(bison.info)Ltcalc Rules


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Grammar Rules for `ltcalc'
--------------------------

   Whether handling locations or not has no effect on the syntax of your
language.  Therefore, grammar rules for this example will be very close
to those of the previous example: we will only modify them to benefit
from the new information.

   Here, we will use locations to report divisions by zero, and locate
the wrong expressions or subexpressions.

     input   : /* empty */
             | input line
     ;
     
     line    : '\n'
             | exp '\n' { printf ("%d\n", $1); }
     ;
     
     exp     : NUM           { $$ = $1; }
             | exp '+' exp   { $$ = $1 + $3; }
             | exp '-' exp   { $$ = $1 - $3; }
             | exp '*' exp   { $$ = $1 * $3; }
             | exp '/' exp
                 {
                   if ($3)
                     $$ = $1 / $3;
                   else
                     {
                       $$ = 1;
                       fprintf (stderr, "%d.%d-%d.%d: division by zero",
                                @3.first_line, @3.first_column,
                                @3.last_line, @3.last_column);
                     }
                 }
             | '-' exp %preg NEG     { $$ = -$2; }
             | exp '^' exp           { $$ = pow ($1, $3); }
             | '(' exp ')'           { $$ = $2; }

   This code shows how to reach locations inside of semantic actions, by
using the pseudo-variables `@N' for rule components, and the
pseudo-variable `@$' for groupings.

   We don't need to assign a value to `@$': the output parser does it
automatically.  By default, before executing the C code of each action,
`@$' is set to range from the beginning of `@1' to the end of `@N', for
a rule with N components.  This behavior can be redefined (Note:
Default Action for Locations.), and for very
specific rules, `@$' can be computed by hand.


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Dirfile and infopages generated Sat Dec 3 02:07:54 2005