[DFDL-WG] Unparsing and choices - (related - Re: Fw: Action 233 (deferred) - "byte order not sufficient..." - draft document on experience with binary format MIL-STD-2045

[Mike Beckerle]

Breaking out this one issue for a separate discussion thread.

Consider the choice below. There is a choice of two different elements
named raw1 and raw2, then a value element which is computed based on one or
the other of the alternatives.

>
> <choice>
>        <!-- length kind pattern is needed to bound length to max of 99 -->
>        <element name="raw1" type="xs:string"
>            dfdl:lengthKind='pattern'
>            dfdl:lengthPattern="[^\x7F]{0,99}"
>            dfdl:terminator="%DEL;"/>
>        <element name="raw2" type="xs:string"
>             dfdl:lengthKind="explicit"
>             dfdl:length="100"/>
> </choice>
> <element name='value' type='xs:string'
>      dfdl:inputValueCalc='{ if (fn:exists( ../raw1 ) then ../raw1 else
> ../raw2 }'/>
>
> Now, given the statement below:

>
> SMH: There is no way to make a choice branch fail when unparsing. (The
> only 'backtracking' when unparsing a choice is when the infoset contains no
> branch at all then the spec states that each branch is examined in turn
> until one is found that successfully applies defaults. But that's not
> really backtracking, as you can statically deduce the branch from the
> schema alone, so the 'default' branch to use can be computed up front).
>
> So is there any way to fix this and make the above choice work?

This is the only thing I can think of, which ends up with entirely separate
elements for parsing and unparsing.

I am not sure this works or should work.

<group name="hidG1">
<sequence>
<!--
   raw1 won't exist on unparsing
  -->
<element name="raw1" type="xs:string"
           dfdl:lengthKind='pattern'
           dfdl:lengthPattern="[^\x7F]{0,99}"
           dfdl:terminator="%DEL;"
           minOccurs='0' dfdl:occursCountKind='implicit'/>
<!--
   raw2 will only occur parsing if raw1 doesn't
   and doesn't exist on unparsing
   -->
<element name="raw2" type="xs:string"
            dfdl:lengthKind="explicit"
            dfdl:length="100"
            minOccurs='0' dfdl:occursCountKind='expression'
            dfdl:occursCount="{ if (fn:exists(../raw1) then 0 else 1 }"/>
<!--
   the outputRaw will only be used when unparsing.
   On parsing it will just get "" as value, and nobody uses the value.
  -->
<element name="outputRaw" type="xs:string"
       dfdl:inputValueCalc='{ "" }'
       dfdl:outputValueCalc="{ if (fn:string-length(../value) eq 100 then
../value else fn:concat(../value, '&#x7F;') }" />
<sequence>
</group>

<!--
    to use, we hide the cruft in a hidden group
   -->
<sequence dfdl:hiddenGroupRef="tns:hidG1"/>
<element name='value' type='xs:string'
     dfdl:inputValueCalc='{ if (fn:exists( ../raw1 ) then ../raw1 else
../raw2 }'/>
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