I may have thought of the reason. If I have a choice of A and B, then
minOccurs=0 for B allows the choice to be empty A|B? but this is the same
as (A|B)? which is allowing the choice itself to be minOccurs=0, which is
not allowed.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
From: Steve Hanson/UK/IBM
To: Mike Beckerle
Cc: "dfdl-wg@ogf.org"
Date: 18/06/2015 10:49
Subject: Re: [DFDL-WG] Action 280 minOccurs='0' choice branch (was:
Re: OCK expression and count of 0 for a choice member....)
Hi Mike
I think the restriction of having minOccurs >= 1 on xs:choice branch arose
for two reasons, though I am unable to find a definitive email trail:
a) If minOccurs = 0 you immediately have two points of uncertainty, so
potentially two discriminators are needed. I'm not sure if this is really
a problem though, because if minOccurs < maxOccurs there are also two
points of uncertainty and it still requires some thought to get
discrimination correct as it varies per occurrence.
b) Interaction with known-to-exist rules. For example, one way to achieve
known-to-exist is to successfully parse an empty representation, which
with minOccurs = 0 may mean that nothing is added to the infoset. I'm not
sure this is actually a problem though. If the branch was successfully
parsed then surely that should discriminate in favour of the branch
regardless of representation.
And even if a) and b) are problematic, the fact exists that you can
trivially negate the restriction by wrapping in xs:sequence.
So I suspect we can drop the restriction altogether, and the 'system' just
works in a consistent manner.
You raised the issue of an element with dfdl:inputValueCalc not being
allowed as a choice branch. I suspect this was added because as soon as
you encounter such as branch you have by definition discriminated in
favour of that branch. But that's ok, you just make that branch the last
in the choice. No different to having a branch that exists just to throw
an error - it too must be last. If such branches are not last, it's a
schema design bug.
Back to Alex's original scenario at the foot of this thread, where his
xs:choice branch element had a dfdl:occursCount expression that evaluated
to 0. According to https://redmine.ogf.org/issues/244 no occurrences are
looked for in the data. That means the occurrences are missing, so
known-not-to-exist and the parser should try the next branch. Below I
said that section 15.1.1 needed updating to correctly reflect section 9.
And I also said we are perhaps missing a definition of what 'missing'
means for an array element?
"(The) spec defines known-to-exist and known-not-to-exist in terms of
occurrences. In (Alex's) choice branch example, it is the element as a
whole we are looking at. That's fine for scalar as element == occurrence
but for an array it's not the same. I think the spec is missing a
definition of what 'missing' means for an array element. I would say that
an array element is missing if all occurrences are missing. And an array
element is not missing if any occurrence has a representation (empty, nil,
normal)."
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle
To: "dfdl-wg@ogf.org"
Date: 02/06/2015 18:41
Subject: [DFDL-WG] Action 280 minOccurs='0' choice branch (was: Re:
OCK expression and count of 0 for a choice member....)
Sent by: dfdl-wg-bounces@ogf.org
I believe this action item remains open still and I would like to revive
the discussion.
I was coding up this aspect of Daffodil and have hit this subject head on.
In section 15 the spec clearly states that the root of a choice branch
cannot be optional, that is cannot have minOccurs="0".
That language is very specific, and it leaves open the possibility of
"effectively optional" things being the roots of choice branches (e.g.,
using OCK 'parsed' or 'expression')
It also allows one to trivially wrap a sequence (having no delimiters,
alignment or skips) around an element (or element ref) carrying
minOccurs="0" so as to simply dodge the restriction.
It was observed in the thread below that we cannot require choice branches
to be scalar elements as there is a need for hidden groups to be branches
of choices, and
for empty sequences carrying only asserts, as another non-element example.
Related: the DFDL spec also specifies that an element that is the root of
a choice branch cannot carry dfdl:inputValueCalc. The spec does NOT
restrict use of dfdl:outputValueCalc on the root of a choice branch, but
the meaning of such is unclear to me.
The existing restriction of "no minOccurs="0" on the root of a choice
branch seems not to accomplish anything. It is only for
occursCountKind='implicit' where this can be meaningful it seems.
Requiring the root of a choice branch to not be "variable occurrence" if
it is an element would accomplish something, but it is not clear this is
needed to eliminate ambiguity or if the ambiguity can be eliminated
without any restriction.
The stable design points I can think of are:
1) root of a choice branch must be scalar (so, only a sequence, choice, or
an element where minOccurs == maxOccurs == 1.)
2) root of a choice branch cannot be optional - for a broad sense of the
word optional - precludes arrays with OCK expression and parsed, and
implicit if minOccurs="0". Fixed length arrays would be allowed.
3) a choice branch must have some syntax
I think we discarded (3) because choice branches that really just reflect
error checking - contain only dfdl:asserts for example - are in use and
serve a useful purpose.
Daffodil's test suite has much use of choice branches that look like this:
<choicie>
.....
<sequence>
<element name="foo" dfdl:inputValueCalc="{....}"/>
</sequence>
</choice>
These have no syntax. This allowing a kind of default-element to be
computed. In most (could be all, I've not searched exhaustively) of these
cases the IVC expression is a constant. But note that the sequence
wrapped around the IVC element is just dodging the restriction that a
choice branch cannot be an IVC element (which is another restriction that
seems unnecessary.)
...mike
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
On Mon, Apr 27, 2015 at 9:30 AM, Steve Hanson wrote:
Mike
A couple of comments:
1) You said below
Optional here means "not required by the DFDL format", as in
occursCountKind cannot be 'parsed' at all, because all occurrences are
then not required, and the min/maxOccurs are only examined for validation
purposes, also occursCountKind cannot be 'implicit' for the same reasons,
and occursCountKind 'expression' also.
OccursCountKind 'implicit' is allowed, because minOccurs is used for
parsing and micOccurs can not be 0.
2) You said below
Wrapping the array element in a sequence doesn't solve the problem unless
the sequence has a required piece of syntax such as an initiator or
terminator, or a hiddenGroupRef to a not-optional (recursively) thing.
A sequence has minOccurs '1' so it does satisfy the spec rule about the
child of a choice being required. Such a sequence could have no syntax and
could contain an element with minOccurs '0' or even be empty. I have seen
DFDL schemas that contain a choice with the last branch being an empty
sequence that contains an assert fn:false() in order to throw a processing
error.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle
To: Alex Wood1/UK/IBM@IBMGB
Cc: "dfdl-wg@ogf.org"
Date: 27/04/2015 13:35
Subject: Re: [DFDL-WG] OCK expression and count of 0 for a choice
member....
Sent by: dfdl-wg-bounces@ogf.org
I believe any use of occursCountKind 'expression' on an element that is
the first element on a branch of a choice should be an SDE.
This is one of the cases where DFDL requires one to introduce an element
that would not be necessary in an ordinary XML schema, but is necessary
because DFDL does not have XML's easily parsed syntax to depend on.
This is my opinion. I think we need to look at whether this restriction is
either
(a) necessary
(b) necessary to avoid excessive complexity in implementations
(c) unnecessary - but is the intention of what is specified already
(despite shortcomings of the prose/description in the spec, which could be
corrected.)
(d) an error in the specification
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
On Mon, Apr 27, 2015 at 5:49 AM, Alex Wood1 wrote:
Hi Mike,
Can you clarify if you are saying that OCK expression should be prohibited
completely on a choice member (as occurrences for OCK expression are
potentially optional regardless of minOccurs value)
Or is your statement that it should cause an SDE specific to the count==0
case?
Kind Regards,
- Alex
Alex Wood -
Software Engineer -
WebSphere Message Broker Development
DFDL Development
MP 211, IBM UK Labs, Hursley Park, Winchester, Hants. SO21 2JN.
Tel: Internal 246272, External 01962 816272
Notes: Alex Wood1/UK/IBM@IBMGB
e-mail: wooda@uk.ibm.com
From: Mike Beckerle
To: Alex Wood1/UK/IBM@IBMGB
Date: 24/04/2015 15:10
Subject: Re: [DFDL-WG] OCK expression and count of 0 for a choice
member....
I think this is an SDE.
Choice branches cannot be optional.
Optional here, does not mean minOccurs == 0, because for many
occursCountKinds, that's never checked unless validation is on, and
validation doesn't guide parsing anyway.
Optional here means "not required by the DFDL format", as in
occursCountKind cannot be 'parsed' at all, because all occurrences are
then not required, and the min/maxOccurs are only examined for validation
purposes, also occursCountKind cannot be 'implicit' for the same reasons,
and occursCountKind 'expression' also.
Wrapping the array element in a sequence doesn't solve the problem unless
the sequence has a required piece of syntax such as an initiator or
terminator, or a hiddenGroupRef to a not-optional (recursively) thing.
Even initiator and terminator are tricky, because in a non-delimited
format, those can be %WSP*; which can match nothing at all; hence, they do
not "require" any syntax.
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
On Fri, Apr 24, 2015 at 9:07 AM, Alex Wood1 wrote:
Hi All,
Please see below for a history of the issue.
This arose from fuzz testing of the IBM DFDL parser which produced a test
with a coutn of 0 for an OCK expression array which was a choice member.
And subsequent reference to the specification.
It was not clear what the correct outcome should be in a choice where the
first member is an array using OCK expression where the count resolves to
0.
a.) resolve the choice to the zero length array
b.) move to the next choice branch
c.) throw an error
Kind Regards,
- Alex
Alex Wood -
Software Engineer -
WebSphere Message Broker Development
DFDL Development
MP 211, IBM UK Labs, Hursley Park, Winchester, Hants. SO21 2JN.
Tel: Internal 246272, External 01962 816272
Notes: Alex Wood1/UK/IBM@IBMGB
e-mail: wooda@uk.ibm.com
From: Steve Hanson/UK/IBM
To: Alex Wood1/UK/IBM@IBMGB
Cc: Andrew Edwards/UK/IBM@IBMGB, Mark Frost/UK/IBM
Date: 24/04/2015 09:19
Subject: Re: OCK expression and count of 0 for a choice member....
When I wrote the paragraph below, the one thing that troubled me was that
the spec defines known-to-exist and known-not-to-exist in terms of
occurrences. In the choice branch example, it is the element as a whole we
are looking at. That's fine for scalar as element == occurrence but for an
array it's not the same. I think the spec is missing a definition of what
'missing' means for an array element. I would say that an array element is
missing if all occurrences are missing. And an array element is not
missing if any occurrence has a representation (empty, nil, normal). With
that in place, my paragraph makes sense, I think.
I believe we have the same issue with 'parsed' and 'stopValue'.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
From: Steve Hanson/UK/IBM
To: Alex Wood1/UK/IBM@IBMGB
Cc: Andrew Edwards/UK/IBM@IBMGB, Mark Frost/UK/IBM@IBMGB
Date: 23/04/2015 18:52
Subject: Re: OCK expression and count of 0 for a choice member....
Here is one interpretation...
A choice is resolved by parsing the branches until one is known-to-exist
as described in section 9.3.3. Section 9.3.1.2 defines known-to-exist (in
the absence of a discriminator, initiator or direct dispatch) as an
occurrence having empty, nil or normal representation. Section 9.3.1.3
defines known-not-to-exist (again in the absence of a discriminator,
initiator or direct dispatchm or an assert) as an occurrence being missing
or causing a processing error. If occursCount is zero no occurrences are
looked for in the data (erratum 5.9) so the element has no representation
and must be missing. Therefore a choice branch containing such an element
is known-not-to-exist.
So in your example, the first choice branch containing myInt is
known-not-to-exist and the parser tries the next branch.
This appears to contradict section 15.1.1 though. I suspect that 15.1.1
was not updated to match section 9.3 when the latter was added.
If you want to make the first choice branch known-to-exist when the count
is zero then I think wrapping myInt in a sequence would work. Or wrapping
myInt in a complex element.
Definitely one to take to the WG though, if only to correct section 15.1.1
to match section 9.
Regards
Steve Hanson
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh@uk.ibm.com
tel:+44-1962-815848
From: Alex Wood1/UK/IBM
To: Steve Hanson/UK/IBM@IBMGB
Cc: Andrew Edwards/UK/IBM@IBMGB, Mark Frost/UK/IBM@IBMGB
Date: 23/04/2015 16:33
Subject: OCK expression and count of 0 for a choice member....
Hi Steve
Just been discussing this with Andy and Mark.
I think the spec
xs:complexType
Where myInt has occursCountKind="expression" occursCount="{../myCount}"
A given instance of this message could have myCount==0
Is this valid?
Should it resolve to 0 occurrences of myInt or move on to myTxt ?
Section15 of the spec says:
The Root of the Branch MUST NOT be optional. That is XSDL minOccurs MUST
BE greater than 0.
But in this case minOccurs is >0.
Assuming this is not an error then in terms of resolving the choice
section 15.1.1 says..
15.1.1 Resolving Choices via Speculation Speculative resolution works as
follows:
1) Attempt to parse the first branch of the choice.
2) If this fails with a processing error
a) If a dfdl:discriminator evaluated to true earlier on this branch then
the parser is 'bound' to this branch and parsing of the entire choice
construct fails with a processing error.
b) If the branch has a dfdl:initiator and the choice has
dfdl:initiatedContent ‘yes’ then the parser is 'bound' to this branch and
parsing of the entire choice construct fails with a processing error. c)
Otherwise we repeat from step 1 for the next branch of the choice.
3) It is a processing error if the branches of the choice are exhausted.
4) If a branch is successfully parsed without error, then that branch's
infoset becomes the infoset for the parse of the choice construct.
So seems like this is 4.) we did not fail to parse myInt...
However talking with mark about real scenarios that this might apply to, a
choice two repeating fields with counts earlier in the data only one of
which must appear. you'd expect 0 of the first means >0 of the second and
visa versa... So you'd probably want 0 myInt allowed the choice to resolve
to myTxt.
Thoughts ?
If you agree we need more clarity in he spec will forward to WG.
Kind Regards,
- Alex
Alex Wood -
Software Engineer -
WebSphere Message Broker Development
DFDL Development
MP 211, IBM UK Labs, Hursley Park, Winchester, Hants. SO21 2JN.
Tel: Internal 246272, External 01962 816272
Notes: Alex Wood1/UK/IBM@IBMGB
e-mail: wooda@uk.ibm.com
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741598.
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741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
Unless stated otherwise above:
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741598.
Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU
