[dfdl-wg] Telecon minutes 2006-04-12

[Mike Beckerle]

DFDL WG call 2006-04-12

Bob McGrath
Geoff Judd
Mike Beckerle

Agenda
- arrays discussion
- issue of defaults 
- outstanding property document discussion (deferred until a call with 
more participants, particularly S. Hanson)

Arrays

We considered the 'string-with-all-lengths-first' example sent by MikeB. 
It uses a @dfdl:index extension to our Xpath expression language. Email 
traffic to the list asked for clarifications of why this is not just the 
Xpath position() function. MikeB to post clarification.

We considered whether the @dfdl:index provides enough/too-much expressive 
power. We agreed to create a 2d example. The parse/read direction seems 
likely to work fine. The write/output direction seems less clear, but 
we'll create a parse/read example first and then consider how/where to 
hang the "inverse index calculations" on it.

Defaults

This is discussion of this issue raised by S. Hanson when he posted the 
'properties' document on 3/23:

    - The issue of defaults poses some questions. We have agreed that
    hard-wired model defaults are not desirable as it means the absence of 
a
    property implies a behaviour. If we wish to change this behaviour then 
we
    are stuck because existing DFDL Schema then behave differently. So the
    proposal in the scoping document is that all properties that are used
    during a parse/serialise must have an explicit value defined somewhere 
in
    the DFDL Schema, typically in a dfdl:defineFormat annotation. However,
    there are properties where we don't want a one-value-fits-all value, 
but at
    the same time do not want to specify a value on every element. 
Example:
    justification, where typically all strings are left justfied, and all
    numbers are right justified.  Example: calendarPattern, where the 
patterns
    for a date, dateTime, time, monthDay, etc are different. One approach 
is to
    duplicate the properties - so we would have textStringJustification,
    textNumberJustification, and so on. You will see that this is what I 
have
    done in the document for justification.  An alternative approach is to 
have
    one property, but to add an additional enum 'schema', which means 
derive
    the default from the logical type. You will see that this is what I 
have
    done for calendarPattern. So calendarPatternKind set to 'schema' for
    xsd:date would yield "yyyy-MM-dd" and for xsd:time would yield
    "hh:mm:ss.sss". Of course, 'schema' can be considered a form of
    hard-wiring, so maybe we also need some properties that define what 
these
    defaults are (they'd only ever be set at dfdl:defineFormat level)?  We 
need
    to decide which of these approaches is preferred, whether they both 
make
    sense, or if there is a better way.

We propose that the general approach here should be to split the 
properties. That is, the textNumberJustification and 
textStringJustification solution be used wherever this problem exists.

We couldn't see why the calendar stuff is sufficiently different. It seems 
that there should be a datePattern, timePattern, dateTimePattern, and so 
forth and these should be independent properties.

Another issue was identified which is exemplified by the separatorEncoding 
property. It is possible for a separator to have a distinct character set 
(obscure, but possible). However, in most cases it does not, and one would 
like separatorEncoding to get its value from the regular encoding 
property. However, if separatorEncoding was to have no binding at all in 
the entire scope then this makes things quite confusing. Adding a binding 
for separatorEncoding at an outer scope would affect lots of enclosed 
separator behavior in unexpected ways.

Proposed solution: when properties are linked like this don't allow things 
to be 'unbound', but rather require a binding which specifically states 
the linkage between the properties. For example: in one of the include 
files at top level there would be a defineFormat where separatorEncoding 
is bound to a distinguished value which means "take the value for this 
property from the encoding property". Scoping would then find this 
distinguished value for separatorEncoding, and then start over from the 
scope of the separator construct searching the scope/context for the 
encoding property. This can be clarified in the next scoping draft and 
should solve the problem. The syntactic device for specifying the value of 
one property to come from another property is TBD. 

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