[DFDL-WG] clarification needed: unparsing and hidden group choices and choices with dfdl:choiceDispatchKey
Steve Hanson
smh at uk.ibm.com
Fri Dec 13 04:29:27 EST 2019
Mike
In the call we said that you would look at this and "come up with a
minimum statement of behaviour for potential inclusion in spec". I thought
I would refresh my memory on the issue.
A couple of observations:
Your case 1). This is what we would do anyway for a missing choice - see
9.4.3.2.
Your case 2). This has made me wonder whether we should have updated
9.4.3.2 when we added the erratum for direct dispatch choice, to do as you
propose for the non-hidden case.
Your case 3). I think is actually case 1).
I think allowing dfdl:outputValueCalc on the head of a choice branch works
as long as it acts just like a default value when unparsing, with no
effects on choice resolution.
Regards
Steve Hanson
IBM Hybrid Integration, Hursley, UK
Architect, IBM DFDL
Co-Chair, OGF DFDL Working Group
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 at ogf.org" <dfdl-wg at ogf.org>
Date: 27/01/2017 18:48
Subject: [DFDL-WG] clarification needed: unparsing and hidden group
choices and choices with dfdl:choiceDispatchKey
Sent by: "dfdl-wg" <dfdl-wg-bounces at ogf.org>
So interesting issue. Comes up as part of our experience implementing the
advanced expression language features: hidden groups, and computed
elements.
We have hidden groups in DFDL.
If a xs:choice is inside a hidden group, then by definition, when
unparsing, elements corresponding to some branch of the choice are not in
the Infoset.
DFDL spec clearly states that dfdl:outputValueCalc is not allowed on the
head of a choice branch.
But elements inside hidden groups must have dfdl:outputValueCalc, or a
default value, otherwise there's no way for them to be filled in as part
of the augmented infoset.
So the only way a choice can be inside a hidden group is if the head of
the choice branches have default values.
This is not sufficient.
I do not think it was our intention to make hidden groups incapable of
holding things complex enough to contain a choice. Rather, the point is to
be able to hide things that have excess complexity needed only for
parsing/unparsing, so that they need not appear in the final Infoset.
So if a choice is allowed inside a hidden group, then a mechanism is
needed by which when unparsing a choice branch is selected based on values
of other parts of the Infoset, and then the elements of that branch are
computed and added to the augmented infoset.
Note that without hidden groups, this problem never arises, as the Infoset
always has to take a position on choice branches by having an unambiguous
element in it.
There are 3 cases:
1. unparsing an xs:choice where for parsing, each branch has a
discriminator, or is resolved by the simple "first successful unparse"
rule.
2. unparsing an xs:choice with dfdl:choiceDispatchKey
3. unparsing xs:choice with dfdl:initiatedContent='yes'
For all cases, I suggest we must remove the restriction on
dfdl:outputValueCalc being disallowed on choice branch head.
For (1). First branch where the branch can be constructed successfully is
added to the augmented infoset. This is a case where an unparse error is
non-fatal, and causes a form of unparser backtracking through
alternatives. Branches would be attempted in schema order.
For (2) I suggest dfdl:unparseChoiceDispatchKey, evaluated at unparse time
only. Produces a choice branch key. That choice branch is then
constructed, and default values or dfdl:outputValueCalcs are used to fill
in values. (BTW: this is the driving use case we have in Daffodil for this
issue.)
For (3).... I have no specific solution. I think it may not actually be a
separate case and has to be treated as (1) or (2).
Comments/Discussion?
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