Handling keywords that are not reserved?

Thu, 17 Jan 91 18:53:17 -0500

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Handling keywords that are not reserved? amb@Apple.COM (1991-01-15)
Handling keywords that are not reserved? meissner@osf.org (1991-01-17)
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Newsgroups: comp.compilers
From: meissner@osf.org
In-Reply-To: amb@Apple.COM's message of 15 Jan 91 23:38:09 GMT
Keywords: parse, yacc, question
Organization: Compilers Central
References: <48132@apple.Apple.COM>
Date: Thu, 17 Jan 91 18:53:17 -0500

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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