[DAFFODIL-2824] warning needed when all contents are optional inside an optional element and dfdl:emptyElementParsePolicy="treatAsEmpty"

• Type: Improvement
• Status: Open
• Priority: Minor
• Resolution: Unresolved
• Affects Version/s: 3.4.0
• Component/s: Middle "End"
• Assignee: Unassigned
• Reporter: Mike Beckerle
• Created: 14/Jun/23 19:16
• Updated: 17/Jan/24 15:40

Description

A common mistake is to have an optional element of complex type which is a sequence of all optional elements.

This requires dfdl:emptyElementParsePolicy="treatAsAbsent", which is NOT the usual default in Daffodil. 

(It is the default in the IBM-portable formats, just not in the standard daffodil-only formats)

Example: (assume context is delimited)

 

<sequence dfdl:separator="%WSP+">
    <element name="required1" type="w:zlString" dfdl:initiator="999"/>
    <element name="optParent" minOccurs="0">
         <complexType>
            <sequence>
              <element name="optChild1" minOccurs="0" dfdl:initiator="//" type="w:string3"/>
              <element name="optChild2" minOccurs="0" dfdl:initiator="[[" type="w:string3" dfdl:terminator="]]"/>
           </sequence>
        </complexType>
     </element>
     <element name="optional2" type="zlString" dfdl:initiator="opt2" minOccurs="0"/>
</sequence>

In the example above, if the data is

 

999
opt2

We want the infoset to be 

 

<required1/>
<optional2/>

But it won't be. It will try to construct this infoset:

 

<required1/>
<optParent/>

 

But you can see that the optParent is deemed present, because an empty element is perfectly allowed by this schema, and dfdl:emptyElementParsePolicy="treatAsEmpty".

Even though none of its children are present. It will then expect another separator, and will fail because there isn't one. 

To fix this, one should set dfdl:emptyElementParsePolicy="treatAsAbsent", and that's what the warning message should suggest. 

A less attractive fix is to add an assertion at the end of the optParent element like this:

 

    <element name="optParent" minOccurs="0">
         <complexType>
            <sequence>
            <element name="optChild1" minOccurs="0" dfdl:initiator="//" type="w:string3"/>
            <element name="optChild2" minOccurs="0" dfdl:initiator="[[" type="w:string3" dfdl:terminator="]]"/>
             <sequence>
                <annotation><appinfo soruce="http://www.ogf.org/dfdl/"/>
                    <!-- 
                      One of the above children must exist or the
                      enclosing parent does not exist
                      -->
                    <dfdl:assert>{ fn:exists(optChild1) or fn:exists(optChild2) }</dfdl:assert>
               </appinfo></annotation>
             </sequence>
           </sequence>
        </complexType>
     </element>