[DFDL-WG] when is the separator expression evaluated?

[Tim Kimber]

A separator as something that applies to the entire group, so I'm 
uncomfortable with the idea of (potentially ) changing it for every member 
of the group.
So I would vote for:
1) The separator is evaluated once per 'data' element; occursIndex
evaluates to index in the 'data' array;

If 2) was desired it could be achieved by setting the terminator on num:
<element name="e2">
  <sequence separator="|" separatorPosition="infix">
    <element name="seps" minOccurs="3" maxOccurs="3"/>
    <element name="data" maxOccurs='10'>
      <sequence>
        <element name="num" maxOccurs='10' terminator="{ 
/e2/seps[dfdl:occursIndex()] }" />
      </sequence>
    </element>
  </sequence>
</element>
..and the infix-ness could be emulated by setting the terminator to "" 
when dfdl:occursIndex() eq count( /e2/seps).

regards,

Tim Kimber, 
Technical Lead for IBM Integration Bus Healthcare Pack
Hursley, UK
Internet:  kimbert at uk.ibm.com
Tel. 01962-816742 
Internal tel. 37246742




From:   Mike Beckerle <mbeckerle.dfdl at gmail.com>
To:     "dfdl-wg at ogf.org" <dfdl-wg at ogf.org>
Cc:     Norm Patrick <npatrick at tresys.com>, Jessie Chab <jchab at tresys.com>
Date:   16/12/2014 22:40
Subject:        [DFDL-WG] when is the separator expression evaluated?
Sent by:        dfdl-wg-bounces at ogf.org



Jessie Chab came up with this interesting case. I am hoping someone else 
remembers somewhere in the spec where this order of evaluation issue is 
taken up in detail. 

Consider:

<element name="e2">
  <sequence separator="|" separatorPosition="infix">
    <element name="seps" minOccurs="3" maxOccurs="3"/>
    <element name="data" maxOccurs='10'>
      <sequence separator="{ /e2/seps[dfdl:occursIndex()] }">
        <element name="num" maxOccurs='10' />
      </sequence>
    </element>
  </sequence>
</element>

So we first parse 3 strings separated by a pipe. After that's parsed,
lets assume our infoset looks like this:

<e2>
  <seps>;</seps>
  <seps>-</seps>
  <seps>#</seps>
</e2>

After that we will have some 'data' elements (separated by pipes) which
each have a sequence of 'num' elements. The question is what are the
valid separators of the 'num' elements. I see two potential 
interpretations.

1) The separator is evaluated once per 'data' element; occursIndex
evaluates to index in the 'data' array; valid data might look something
like:

;|-|#|a;b;c;d|e-f-g-h|i#j#k#l

Note that this means the size of the data array must be less than or
equal to the size of the seps array (though that could be worked around 
using mod 3 arithmetic.)

2) Everytime we need to look for a separator between a num element, we
reevaluate the separator expression. This means the occursIndex()
references the index in the 'num' array, and so valid data might look
something like:

;|-|#|a;b-c#d|e;f-g#h|i;j-k#l

Note that this means the size of the num array must be less than or
equal to the size of the seps array.

I recall we were considering an argument to dfdl:occursIndex() to make 
exactly this kind of issue clear. I believe we decided against it, as we 
weren't able to pin down the semantics quite clearly.  E.g., in the above, 
how would you add an argument to the dfdl:occursIndex(...) call that 
points to the num array, which isn't even in scope at that point?

I know we say somewhere in the spec that separator can be defined, in say, 
the default format of some other schema file. It can be an expression, and 
that expression isn't evaluated until some sequence which has that 
separator in scope. Which means the expression can refer to path steps and 
such that are meaningless at the point where it appears lexically, but 
will be meaningful for a sequence where that separator expression is in 
scope. 

But this problem is slightly different. The question is whether the 
evaluation is per-item of the sequence, or once for the sequence.


...mikeb

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology | 
www.tresys.com
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