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XML-L  May 2001

XML-L May 2001

Subject:

Re: XML DTD syntax

From:

Peter Flynn <[log in to unmask]>

Reply-To:

[log in to unmask]

Date:

Sat, 19 May 2001 21:25:48 +0100

Content-Type:

text/plain

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Parts/Attachments

text/plain (100 lines)

On Thu, 17 May 2001, you wrote:
> For starters, it sounds like you and I are in agreement that mixed content
> is necessary. This is in contrast to other things have read, however. At
> least one widely-read (and otherwise excellent, in my opinion) book on XML
> states that mixed content should be used only as a crutch when converting
> old text data to XML.

They're wrong. That applies only to XML used to represent
database-style data, not to its use for continuous text.

The view you report is a result of some authors forgetting
that XML can be used for text. In the courses I run, many
e-commerce implementors are shocked to find you can use XML
to write text documents, even do your documentation, because
they simply haven't come across the concept before. Equally
many publishing people are baffled to see XML being used to
hold data being sent from one transactional engine to another,
with no DTD and no stylesheet.

> In my specific case, I've found ways to use mixed content when needed, but
> as a result, our DTD is more permissive than I'd like it to be in a number
> of places. For example, we have an element called <filter> that can be used
> to tag content as specific to a given product, market, or media, as follows:
>
>
> <p>This sentence appears in both help and the book.
>     <filter><media help="exclude"/>
>         For more information, see chapter xref...
>     </filter>
> </p>
>
> The filtered content can be a word or sentence within a paragraph (as
> above), or multiple paragraphs or other block of content. Ideally I'd like
> the DTD to allow only one media, product, or market tag, to eliminate the
> possiblity of conflicting tags:

This class of information is called Effectivities. It's very
widely used in industrial DTDs for marking material which is
used only in certain products. DocBook does a lot of this, to
flag passages which apply only to certain circumstances, but
it does it with attributes (as do most other systems I have
seen). Try something like a "media" attribute on all element
types: then you have to add only one extra element for use in
mixed content (which you could call "filter") to cater for those
cases where what you want to catch is some arbitrary string in
mid-paragraph.

That way you only have one attribute name to track. You can make
it a name token group so that your authors just get a drop-down
of valid media to flag, and you can hide the list in an external
entity so you have only one object to maintain, eg

<?xml version="1.0"?>
<!DOCTYPE test [
<!ENTITY % valid_media SYSTEM "media.list">
<!ELEMENT test (foo)+>
<!ATTLIST test medium (%valid_media;) "all">
<!ELEMENT foo (#PCDATA)>
<!ATTLIST foo medium (%valid_media;) "all">
]>
<test>
  <foo medium="email"></foo>
</test>

where media.list says something like

     radio|tv|newspaper|magazine|web|email|news|all|none

However, if you want to be able to select more than one medium,
it gets trickier: name token groups only let you select one
value. To allow more, make the attribute class ENTITIES and use
ENTITY declarations for all valid media.

<?xml version="1.0"?>
<!DOCTYPE test [
<!ELEMENT test (foo)+>
<!ATTLIST test medium ENTITIES #IMPLIED>
<!ELEMENT foo (#PCDATA)>
<!ATTLIST foo medium ENTITIES #IMPLIED>
<!NOTATION medium SYSTEM "medium.txt">
<!ENTITY radio SYSTEM "radio.txt" NDATA medium>
<!ENTITY tv SYSTEM "tv.txt" NDATA medium>
<!ENTITY newspaper SYSTEM "newspaper.txt" NDATA medium>
<!ENTITY magazine SYSTEM "magazine.txt" NDATA medium>
<!ENTITY web SYSTEM "web.txt" NDATA medium>
<!ENTITY email SYSTEM "email.txt" NDATA medium>
<!ENTITY news SYSTEM "news.txt" NDATA medium>
<!ENTITY all SYSTEM "all.txt" NDATA medium>
<!ENTITY none SYSTEM "none.txt" NDATA medium>
]>
<test>
  <foo medium="radio tv news"></foo>
</test>

I don't think any parser ever checks on the existence of
unparsed external entities, but you could use the files
to hold a brief textual description of exactly what classifies
as "a newspaper", for example.

///Peter

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