[DFDL-WG] dfdl:prefixLengthType with dfdl:assert on it

[Mike Beckerle]

I think we have to perhaps specify what a path expression like "." means if
on the simple type definition used as a prefixLengthType.

It doesn't refer to the element itself, but to the "quasi-element" (that's
what we call them internally to Daffodil) which is the prefix value itself.
At least that's what I need it to refer to for the example to work and
cause an assertion failure on bad prefix values.

On Wed, Feb 16, 2022 at 12:36 PM Steve Hanson <smh at uk.ibm.com> wrote:

> Hi Mike
>
> Unfortunately I can't test this with IBM DFDL, as one of our limitations
> is that you can't specify assert or discriminator on a global element or
> global type, and by definition a prefix length type must be global [image:
> :-)].  However, although the DFDL spec does not explicitly say that
> assert or discriminator *can* be used  on such a simple type, that's not
> the way the spec usually works; it usually says when you *can't* do
> something. So, given that there is no wording to say you can't, both are
> allowed.
>
> Regards
>
> Steve Hanson
>
> IBM Integration, Hursley, UK
> Architect, IBM DFDL
> Co-Chair, OGF DFDL Working Group
> smh at uk.ibm.com
> tel:+44-7717-378890
> Note: I work Tuesday to Friday
>
> -----Original Message-----
> *From*: Mike Beckerle <mbeckerle at apache.org
> <Mike%20Beckerle%20%3cmbeckerle at apache.org%3e>>
> *Reply-To*: mbeckerle at apache.org
> *To*: DFDL-WG <dfdl-wg at ogf.org <DFDL-WG%20%3cdfdl-wg at ogf.org%3e>>
> *Subject*: [EXTERNAL] [DFDL-WG] dfdl:prefixLengthType with dfdl:assert on
> it
> *Date*: Tue, 15 Feb 2022 17:02:31 -0500
>
> I believe the DFDL spec does not specify whether the prefixLengthType for
> an element of lengthKind 'prefixed' can have assertions, and
> if those assertions are evaluated, and when.
>
> Consider this example below. We really want the assert on the
> bufferPayload type to be evaluated before the length computed from the
> prefix is used as a length because we'd like good diagnostic behavior that
> points out that the prefix value is way out of realistic range.
>
> In many formats we see 4-byte length prefixes, but the range of values for
> specific fields is relatively tiny, often even just 1 to 100. Enforcing
> this via the assert expressed on the prefixLengthType's simpleType
> definition makes this easy.
>
> I could go either way on whether a prefixLengthType can carry a
> discriminator. But an assert should definitely be allowed.
>
>         ...
>         <complexType>
>           <sequence>
>             <element name="payload" type="xs:hexBinary"
>                      dfdl:lengthKind="prefixed"
>                      dfdl:prefixIncludesPrefixLength="no"
>                      dfdl:prefixLengthType="p8:bufferPayload"/>
>           </sequence>
>         </complexType>
>         ...
>
>
> <simpleType name="bufferPayload">
>   <annotation>
>     <appinfo source="http://www.ogf.org/dfdl/">      <dfdl:assert>{ dfdl:checkConstraints(.) }</dfdl:assert>
>     </appinfo>
>   </annotation>
>   <restriction base="p8:unsignedLong">
>     <minInclusive value="0"/>
>     <maxInclusive value="32768"/>
>   </restriction>
> </simpleType>
>
> --
>
>   dfdl-wg mailing list
>
>   dfdl-wg at ogf.org
>
>   https://www.ogf.org/mailman/listinfo/dfdl-wg
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.ogf.org/pipermail/dfdl-wg/attachments/20220216/3f9b1493/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: face-smile.png
Type: image/png
Size: 871 bytes
Desc: not available
URL: <http://www.ogf.org/pipermail/dfdl-wg/attachments/20220216/3f9b1493/attachment-0001.png>