[DFDL-WG] Clarification needed " .[1] " expression on an array element

[Steve Hanson]

Section 18 only says that about arrays, because we don't allow more 
complicated predicates that you can have with XPath, such as 
/class/student[@rolllno = 493].

But saying "only used for indexing arrays" is a bit misleading. The 
infoset has no concept of an array, it just has elements that can have the 
same name, so the predicate is selecting one of those.

Take these three schema snippets...
.
1) xs:element name="x" minOccurs="3" maxOccurs="3" ...

2) xs:element name="x" minOccurs="1" ...
    xs:element name="x" minOccurs="1" ...
    xs:element name="x" minOccurs="1" ...

3) xs:element name="x" minOccurs="1" ...
    xs:element name="y" minOccurs="0" ...
    xs:element name="x" minOccurs="1" ...
    xs:element name="z" minOccurs="0" ...
    xs:element name="x" minOccurs="1" ...

Each of these can give rise to an infoset containing ...

  "x" : "value1"
  "x" : "value2"
  "x" : "value3"

A predicate of x[2] returns the 2nd "x" value in all cases.

Maybe we should take an erratum on section 18 to clarify that. 

To your question ... if other XPath tools allow it and give an obvious 
behaviour then we should follow that ... so that's (b).

I'll add to agenda for next WG call.

Regards
 
Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
smh at uk.ibm.com
tel:+44-1962-815848
mob:+44-7717-378890
Note: I work Tuesday to Friday 



From:   "Mike Beckerle" <mbeckerle.dfdl at gmail.com>
To:     "DFDL-WG" <dfdl-wg at ogf.org>
Date:   29/06/2021 16:31
Subject:        [EXTERNAL] [DFDL-WG] Clarification needed " .[1] " 
expression on an array  element
Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>



Apache Daffodil has this bug report: 
https://issues.apache.org/jira/browse/DAFFODIL-2182

It calls for getting clarification from the DFDL WG.

Suppose I have an array element named "a" and it contains an assert.

The assert expression can contain the subexpression " .[1] " 

Section 18 does say predicates (i.e., the square brackets) are only used 
for indexing arrays. It says it is an SDE if the predicate value doesn't 
evaluate to an integer index. But it does not say that one cannot put the 
predicate brackets on scalars, context elements, relative path steps, 
etc., however, this can only be meaningful if the predicate value 
evaluates to 1. The grammar given for the expression language explicitly 
does allow " .[1] " to be syntactically allowed. 

The question is does the ".[1]" mean:

(a) the whole array, so that ".[1]" refers to the first element of the 
array, so equivalent to "../a[1]"
(b) the current element, so equivalent to "../a[dfdl:occursIndex()]"
(c) SDE the expression is illegal as "." is never an array, and only 
arrays can be indexed with predicates.

I believe (b) is consistent with XPath behavior for XML documents.  An 
online XPath tool verifies this. 

That means the only way to refer to other array element items in the 
infoset is to reference the array itself via ../a[N]. 

This has the drawback of not being polymorphic - the array element name 
must be used as a path step. That may well be livable as a restriction. 

-mikeb

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Owl Cyber Defense | 
www.owlcyberdefense.com
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