11.5. Using Only Synthesized Attributes

S-attributed equations

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A set of attribute equations is S-attributed if all of its attributes are synthesized.

S-attributed equations are the easiest to use with a bottom-up parser, since they can be handled on a single bottom-to-top pass.

Abstract syntax trees can help you avoid inherited attributes

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Normally, you only need synthesized attributes to create abstract syntax trees. Then, once you have the tree, you can traverse it up and down as many times as you have a mind to.

Synthesized attributes of embedded actions in Bison

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Recall that Bison allows you to intersperse an action among the tokens and nonterminals on the right-hand side of a production. Those embedded actions can have synthesized attributes.

If the synthesized attribute uses field intval of YYSTYPE, define $<intval>$. To use that in other actions for the same production, use $<intval>k, where k is the position of the action.

For example, suppose that you want an action that sets a context and remembers the current line number.

expr : TOK_FOO
         {insideFoo = 1;
          $<intval>$ = line_number;
         }
       expr  ','  expr  TOK_ENDFOO
         {
           $$ = handleFoo($3, $5, $<intval>2);
         }
     ;