jenglish at crl.com
Fri Apr 11 21:33:03 BST 1997
Peter at ursus.demon.co.uk (Peter Murray-Rust) wrote:
> For discussion assume that the declarations belong to MYDTD
> David Megginson writes:
> > <?ArcBase html?> <?ArcBase math?>
> So in C++ terms MYDTD has two base classes, HTML and MATH.
It may be easier to understand architectural forms if you
_don't_ think of it in C++ terms... The mechanism is more like,
in Java, declaring that a class (DTD) implements an interface
(architecture) than inheriting an implementation from a base
Even this is not really accurate, since in SGML elements don't
_do_ anything, they just _are_; it's up to the application to
interpret them. The architectural mapping simply tells applications
that know how to process the architectural forms ("meta"-DTD)
how to interpret instances conforming to a document type ("real" DTD)
based on that architecture.
> > <!ELEMENT para ...>
> > <!ATTLIST para
> > html NAME #FIXED "p">
> This means that PARA (in MYDTD) inherits properties from P (in HTML). Does
> that mean it automatically inherits the content model and the attributes?
I'm not sure I understand the question. If you're asking if
it's now possible to write:
<PARA ALIGN=CENTER> ... </PARA>
(where the ALIGN attribute is defined in the HTML meta-DTD
but not in MYDTD), the answer is no. However, when it's
interpreted by an HTML-architecture-aware processor, any
"P" attributes with a default or #FIXED value in the meta-DTD
will be (considered to be) present.
> For example, the content model of P is
> which expands to
> (#PCDATA | IMG | BR | EM)* <!-- several missed out -->
> Can PARA assume these or does it have to map each of these (e.g. IMG) onto
> <!ELEMENT image ...>
> <!ATTLIST image
> html NAME #FIXED "img>
A "PARA" element can contain whatever is allowed by its content
model in MYDTD. The only restriction is that when the children
of a PARA element are mapped to HTML, the result must conform
to "P"s content model in the meta-DTD.
> similarly does PARA have to explicity declare the same attributes as P in HTM
Only if it wants to use those attributes in the source document.
> > <!ELEMENT sym ...>
> > <!ATTLIST symbol
> > html NAME #FIXED "sym"
> > math NAME #FIXED "symbol">
> This is - presumably - multiple inheritance in that SYMBOL inherits something
> from HTML.SYM and MATH.SYMBOL (though I'm not clear what). How does the
> content of SYMBOL relate to either of its two base classes?
When an HTML-architecture processor encounters a SYMBOL element,
it treats it like an HTML "SYM" element (which does
not exist AFAIK, so this would probably be an error).
When a MATH-architecture processor encounters a SYMBOL element
(in a document conforming to MYDTD) it treats it like a
SYMBOL element (as defined in the MATH meta-DTD).
jenglish at crl.com
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