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   Re: lex, yacc, and xml

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  • From: Richard Tobin <richard@cogsci.ed.ac.uk>
  • To: ht@cogsci.ed.ac.uk (Henry S. Thompson), xml-dev@ic.ac.uk
  • Date: Sat, 27 Dec 1997 18:11:12 GMT

> The currently available LT XML release (http://www.ltg.ed.ac.uk/software/xml/) 
> uses a lex/yacc parser.

The ugliest part of this code is the DTD parsing, because you want
(say) SYSTEM returned in some places as a keyword, and in others as a
name.  To achieve this, the yacc layer has to be constantly setting
the lexer mode ("lexical tie-ins").  Contrast this with C (surprise!)
where you can't have a variable with the same name as a keyword.

As Henry said, the performance is one reason why we switched to a plain
C parser.  Another is the question of 16-bit characters, though this
could probably have been kludged since all the syntactically important
characters are < 128.

-- Richard

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