[DFDL-WG] What is Rationale why Nillable complex type elements can only have '%ES; ' as their dfdl:nilValue property

[Mike Beckerle]

I guess I really don't see the need for the restriction.

We do need a restriction for complex types that nilKind must be
'literalValue' or 'literalCharacter', not 'logicalValue' because there are
no logical values for complex types. But literalValue and literalCharacter
are representational. They're not about the value of an element.

The representation of the complex type can be nilled with a nil
representation of "-" without introducing any concept of "value" for the
complex type element. Processing always proceeds to check the nil
representation first, before checking anything else, and has to even now,
given that we do allow %ES; as nil value for a complex type element.

Did this %ES; only policy stem from the fact that we didn't really
understand (early on) that nilled is an orthogonal non-value flag in the
infoset elements?

The fact that the workaround is this:

<choice>
    <element name="noValue" type="xs:string" dfdl:lengthKind="explicit"
dfdl:length="0" dfdl:initiator="-"/>
    <element name="myComplexType">
       .... non nillable complex type definition ....
    </element>
</choice>

essentially this argues that the nil representation can be checked for
first, and there's no real reason why the element of complex type can't be
nillable with nilValue "-".

On Wed, Dec 1, 2021 at 4:39 PM Steve Hanson <smh at uk.ibm.com> wrote:

> See spec section 13.15. "to avoid the concept of a complex element having
> a value, which does not exist in DFDL".  The parser would not know to treat
> the '-' as the nil value for the complex element, or the content of the
> first child? Allowing just %ES; avoids that.
>
> Regards
>
> Steve Hanson
>
> IBM Hybrid Integration, Hursley, UK
> Architect, *IBM DFDL*
> <http://www.ibm.com/developerworks/library/se-dfdl/index.html>
> Co-Chair, *OGF DFDL Working Group* <http://www.ogf.org/dfdl/>
> *smh at uk.ibm.com* <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:        01/12/2021 21:07
> Subject:        [EXTERNAL] [DFDL-WG] What is Rationale why Nillable
> complex type elements can only have '%ES; ' as their dfdl:nilValue property
> Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>
> ------------------------------
>
>
>
> DFDL has this seemingly ad-hoc restriction.
>
> Users naturally want to model a complex element where "-" (dash) means the
> whole complex element is nilled, and if not "-" then we parse and produce a
> complex element.
>
> What is the rationale for this restriction?
> --
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