Accessing Values in Enclosing Rules

An action can refer to values returned by actions to the left of the current rule. The mechanism is the same as ordinary actions, $ followed by a digit.

sent	   	: adj noun verb adj noun 
         	{ 
            		look at the sentence ...  
          } 
          ; 
adj	      : THE 
         	{ 
           			$$ = THE; 
         	} 
          | YOUNG 
          { 
           			$$ = YOUNG; 
          } 
            	...  
            		; 
noun	    	: DOG 
        		{ 
           			$$ = DOG; 
         	} 
         	| CRONE 
          { 
           			if ( $0 = = YOUNG ) 
           			{ 
              				(void) printf( "what?\n" ); 
             	} 
           			$$ = CRONE; 
          } 
        		; 
...

In this case, the digit can be 0 or negative. In the action following the word CRONE, a check is made that the preceding token shifted was not YOUNG. Notice, however, that this is only possible when a great deal is known about what might precede the symbol noun in the input. Nevertheless, at times this mechanism prevents a great deal of trouble, especially when a few combinations are to be excluded from an otherwise regular structure.