[DFDL-WG] Unparsing and outputValueCalc

Sat Jun 21 07:17:45 CDT 2008

Revised per our discussions last week on the call.

Unparsing

Definition: augmented infoset. When unparsing one begins with the DFDL
schema and conceptually with the logical infoset. As the values of items are
filled in by defaulting, and by use of the DFDL outputValueCalc property,
these new item values augment the infoset. The resulting infoset is called
the augmented infoset.

Definition: an element declaration in the schema describes a potentially
represented item if that element declaration does not have an inputValueCalc
property. Whether the element declaration describes an item that is actually
represented or not depends on whether the element declaration is for a
required or optional element, and whether the element has a corresponding
value in the augmented infoset.

When unparsing, an element declaration and the infoset are considered as
follows: 

a)       If the element declaration has a dfdl:outputValueCalc property then
the expression which is the dfdl:outputValueCalc property value is evaluated
and the resulting value becomes the value of the element item in the
augmented infoset. Any pre-existing value for the infoset item is superseded
by this new value.

References to other augmented infoset items from within the outputValueCalc
expression must obtain their values from the augmented infoset directly
(when the value is already present) or by recursively using these methods
(a) and (b) as needed. 

b)       If the element declaration has no corresponding value in the
augmented infoset, and the element declaration is for a required item, and
it has a default value specified, then an element item having the default
value is created in the augmented infoset. 

c)       If any infoset item's value is requested recursively as a part of
(a) above and (a) does not apply, and the corresponding value is not
present, and (b) does not apply then it is a processing error. 

Given this augmented infoset, then if the potentially represented element
declaration has a corresponding infoset item then that item is serialized
according to its DFDL properties. If the element declaration is for a
required item, and there is no value in the augmented infoset then it is a
processing error. 

Because rule (a) above is used even if the augmented infoset item already
exists and has a value, it is possible for an outputValueCalc expression to
be evaluated multiple times. DFDL implementations are free to cache values
and avoid this repeated evaluation for efficiency, as the semantics of DFDL
require that the outputValueCalc expression return the same value every time
it is evaluated. 

In expressions, the function dfdl:length() can be called to determine the
representation length of an item. If an element declaration is not
potentially represented, then dfdl:length() is defined to return 0.

  _____  

From: Mike Beckerle [mailto:mbeckerle.dfdl at gmail.com] 
Sent: Thursday, June 12, 2008 12:01 AM
To: dfdl-wg at ogf.org
Subject: Unparsing and outputValueCalc

Based on our discussions on the call today, I was thinking about the
definition of output "unparsing" and how outputValueCalc is dealt with. 

I believe the following language is sufficient to explain how such
expressions are evaluated.

When unparsing, an element declaration in the schema must have a
corresponding value in the infoset. 

If one exists then that value is serialized based on its properties. If
there is no corresponding value in the infoset then a value is computed as
follows:  

d)       If the element declaration is required, and has a default value
specified, then an element item having the default value is created in the
infoset

e)       If the element declaration has an outputValueCalc property then the
expression which is the property value is evaluated and the resulting value
becomes the value of the element item in the infoset. References to other
infoset elements from within the outputValueCalc expression must obtain
their values from the infoset directly (when the value is already present)
or by recursively using these methods (a) and (b) as needed. 

f)         If any infoset element's value is requested and neither (a) nor
(b) applies, then it is a processing error.

Seems ok to me. This is the ordinary stuff of language specification.

The function dfdl:length() needs some additional discussion. 

I think we can restrict dfdl:length() to accept only paths to element info
items. I.e., the first argument must be an explicit path. 

(Alternatively, we can make the dfdl:length() be a member of an info item,
as in ../x.dfdl:length('bytes')  - of however we want to notate obtaining
the length from a path, instead of dfdl:length(../x, 'bytes'). Either
notation style is ok with me.)

The path for the dfdl:length must be to an element which has representation.
That is, it cannot have the inputValueCalc property.  This insures that it
is meaningful to ask for the dfdl:length, that is the representation length
of the item measured in the requested units.

There is already an Xpath function count() which returns the number of
occurrences of an item.  Both count() and dfdl:length() potentially imply
buffering in the unparser implementation. 

Comments?

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