Re: Usefulness and Limitations of XML
Dear Chris and Matthew
Perhaps I can put my oar in here as someone who has made significant use of
both (E)BNF and XML.
EBNF is a language used to express the formal grammar of (predominantly)
computer processable languages such as programming languages. As such it is
suited to defining a language such as KIF. (It is also, incidentally, the
language in which XML was defined.) EBNF has nothing to say about the
semantics of the language, it simply defines the sequences of characters
that form grammatically legal sentences in the language.
XML is not designed per se as a language for defining other languages,
although there is nothing that precludes it being used to document such a
definition. Despite being an advocate of XML I would not particularly
recommend it as a substitute for EBNF in defining the grammar of a language
such as KIF. As witness to this EBNF is defined in EBNF, XML is not defined
in XML.
In the context of SUO XML might well have a significant role to play, but
not one that pits it against EBNF. If we assume, for the moment, that KIF
is chosen as the language for making formal (machine-processable?)
statements about the terms in the SUO at the level of FOL (with whatever
extensions are deemed necessary) it would seem likely that such statements
will be augmented by other material (including natural language
explanations, etc.). The point at which XML really comes into its own is
when we wish to start combining statements in KIF with other material and
arrive at a structured document. (Note that there is no particular problem
in embedding statements in KIF in an XML document).
The structure of an XML document can be defined in two ways using either a
DTD (Document Type Definition) which defines the allowed nesting pattern of
XML elements within a document and the attributes that may be used to
describe those elements or a Schema which owes more to the concepts of data
modelling and less to those of publication. A Schema provides more
expressive power for defining datatypes and structural constraints.
Chris Angus
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