3.5.3 Default Action for Locations

Actually, actions are not the best place to compute locations. Since locations are much more general than semantic values, there is room in the output parser to redefine the default action to take for each rule. The YYLLOC_DEFAULT macro is invoked each time a rule is matched, before the associated action is run. It is also invoked while processing a syntax error, to compute the error’s location. Before reporting an unresolvable syntactic ambiguity, a GLR parser invokes YYLLOC_DEFAULT recursively to compute the location of that ambiguity.

Most of the time, this macro is general enough to suppress location dedicated code from semantic actions.

The YYLLOC_DEFAULT macro takes three parameters. The first one is the location of the grouping (the result of the computation). When a rule is matched, the second parameter identifies locations of all right hand side elements of the rule being matched, and the third parameter is the size of the rule’s right hand side. When a GLR parser reports an ambiguity, which of multiple candidate right hand sides it passes to YYLLOC_DEFAULT is undefined. When processing a syntax error, the second parameter identifies locations of the symbols that were discarded during error processing, and the third parameter is the number of discarded symbols.

By default, YYLLOC_DEFAULT is defined this way:

# define YYLLOC_DEFAULT(Cur, Rhs, N)                      \
do                                                        \
  if (N)                                                  \
    {                                                     \
      (Cur).first_line   = YYRHSLOC(Rhs, 1).first_line;   \
      (Cur).first_column = YYRHSLOC(Rhs, 1).first_column; \
      (Cur).last_line    = YYRHSLOC(Rhs, N).last_line;    \
      (Cur).last_column  = YYRHSLOC(Rhs, N).last_column;  \
    }                                                     \
  else                                                    \
    {                                                     \
      (Cur).first_line   = (Cur).last_line   =            \
        YYRHSLOC(Rhs, 0).last_line;                       \
      (Cur).first_column = (Cur).last_column =            \
        YYRHSLOC(Rhs, 0).last_column;                     \
    }                                                     \
while (0)

where YYRHSLOC (rhs, k) is the location of the kth symbol in rhs when k is positive, and the location of the symbol just before the reduction when k and n are both zero.

When defining YYLLOC_DEFAULT, you should consider that:

• All arguments are free of side-effects. However, only the first one (the result) should be modified by YYLLOC_DEFAULT.
• For consistency with semantic actions, valid indexes within the right hand side range from 1 to n. When n is zero, only 0 is a valid index, and it refers to the symbol just before the reduction. During error processing n is always positive.
• Your macro should parenthesize its arguments, if need be, since the actual arguments may not be surrounded by parentheses. Also, your macro should expand to something that can be used as a single statement when it is followed by a semicolon.