[DFDL-WG] Problem: simple format that is impossible to model

[Mike Beckerle]

Yes there is the nillable technique, but my simplified example data format
was too simplified.

in the real format that I derived my simple example from, nillability is
used for other purposes. In that format generally elements are nillable
with nilValue="%WSP*;-%WSP*;".  That is, the format needs to distinguish
explicitly nilled values from string values, including empty string values.

I know I'm not the only user who thinks one should be able to model this
simple data format without needing to use nillability.

For example, if you look at the CSV schema on DFDLSchemas on github, the
elements for rows of data are not nillable, even though adjacent commas are
routine in CSV files.
Of course a CSV schema for a fixed-number-of-columns doesn't have a
variable number of elements in the rows, so the elements in the rows are
all required, not optional.  Still I think you can't tolerate adjacent
commas without using nillability if you want the data to both parse and
unparse.  I have wanted to enhance the CSV schema on github to show more
variations on the CSV-like theme for a while, because I have recently
created many CSV-like data schemas, and a common theme to them is that
there are a variety of representations of nilled such as "N/A none -"
(these were human-created spreadsheet 'documents' exported as CSV, not
machine-generated CSV data sets),  and in some of these empty strings are
legit "normal" values. I have had the good fortune that these formats were
parse-only, as they would not have faithfully unparsed.

The problem ultimately boils down to there is no way in DFDL to say "treat
empty strings as just normal strings".The use of initiators/terminators and
dfdl:emptyValueDelimiterPolicy="both" doesn't fix this, because that
doesn't give you a NormalRep, it gives you EmptyRep.

As well there is ambiguity in the spec between the sections 9.2.5 and 9.2.3
- 9.2.4, as to whether zero-length string/hexBinary with no framing is
NormalRep or EmptyRep.

The fact that we have a property named dfdl:emptyValueDelimiterPolicy
suggests that an element, regardless of type, is EmptyRep if the content is
zero length and the initiators/terminators match the EVDP policy.
That suggests that section 9.2.5 is simply incorrect - a NormalRep cannot
be zero-length for string or hexBinary if there is no framing. Such would
always be an EmptyRep.
That would leave the nillable mechanism as the only way to deal with
zero-length strings that need to be retained in the infoset.

While it is good to fix that ambiguity, I find this not really an adequate
solution. I can't deal with my slash-delimited format that uses nillable
for other purposes in any reasonable way. I need a way to say "treat
zero-length strings as normal values".

I suggest we modify the recently proposed dfdlx:emptyElementParsePolicy
property to encompass the added variation we need. So the values of the
property would be:

   - treat zero-length for all types as AbsentRep always (we were calling
   this "treatAsMissing", or "treatAsAbsent" - this is the IBM DFDL behavior
   today as I understand it.)
   - treat zero-length for all types as EmptyRep always (we were calling
   this "treatAsEmpty" - this is the DFDL Spec behavior as written today as
   revised by current errata and with the correction mentioned above to remove
   the ambiguity.)
   - treat zero-length for string/hexBinary as NormalRep, all other types
   as EmptyRep (Suggest  "treatAsNormalOrEmpty". The rationale for this enum
   name is since the other types than string/hexBinary can't have zero length
   NormalRep, they must be EmptyRep. I read the enum name as "treat as
   NormalRep when possible otherwise treat as EmptyRep".  Another possible
   enum name might be "preferNormalToEmpty".)

Section 9.2.5 would be clarified to say that zero-length NormalRep is
possible for string/hexBinary if there is no framing and
dfdlx:emptyElementParsePolicy is 'treatAsNormalOrEmpty'.

Sections 14.2.2 and 14.2.3 may need a one-line clarification added that
when zero-length string/hexBinary is being treated as NormalRep, then they
are "normal" not "empty", and since they are not EmptyRep suppression of
zero-length and separators would not occur for trailingEmpty,
trailingEmptyStrict, or anyEmpty. (Which should be intuitive given the enum
names use the word "Empty")

It would/could be an SDE (or maybe warning) if this latter
"treatAsNormalOrEmpty" was specified for a potentially required element
(scalar or minOccurs > 0) of type string or hexBinary of variable length
(so possibly zero) with a default specified other than default="", because
such a default value could never be used, as zero length would be
considered NormalRep and so would not trigger use of the default value.
I.e., SDE like "Default value for element X can never be used because...."

Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
www.tresys.com
Please note: Contributions to the DFDL Workgroup's email discussions are
subject to the OGF Intellectual Property Policy
<http://www.ogf.org/About/abt_policies.php>



On Fri, Sep 27, 2019 at 4:39 AM Steve Hanson <smh at uk.ibm.com> wrote:

> As there are no initiators or terminators, and your example infoset calls
> everything 'field', I am assuming that the element looks logically like:
>
> <xs:element name="field" type="xs:string" minOccurs="0"
> maxOccurs="unbounded" />
>
> You want to preserve the position of the occurrences in the infoset so
> that they re-appear on output. The agreed way to do this is:
>
> <xs:element name="field" type="xs:string" minOccurs="0"
> maxOccurs="unbounded" nillable="true" dfdl:nilKind="literalValue"
> dfdl:nilValue="%ES;" />
>
> 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:        26/09/2019 19:11
> Subject:        Re: [DFDL-WG] Problem: simple format that is impossible
> to model
> Sent by:        "dfdl-wg" <dfdl-wg-bounces at ogf.org>
> ------------------------------
>
>
>
>
> To start discussion on my own issue.....
>
> The problem here may be that for a string (or hexBinary), if there is no
> initiator/terminator, there is no way to distinguish EmptyRep from
> NormalRep. I.e., an empty string is a "normal" value for a string.
>
> Sections 9.2.3 and 9.2.4 seem to define EmptyRep and NormalRep such that
> an empty string will be a EmptyRep, not a NormalRep.
>
> However section 9.2.5 on zero-length says:
>
>    "The normal representation can be a zero-length representation if the
> type is xs:string or xs:hexBinary and there is no framing."
>
> That suggests that when there is no framing, a zero-length string is
> NormalRep, not EmptyRep, which is the opposite conclusion from what is in
> sections 9.2.3 and 9.2.4.
>
> If this latter clarification is correct, then my format *should* work as I
> expect, because the empty string elements will be considered NormalRep and
> infoset values will be created for them.
> It simply doesn't work because of a bug in daffodil which has not
> interpreted this correctly.
>
> ...mikeb
>
>
>
> Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
> *www.tresys.com* <http://www.tresys.com>
> Please note: Contributions to the DFDL Workgroup's email discussions are
> subject to the *OGF Intellectual Property Policy*
> <http://www.ogf.org/About/abt_policies.php>
>
>
>
> On Thu, Sep 26, 2019 at 1:47 PM Mike Beckerle <*mbeckerle.dfdl at gmail.com*
> <mbeckerle.dfdl at gmail.com>> wrote:
> I have a dead-simple little format:
>
>     data/data/data/data
>     data/data/data/data
>
> it is lines of "/" separated strings. All elements are optional.
>
> I simply want this:
>
>    data//data
>
> to round trip. For that to happen I need it to parse into
>
>    <field>data</field><field></field><field>data</field>
>
> That is, I require that empty field element in the middle to be created
> and put into the infoset.
>
> I can find no way to do this.
>
> The strings have no initiator/terminator, so
> dfdl:emptyValueDelimiterPolicy is not relevant. All the elements are
> optional, so default values aren't relevant.
>
> The spec states:
>
> 9.4.2.2      Simple element (xs:string or xs:hexBinary)
> Required occurrence: If the element has a default value then an item is
> added to the infoset using the default value, otherwise an item is added to
> the Infoset using empty string (type xs:string) or empty hexBinary (type
> xs:hexBinary) as the value.
> Optional occurrence: If dfdl:emptyValueDelimiterPolicy is not 'none'*[12]*
> <https://daffodil.apache.org/docs/dfdl/#_ftn12> then an item is added to
> the Infoset using empty string (type xs:string) or empty hexBinary (type
> xs:hexBinary) as the value, *otherwise nothing is added to the Infoset*.
>
>
> There are errata/actions to clarify wording here around
> dfdl:emptyValueDelimiterPolicy being in effect or not (because there is no
> initiator/terminator for it to use as opposed to the property in isolation
> just being 'none').
> But that doesn't change anything about this issue.
>
> If this very simple format is not possible, then we need a property or new
> property enum value that makes it possible.
>
> Thoughts?
>
>
> Mike Beckerle | OGF DFDL Workgroup Co-Chair | Tresys Technology |
> *www.tresys.com* <http://www.tresys.com>
> Please note: Contributions to the DFDL Workgroup's email discussions are
> subject to the *OGF Intellectual Property Policy*
> <http://www.ogf.org/About/abt_policies.php>
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