ietf-xml-mime
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Re: DTD media type?

1999-04-13 03:34:04
Martin J. Duerst wrote:

Makoto, I think the things you have mentioned in your mail are quite 
important.
But my guess is that many of the participants in this discussion don't have
the necessary background. Is there somewhere a web page where all these terms
are defined and explained with simple examples, or something similar?

I am obsessed by XML...

Here is an imprecise-but-not-totally-inappropriate analogy.

XML documents are source programs.  External DTD subsets are 
header files that contain declarations and are referenced 
at the beginning of XML documents (note: at most one external 
DTD subset is referenced by one XML document).

External parsed entities are include files that contain source 
program fragments and are referenced in the middle of XML documents.  
External parameter entities are include files that contain declarations 
and are referenced by external DTD subsets. 

       XML document ----------------> external DTD subset 
        |                              |-->  external parameter entity
        |
        |--> external parsed entity 

The BNF for XML documents and that for external DTD subsets are quite 
different.  On the other hand, some external parsed entities are 
legal XML documents, and some external parameter entities are legal 
external DTD subsets.

Viewers for XML documents and viewers for external DTD subsets are 
quite different.

(Please forgive my white lie.  I think that this is enough for our 
 discussion. > XML experts)

If you would like to know the truth, the best document on the WWW is 
Tim's annotated XML.  Its URL is:

        http://www.xml.com/axml/testaxml.htm

DTDs, external DTD subsets, and internal DTD subsets are defined in 
2.8 of XML 1.0 (http://www.xml.com/axml/target.html#dt-doctype)

        [Definition:] The XML document type declaration contains or 
        points to markup declarations that provide a grammar for a class of 
        documents. This grammar is known as a document type definition, or 
        DTD. The document type declaration can point to an external subset (a 
        special kind of external entity) containing markup declarations, or 
        can contain the markup declarations directly in an internal subset, 
        or can do both. The DTD for a document consists of both subsets taken 
        together.


Entitites are defined in Section 4 of XML 1.0. 
(http://www.xml.com/axml/target.html#sec-physical-struct)

Parameter entity

        [Definition:] Parameter entities are parsed entities for use within the 
        DTD. These two types of entities use different forms of reference and 
are 
        recognized in different contexts. Furthermore, they occupy different 
namespaces; a 
        parameter entity and a general entity with the same name are two 
distinct 
        entities. 

Parsed entity

        Entities may be either parsed or unparsed. [Definition:] A parsed 
entity's 
        contents are referred to as its replacement text; this text is 
considered an 
        integral part of the document.


Hope this helps.

Cheers,

Makoto
 
Fuji Xerox Information Systems
 
Tel: +81-44-812-7230   Fax: +81-44-812-7231
E-mail: murata(_at_)apsdc(_dot_)ksp(_dot_)fujixerox(_dot_)co(_dot_)jp

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