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From: enag@ifi.uio.no (Erik Naggum)
Newsgroups: comp.text.sgml
Subject: Re: A floating paired tag...
Message-ID: <ENAG.91Jun9020802@gyda.ifi.uio.no>
Date: 9 Jun 91 00:08:06 GMT
References: <932@bcstec.boeing.com>
Sender: enag@ifi.uio.no (Erik Naggum)
Organization: Naggum Software, Oslo, Norway
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In-Reply-To: ruben@bcstec.boeing.com's message of 7 Jun 91 21: 07:15 GMT
Originator: enag@gyda.ifi.uio.no

Reuben Wachtfogel <ruben@bcstec.boeing.com> writes:
|
|   This may be very basic question but I really did
|   try to find the answer independently and I'm stumped.

Thanks for this courtesy to other news readers.  

|   I wish to define a bracketing structure that could be inserted
|   around any of:
|   1) blah
|   2) <b1><\b1>
|   3) <a1><\a1>
|   4) <a><\a>

If I may make some assumptions about what your problem is, here's my
thoughts.

<assumption>
The bracketing, floating element will be used to encode some
information _about_ the data content found inside the elements it
embraces.
<solution>
This suggests itself to attributes, and since you wish to have the
attribute value remain across sub-elements, you can either (1) let the
value be #IMPLIED by the application, so that the application will
keep track of the nested elements, or (2) let the value be #CURRENT.
<discussion>
The disadvantage with (1) is that you need a special mechanism in the
application software to handle this (and other attributes).  The
disadvantage with (2) is that #CURRENT attribute values do not respect
hierarchies, but are "current" for a given attribute when the element
to which it was specified ends.

I'm unclear on this point, myself, and would appreciate comments.
Given the document type declaration fragment
    <!ELEMENT outer (#PCDATA|inner)*>
    <!ELEMENT inner (#PCDATA)>
    <!ATTLIST (outer,inner) language CDATA #CURRENT>
and the document instance fragment
    <outer language=chinese>
	ha che
	<inner language=english>
	    good food
	</inner>
	kuluyuk
    </outer>
what is the value of the language attribute when "kuluyuk" is seen?

It seems clear that if the fragment ends with
	</inner>
	<inner>
	    sweet-sour pork
	</inner>
    </outer>
the second inner will have the language attribute value "english".

<assumption>
The information you need to encode is somehow external to the document
in which it applies, and should be regarded as a different "view" of
the document.
<solution>
For this purpose, you could define a concurrent document type which
deals with this information, and not the other structural information.
Presented this way, the elements of one document type does not interfere
with the other.
<discussion>
One clear disadvantage with this approach is that you would not be
able to relate the two document types very easily.  For instance, the
first document type's elements need not embrace complete elements of
the second.  Depending on your needs, this may be what you want.

<assumption>
There are external requirements that force you to use an element for
this purpose, and you very badly need to have defined regions within
which the attributes are declared, so badly that you could dispense
with the rigidity of the markup.
<solution>
You could declare the element with content model ANY, and enforce the
rigidity in the application, instead:
    <!ELEMENT language ANY>
    <!ATTLIST language language CDATA #IMPLIED>
If you do this, you can open a new element inside a <language> element
and proceed inside that element according to its content model, and
then close the <language> element.
<discussion>
I don't recommend this.  An alternate solution would be to list all
the valid sub-elements that language could possible embrace, e.g.
    <!ELEMENT language (a|b1|b2|b3)>
but this quickly becomes difficult to maintain and understand,
although it's better from a purist's view.  Note: this works only if
you allow <language> to embrace only _one_ element at a time.  If you
need to embrace several elements things quickly get very difficult.

I need more information on your problem to be able to evaluate each of
these or perhaps think of other solutions.  I can't readily see an
elegant way to do this without floating attributes (such as #CURRENT)
or a concurrent document, but there are many ways to specify attribute
values, including link attributes.

What we need (which may exist, but I may just have overlooked it), is
a means to supply attribute values for the extent of an element and
all sub-elements thereof.  #CURRENT, in my reading, doesn't support
this.  I'd be happy to be proven wrong on this one.  I'd especially
like to know what's the right interpretation of a #CURRENT attribute
which is changed inside an element.

I'm sorry that I can't test this because the parser I've written is
based on my own fuzzy assumptions on the treatment of CURRENT values.
Can anybody help, either with theory or with practice?

</Erik>
--
Erik Naggum             Professional Programmer            +47-2-836-863
Naggum Software             Electronic Text             <ERIK@NAGGUM.NO>
0118 OSLO, NORWAY       Computer Communications        <enag@ifi.uio.no>