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.\" $OpenBSD: ssa,v 1.1 2002/12/03 21:36:35 mickey Exp $
.\"
.\" Copyright (C) Caldera International Inc. 2001-2002.
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code and documentation must retain the above
.\" copyright notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed or owned by Caldera
.\" International, Inc.
.\" 4. Neither the name of Caldera International, Inc. nor the names of other
.\" contributors may be used to endorse or promote products derived from
.\" this software without specific prior written permission.
.\"
.\" USE OF THE SOFTWARE PROVIDED FOR UNDER THIS LICENSE BY CALDERA
.\" INTERNATIONAL, INC. AND CONTRIBUTORS ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL CALDERA INTERNATIONAL, INC. BE LIABLE FOR ANY DIRECT,
.\" INDIRECT INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES
.\" (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
.\" SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
.\" STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING
.\" IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
.\" POSSIBILITY OF SUCH DAMAGE.
.\"
.\" @(#)ssa 8.1 (Berkeley) 6/8/93
.\"
.SH
Appendix A: A Simple Example
.PP
This example gives the complete Yacc specification for a small desk calculator;
the desk calculator has 26 registers, labeled ``a'' through ``z'', and accepts
arithmetic expressions made up of the operators +, \-, *, /,
% (mod operator), & (bitwise and), | (bitwise or), and assignment.
If an expression at the top level is an assignment, the value is not
printed; otherwise it is.
As in C, an integer that begins with 0 (zero) is assumed to be octal;
otherwise, it is assumed to be decimal.
.PP
As an example of a Yacc specification, the desk calculator
does a reasonable job of showing how precedences and ambiguities
are used, and demonstrating simple error recovery.
The major oversimplifications are that the
lexical analysis phase is much simpler than for most applications, and the
output is produced immediately, line by line.
Note the way that decimal and octal integers are read in by the grammar rules;
This job is probably better done by the lexical analyzer.
.sp
.nf
.ta .5i 1i 1.5i 2i 2.5i
%{
# include <stdio.h>
# include <ctype.h>
int regs[26];
int base;
%}
%start list
%token DIGIT LETTER
%left \'|\'
%left \'&\'
%left \'+\' \'\-\'
%left \'*\' \'/\' \'%\'
%left UMINUS /* supplies precedence for unary minus */
%% /* beginning of rules section */
list : /* empty */
| list stat \'\en\'
| list error \'\en\'
{ yyerrok; }
;
stat : expr
{ printf( "%d\en", $1 ); }
| LETTER \'=\' expr
{ regs[$1] = $3; }
;
expr : \'(\' expr \')\'
{ $$ = $2; }
| expr \'+\' expr
{ $$ = $1 + $3; }
| expr \'\-\' expr
{ $$ = $1 \- $3; }
| expr \'*\' expr
{ $$ = $1 * $3; }
| expr \'/\' expr
{ $$ = $1 / $3; }
| expr \'%\' expr
{ $$ = $1 % $3; }
| expr \'&\' expr
{ $$ = $1 & $3; }
| expr \'|\' expr
{ $$ = $1 | $3; }
| \'\-\' expr %prec UMINUS
{ $$ = \- $2; }
| LETTER
{ $$ = regs[$1]; }
| number
;
number : DIGIT
{ $$ = $1; base = ($1==0) ? 8 : 10; }
| number DIGIT
{ $$ = base * $1 + $2; }
;
%% /* start of programs */
yylex() { /* lexical analysis routine */
/* returns LETTER for a lower case letter, yylval = 0 through 25 */
/* return DIGIT for a digit, yylval = 0 through 9 */
/* all other characters are returned immediately */
int c;
while( (c=getchar()) == \' \' ) { /* skip blanks */ }
/* c is now nonblank */
if( islower( c ) ) {
yylval = c \- \'a\';
return ( LETTER );
}
if( isdigit( c ) ) {
yylval = c \- \'0\';
return( DIGIT );
}
return( c );
}
.fi
.bp
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