6.5.3 S-Attributed Attribute Grammars

Attribute in an S-attributed grammar can be evaluated at parse time by a bottom-up parser. Interestingly, these grammars form a subset of the L-attributed grammars.

      An attribute grammar is S-attributed if and only if:

• It is L-attributed.
• Nonterminals have only synthesized attributes.

           This allows attributes to be evaluated during LR-parsing.

      L-attributed and S-attributed grammars allow efficient evaluation---either during parsing or as a single pass after parsing.

      In Section 6.6, we see another method and a different attribute restriction that allows attribute evaluation during top-down parsing.

     For the next few sections, we focus on restrictions to attributes that allow for efficient attribute evaluation.