12.3.2 So many keywords, so many problems ...

Keywords can be potential problems since they are (generally) not available for use as identifiers. Only those keywords that are defined in ISO 7185 Pascal are unproblematic because no valid program should ever use them as identifiers.

To cope with this problem, GPC does several things:

• If a dialect option is set, only keywords of the specified dialect are enabled. All possible keywords, together with their dialects, are defined in predef.def. However, compiling with dialect options is usually not recommended, so this is no good general solution.
• The user can turn off individual keywords using the compiler directive {$disable-keyword}. This makes sure that every conflict with a user's identifier can be avoided, but with extra work on the part of the user.
• The parser used to enable and disable keywords in certain syntactic contexts. However, this was rather fragile since it interacts with the parser's read-ahead, and it requires attention on every related change in the parser. Therefore, this mechanism was removed.
• All non-ISO-7185 keywords are now treated as “weak”. This means, they are only recognized as keywords if no current declaration of this name exists. However, so that this can work, it must be possible to create new declarations of this name in the first place – at this point, no declaration exists yet, so the name is recognized as a keyword.

  This is solved by listing these keywords in the new_identifier rule of the parser. This means, first the lexer recognizes them as keywords, then the parser “turns them back” into identifiers. The advantage, compared to explicit enabling and disabling of keywords, is that bison automatically finds the places in which to apply the new_identifier rule, i.e. treat them as plain identifiers.

  Of course, there is a catch. Since the keyword tokens are listed in new_identifier, they can conflict with occurrences of the actual keywords (bison will find such cases as S/R or R/R conflicts). Such conflicts have to be sorted out carefully and either removed or left to GLR handling. Fortunately, for many keywords, removing the conflicts turned out quite easy – in some cases no conflicts arose at all.