Unspecified #IMPLIED attributes in Java (was: Goals: XML ...)

[Peter Murray-Rust]

At 16:32 18/12/97 +0000, Toby Speight wrote:
>Peter> Peter Murray-Rust <URL:mailto:peter at ursus.demon.co.uk>
>
>> In article <3.0.1.16.19971218152122.29672990 at pop3.demon.co.uk>,
>> Peter wrote:
>
>Peter> Is it the question of the return value of a non-existent
>Peter> attribute.  IOW what does
>Peter>
>Peter> <!ATTLIST FOO PLUGH CDATA #IMPLIED>
>Peter>
>Peter> <FOO BAR="baz" XYZZY="" BLORT="six      spaces"/>
>Peter>
>Peter> return for
>Peter> String s = element.getAttval("PLUGH");// could be "", or null
>
>David has answered the original question (what is isSpecified() for in the
>Java simple API?), but I thought I'd mention that DSSSL's attribute-string
>function returns #f for PLUGH; the Java equivalent of this is of course,
>null.  I think this is the Right Thing to do; it's sometimes important to
>tell the difference between <Fu bargh=""/> and <Fu/>.

I agree that it is the Right Thing to do. If everyone else agrees it is the
Right Thing to do I will be very happy.  If 10% agrees and the other 90%
don't know what we are on about, we need to make sure they can't Go Wrong :-)


>
>The first case is often used to mean a known, empty value; the second
>to mean "not known" or "not applicable".
>
>Concrete example: I'm a rock climber, and I keep a record of all my
>climbing in XML format.  Climbs are defined as

How exciting - I used to be (not a very good one).

[...]
>
>climbs.xml>  <climb grade="VD" stars="" style="2"
>climbs.xml>   with="&p-hkm;">Difficult Crack</climb>
>
>Here, the lack of stars is explicit - it's not a high-quality climb.
>Whereas
>
>climbs.xml> <climb grade="D" with="&p-ejk;, &p-dmj;"
>climbs.xml>  style="l">King's Chimney</climb>
>
>is a climb in a part of Britain where the star system isn't used, and
>so I omitted the attribute - even though it probably deserves a star
>or two.

Not according to MacInnes' star system; he gives it zero stars :-).
Seriously, if we adopt this system then we should make every effort to
promote it. One difficult area is in editors - how do you signal the
difference between 
"" and null when entering a value in a box? You either have to get them to
input NULL (yukk) or add another button for "IMPLIED" (Ugh).

I'd like some other expert opinion on this. It's a tricky area if we get it
wrong.

	P.

Peter Murray-Rust, Director Virtual School of Molecular Sciences, domestic
net connection
VSMS http://www.nottingham.ac.uk/vsms, Virtual Hyperglossary
http://www.venus.co.uk/vhg

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