revisit: nillable complex types and %ES restriction
The Daffodil user community is complaining about the DFDL v1.0 restriction that nillable complex types can only have %ES; (empty data) as their nilled representation. Multiple important data standards find this restriction quite awkward. It has significant negative impact on usability because it eliminates polymorphism of paths. The path needed to refer to an element value has different ending element path steps depending on whether it is nilled or not. The value of having DFDL create an XML infoset is lost if something as basic as this cannot be done in such a way that path expressions work alike to how they would work in regular XML/XSD. Is there a significant reason why this restriction cannot be lifted? If not we would probably plan to define a dfdlx namespace property that can be used to enable this alternate behavior, and then we could prototype this new fuctionality in Daffodil and propose this for DFDL 2.0 revision.
Mike, you asked this same question on 1st December, and I responded with the following ... sounds like it got lost in the Christmas post :) I need to do some archaeology here but I'm fairly sure it is to do with our rules for establishing representation. To establish representation, you need to have extracted the data from the bitstream using lengthKind. The only length that makes sense for a complex element is zero, which is why only %ES; is allowed. (Note that a complex element with lengthKind='implicit' and nillable='true' is a SDE). Your suggestion of treating nil value on a complex element as a delimiter sidesteps that nicely but scuppers the algorithm for establishing representation, and possibly known-to-exist versus known-not-to-exist. I don't have time right now to do the archaeology, but this is at best a candidate for 2.0, and definitely not an erratum on 1.0. It certainly wasn't an ad-hoc restriction. I will forward you the complete thread. Regards Steve Hanson IBM Hybrid Integration, Hursley, UK Architect, IBM DFDL Co-Chair, OGF DFDL Working Group smh@uk.ibm.com tel:+44-1962-815848 mob:+44-7717-378890 Note: I work Tuesday to Friday From: "Mike Beckerle" <mbeckerle@apache.org> To: "DFDL-WG" <dfdl-wg@ogf.org> Date: 25/01/2022 17:47 Subject: [EXTERNAL] [DFDL-WG] revisit: nillable complex types and %ES restriction Sent by: "dfdl-wg" <dfdl-wg-bounces@ogf.org> The Daffodil user community is complaining about the DFDL v1.0 restriction that nillable complex types can only have %ES; (empty data) as their nilled representation. Multiple important data standards find this restriction quite awkward. It has significant negative impact on usability because it eliminates polymorphism of paths. The path needed to refer to an element value has different ending element path steps depending on whether it is nilled or not. The value of having DFDL create an XML infoset is lost if something as basic as this cannot be done in such a way that path expressions work alike to how they would work in regular XML/XSD. Is there a significant reason why this restriction cannot be lifted? If not we would probably plan to define a dfdlx namespace property that can be used to enable this alternate behavior, and then we could prototype this new fuctionality in Daffodil and propose this for DFDL 2.0 revision. -- dfdl-wg mailing list dfdl-wg@ogf.org https://www.ogf.org/mailman/listinfo/dfdl-wg Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
participants (2)
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Mike Beckerle -
Steve Hanson