Parsing with yacc

Support for arbitrary value types

By default, the values returned by actions and the lexical analyzer are integers. yacc can also support values of other types including structures. In addition, yacc keeps track of the types and inserts appropriate union member names so that the resulting parser is strictly type checked. The yacc value stack is declared to be a union of the various types of values desired. You declare the union and associate union member names with each token and nonterminal symbol having a value. When the value is referenced through a $$ or $n construction, yacc will automatically insert the appropriate union name so that no unwanted conversions take place.

There are three mechanisms used to provide for this typing. First, there is a way of defining the union. This must be done by the user since other subroutines, notably the lexical analyzer, must know about the union member names. Second, there is a way of associating a union member name with tokens and nonterminals. Finally, there is a mechanism for describing the type of those few values where yacc cannot easily determine the type.

To declare the union, you include

   %union
   {
      body of union
   }

in the declaration section. This declares the yacc value stack and the external variables yylval and yyval to have type equal to this union. If yacc was invoked with the -d option, the union declaration is copied into the y.tab.h file as YYSTYPE.

Once YYSTYPE is defined, the union member names must be associated with the various terminal and nonterminal names. The construction

   <name>

is used to indicate a union member name. If this follows one of the keywords %token, %left, %right, and %nonassoc, the union member name is associated with the tokens listed. Thus, saying

   %left  <optype>  '+'  '-'

causes any reference to values returned by these two tokens to be tagged with the union member name optype. Another keyword, %type, is used to associate union member names with nonterminals. Thus, one might say

   %type  <nodetype>  expr  stat

to associate the union member nodetype with the nonterminal symbols expr and stat.

There remain a couple of cases where these mechanisms are insufficient. If there is an action within a rule, the value returned by this action has no a priori type. Similarly, reference to left context values (such as $0) leaves yacc with no easy way of knowing the type. In this case, a type can be imposed on the reference by inserting a union member name between < and > immediately after the first $. The example below

   rule   :   aaa
                    {
                       $<intval>$ = 3;
                    }
                    bbb
          {
              fun( $<intval>2, $<other>0 );
          }
          ;

shows this usage. This syntax has little to recommend it, but the situation arises rarely.

A sample specification is given in ``An advanced example''. The facilities in this subsection are not triggered until they are used. In particular, the use of %type will turn on these mechanisms. When they are used, there is a fairly strict level of checking. For example, use of $n or $$ to refer to something with no defined type is diagnosed. If these facilities are not triggered, the yacc value stack is used to hold ints.