A compiler must do more than recognize whether a sentence belongs to the language of a grammar – it must do something useful with that sentence. The semantic actions of a parser can do useful things with the phrases that are parsed.

In a recursive-descent parser, semantic action code is interspersed with the control flow of the parsing actions. In a parser specified in Yacc, semantic actions are fragments of C program code attached to grammar productions.

SEMANTIC ACTIONS

Each terminal and nonterminal may be associated with its own type of semantic value. For example, in a simple calculator using Grammar 3.35, the type associated with exp and INT might be int; the other tokens would not need to carry a value. The type associated with a token must, of course, match the type that the lexer returns with that token.

For a rule A → B C D, the semantic action must return a value whose type is the one associated with the nonterminal A. But it can build this value from the values associated with the matched terminals and nonterminals B, C, D.

RECURSIVE DESCENT

In a recursive-descent parser, the semantic actions are the values returned by parsing functions, or the side effects of those functions, or both.