In the <I>yacc</I> input file, concrete syntax is translated into abstract syntax using the
<EM>operator</EM> functions.
Note also the type annotation of concrete symbols, as in <I>%token </I><<I>yt_casestring</I>><I> ID</I>.
This type is a union selector of the generated <I>YYSTYPE</I>.
<A NAME="1043"> </A><A NAME="1044"> </A>
<PRE><HR><!--lgrindfile: yacc_input_example.y 12:44 Oct 26 1992-->
<I>/* Concrete syntax */</I>
%{
<B>#include</B> <code>"k.h"</code>
<BR>
funnytree thetree;
%}
<BR>
%token <yt_casestring> ID
<BR>
%type <yt_funnytree> tree
%%
<BR>
theroot:tree { thetree = $1;};
<BR>
tree: ID { $$ = Str($1);}
| <code>'('</code> tree tree <code>')'</code> { $$ = Cons($2, $3);}
;
<HR></PRE>
<P>
The typical way of using <I>lex</I><A NAME="1440"> </A> is to have the lexical analyser generate tokens
for the parser and build values of the phylum <I>casestring</I>.
The following is the complete <I>lex</I> input.
<A NAME="1050"> </A><A NAME="1051"> </A>
<PRE><HR><!--lgrindfile: lex_input_example.l 15:00 Sep 28 1992-->
<I>/* Lexemes */</I>
%{
<B>#include</B> <code>"k.h"</code>
<B>#include</B> <code>"y.tab.h"</code>
%}
%%
[a-zA-Z0-9]+ { yylval.yt_casestring = mkcasestring(yytext); <B>return</B> ID;}
[\t\n ] { ; } <I>/* skip the white space */</I>
. { <B>return</B> yytext[0]; }
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