Is the order of reduction defined in Yacc?

Question

This is more of an "in principle" question than a practical one. Is the order in which Yacc reduces productions, and reads new tokens from the lexer defined. That is, if I had the following set of tokens:

INTEGER_BEGIN
INTEGER_VALUE
LESS_THAN
INTEGER_BEGIN
INTEGER_VALUE

Can Yacc, within its semantics, read the LESS_THAN token from the lexer, before it reduces INTEGER BEGIN INTEGER_VALUE to a single thing, given a set of productions like:

expr : expr LESS_THAN expr
     | integer

integer : INTEGER_BEGIN INTEGER_VALUE

Do the rules for this change if these are defined with semantic actions?

Answer 1

Yes it can. Yacc creates an LALR(1) parser -- the (1) means 1 token of lookahead -- so it may read ahead 1 token past the end of the tokens for a rule before reducing that rule. The existence of semantic actions is irrelevant, as the semantic action is just some C code to run just before reducing a rule.

Note that there's no guarantee that it will ALWAYS read ahead a token. The parser created by yacc or bison sometimes uses 'default reductions' -- states where it can reduce a rule WITHOUT having to read the next token first. This happens whenever the reduction of a rule is independent of the next token.

In this specific example, a default reduction could be used for the integer rule, so it might reduce it without lookahead, but again there's no guarantee -- default reductions are an optimization used by some (but not all) implementations of yacc.