[DFDL-WG] Unparsing and outputValueCalc

Wed Jun 25 06:14:53 CDT 2008

Mike

Augmented infoset should include hidden elements. Although these almost 
certainly have an outputValueCalc by including hidden elements we could 
use the same concept on parsing to describe the infoset used by the 
expression language.

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

        0)      If the element declaration has a dfdl:inputValueCalc 
property then the infoset value is ignored and nothing is output

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superseded by this new value.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. 

Alan Powell

 MP 211, IBM UK Labs, Hursley,  Winchester, SO21 2JN, England
 Notes Id: Alan Powell/UK/IBM     email: alan_powell at uk.ibm.com 
 Tel: +44 (0)1962 815073                  Fax: +44 (0)1962 816898

From:
"Mike Beckerle" <mbeckerle.dfdl at gmail.com>
To:
<mbeckerle.dfdl at gmail.com>, <dfdl-wg at ogf.org>
Date:
21/06/2008 13:18
Subject:
Re: [DFDL-WG] Unparsing and outputValueCalc

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.

Mike Beckerle | OGF DFDL WG Co-Chair | CTO | Oco, Inc.
Tel:  781-810-2100  | 504 Totten Pond Road, Waltham MA 02451 | 
mbeckerle.dfdl at gmail.com 

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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