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[XML-DEV Mailing List Archive Home] [By Thread] [By Date] [Recent Entries] [Reply To This Message] RE: Occurrence Question
--- "Tolkin, Steve" <Steve.Tolkin@F...> ha scritto:
> Can you please clarify what you meant by "there is
> no difference between
> empty and absent in xml."
Yes sure, I can try, I have various thoughts
on the argument, I put them out.
xml tree is made mainly of sequences of nodes,
these sequences can be empty or filled but
there is no third option (null or absent)
in this construct.
In DOM child nodes are a Node*List* that
is always present,by default empty.
The difference between null and empty is common
in programming languages and databases.
An object and a record can be seen as a map,
an associative array, one gets the values
from it by name.
The children of an xml element are a single
sequence containing items with possibly
different names, is very different from
an object or a record.
A text field of a DB record can usually be
null or an empty string, an object field
value might be null if the language uses
strings as references, like in java. In
Basic there is no null string, some Basic
use a placeholder for communicating
with databases.
XML attributes are different because they
are not a sequence but more like a map
(in DOM a NamedNode*Map*) and one can
distinguish between the three cases for
attribute b:
<a>
<a b=''>
<a b='ccccc'>
An attribute can be used more easily to
serialize the null value of a field,
but there are some differences.
In a map it might be possible to distinguish
between an entry {key,value} with null value
and an absent entry, this might be relevant
when merging maps (as happens for DB
updates)
Something like this is (luckily?) not present in xml.
<a b='<null/>'/>
When there is this empty/null difference
there _might_ be three states for a 'value':
null/empty/present
or even 4 considering [absent] and [present but null]
as different. This last possibility is only
in open maps, not in objects or records where the
keys are fixed.
xml elements 'collections' and xml text content
cannot be mapped to a null or an 'empty value'
consistently, you need to choose one of the two
possibly helping yourself with some other info.
Text of an element does not really exists
as property of an xml element,
in DOM might be seen as the the sum of
the node values of the child nodes of type text
if there are no child elements.
Also 'collections' do not exist,
there is only one sequence,
the sequence of the child nodes,
you need to interpret the sequence iterating
it and selecting the right nodes to be put
in a collection, you cannot just say
"give me a certain collection" as with objects.
> I think there is a difference. Suppose the empty
> element <Bonus> is
> used as a "marker", meaning: If and only if <Bonus/>
> occurs then the
> person is eligible to receive a bonus.
> Clearly there is a difference between empty and
> absent.
Yes but there is no third option.
In your example the element is present:
is the content, the child nodes sequence,
to be empty. The other case is the absent element.
If the bonus is a boolean primitive (true/false)
then an element absent/present might be a good marker.
But a boolean could be also a java.lang.Boolean
allowing null value, or a boolean field of a DB
record, also allowing null value.
The null value might mean that the
bonus-situation has not been decided yet
or might be used to unset a true/false situation.
If one doesn't have nulls might use two values
or create an enumeration, use xsi:nil [1] or other
trick.
In general one needs a convention so that is
pssible to interpret the xml data and
reproduce this concept and
you cannot just 'read it' from the data.
in my opinion the absence of a
difference between null an empty and also
the absence of an explicitly serialized
container for collections gives big advantages
in compatibility between _serialized instances
of objects_ since the same serialized form
can be 'interpreted' as an instance of different
structures. For example one single element
can be a single occurrence field but also
the first item of a future collection.
So for me using xml for object serialization
is better than binary for compatibility,
what is not good instead is using xml or
xml schema to describe objects,
beacause some structures cannot be mapped.
Greetings
Michele
[1] http://www.w3.org/TR/xmlschema-1/#xsi_nil
http://xsd.stylusstudio.com/2001Nov/post02006.htm
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