Handling keywords that are not reserved?

A. Michael Burbidge writes:


|I am writing a parser for MPW's Object Pascal. Some of the extension keywords
|to standard Pascal are not reserved. I am using yacc and having some trouble
|figuring out a clean way to handle non-reserved keywords.


And John writes:


| [There isn't any clean way to handle non-reserved keywords in yacc. You
| have to feed information from the parser back to the lexer to tell it when
| a token can be a keyword and when it can't. -John]


You can do it in other ways as well. I recall that some people used
the following to parse PL/1 (which has no reserved words at all).
You have a token for each keyword, and an everything else keyword.
Then you have an identifier reduction like:


<identifier>: other
| keyword1
| keyword2
| keyword3 ... ;


You put this at the end of the file, so that yacc's default rules (of
chosing the shift over a reduce rule in case of a conflict in a
shift/reduce conflict) will give you what you want. In order to
eliminate the shift/reduce conflicts, you then need to have
productions like:


<identifier_except_keyword1>: other
| keyword2
| keyword3 ... ;


<identifier_except_keyword2>: other
| keyword1
| keyword3 ... ;


You do have to make sure that in general the rules with the special
keywords come before the more general rules.


--
Michael Meissner email: meissner@osf.org phone: 617-621-8861
Open Software Foundation, 11 Cambridge Center, Cambridge, MA, 02142
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