Look-Ahead Tokens ================= The Bison parser does _not_ always reduce immediately as soon as the last N tokens and groupings match a rule. This is because such a simple strategy is inadequate to handle most languages. Instead, when a reduction is possible, the parser sometimes "looks ahead" at the next token in order to decide what to do. When a token is read, it is not immediately shifted; first it becomes the "look-ahead token", which is not on the stack. Now the parser can perform one or more reductions of tokens and groupings on the stack, while the look-ahead token remains off to the side. When no more reductions should take place, the look-ahead token is shifted onto the stack. This does not mean that all possible reductions have been done; depending on the token type of the look-ahead token, some rules may choose to delay their application. Here is a simple case where look-ahead is needed. These three rules define expressions which contain binary addition operators and postfix unary factorial operators (`!'), and allow parentheses for grouping. expr: term '+' expr | term ; term: '(' expr ')' | term '!' | NUMBER ; Suppose that the tokens `1 + 2' have been read and shifted; what should be done? If the following token is `)', then the first three tokens must be reduced to form an `expr'. This is the only valid course, because shifting the `)' would produce a sequence of symbols `term ')'', and no rule allows this. If the following token is `!', then it must be shifted immediately so that `2 !' can be reduced to make a `term'. If instead the parser were to reduce before shifting, `1 + 2' would become an `expr'. It would then be impossible to shift the `!' because doing so would produce on the stack the sequence of symbols `expr '!''. No rule allows that sequence. The current look-ahead token is stored in the variable `yychar'. Note: Special Features for Use in Actions.
Dirfile and infopages generated Sat Dec 3 02:07:54 2005