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

[Mike Beckerle]

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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