[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: The remarkable similarities between XSLT and Flex/Lex
Hi Folks, The below message was sent to me privately. I was given permission to share it with the list. Regarding XSLT's similarity to Lex: It's not really about a single tool like Lex. Before XML there was SGML, which XML was supposed to "simplify". SGML included a schema language (DTD), which defines the hierarchical structure of a document using regular expressions over elements. There was also a strange unnecessary constraint on these expressions called "ambiguity", which *everybody* who wrote SGML software needed to understand, and so the idea of applying formal language techniques to SGML was inevitable. Long before XSLT, there were a variety of attempts to define languages that would allow users to specify an automatic translation from SGML into printed form. Many of these languages were context-free grammars at their core, with translation rules as actions. This is called "syntax-directed translation" and was a well-known concept long before that. With SGML, though, the problem of syntax-directed translation is different than it is in other contexts, and more difficult in many ways, because the basic structures in the input are very easy to parse -- elements are delimited after all -- but the input was a semantically marked up text and the output was a published document that had to follow all the ambiguously-defined stylistic rules that people use when they actually do typography. This meant that complicated grammars, over *element trees* instead of linear text, and lots of other ideas, needed to be applied. Lots of companies put a lot of work into it. So by the time XSLT came around, everyone on the committee as already familiar with a lot of this history from SGML processing, which was based on a lot of work rooted in the same formal language theory that goes into lexers and parsers, and that is why some of XSLT looks a lot like Lex. Unfortunately, XSLT kind of [expletive deleted]. When the standard was written, the problem itself had not really been solved by industry in a really acceptable way (and it still hasn't been!), and the W3C committee fell into the trap of trying to innovate instead of codifying best practice.
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