[DFDL-WG] DFDL Statement Evaluation Timing (Assert, Discriminator, SetVariable, NewVariableInstance)
Steve Hanson
smh at uk.ibm.com
Tue Oct 30 11:00:45 EDT 2012
Suman - we could make that a limitation - but why is that any different to
putting a relative expression in a dfdl:xxxx property on a global object -
we don't disallow that.
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Suman Kalia <kalia at ca.ibm.com>
To: Tim Kimber/UK/IBM at IBMGB,
Cc: dfdl-wg at ogf.org, dfdl-wg-bounces at ogf.org, Mike Beckerle
<mbeckerle.dfdl at gmail.com>, Steve Hanson/UK/IBM at IBMGB
Date: 30/10/2012 14:46
Subject: Re: [DFDL-WG] DFDL Statement Evaluation Timing (Assert,
Discriminator, SetVariable, NewVariableInstance)
>>> A single dfdl:discriminator annotation may appear on an element
reference, or on the global element declaration it refers to, or on the
simple type appearing immediately within or referenced from the global
element declaration. But only one of those places. In addition, if a
discriminator occupies one of those places, then no dfdl:assert
annotations may appear in any of those locations.
I really have an issue with putting relative path on global elements,
types in any of the discriminator, asserts etc.. because there is no
context available.. the right place is the element reference ... I don't
have an issue with specifying value expressions , concrete set of values (
glorified pattern facet) and it should be constrained to types.. this
would be in-line with XML Schema spec and I would prefer not to move away
from it..
Suman Kalia
IBM Canada Lab
WMB Toolkit Architect and Development Lead
Tel: 905-413-3923 T/L 313-3923
Email: kalia at ca.ibm.com
For info on Message broker
http://www.ibm.com/developerworks/websphere/zones/businessintegration/wmb.html
From: Tim Kimber <KIMBERT at uk.ibm.com>
To: Steve Hanson <smh at uk.ibm.com>, Mike Beckerle
<mbeckerle.dfdl at gmail.com>,
Cc: dfdl-wg at ogf.org
Date: 10/30/2012 10:03 AM
Subject: Re: [DFDL-WG] DFDL Statement Evaluation Timing (Assert,
Discriminator, SetVariable, NewVariableInstance)
Sent by: dfdl-wg-bounces at ogf.org
I agree with most of that. I certainly would want newVariableInstance to
be evaluated before a complex element is parsed.
The complication with the setVariable rule is that an absolute path may be
equivalent to a path which uses the self-axis but it is not possible, in
general, to determine this by static analysis of the xsd. Furthermore, one
XPath expression can contain zero to many path refs, any or all of which
might ( or might not ) start with the self-axis.
I can only see two possible solutions to this.
- Asserts/discriminators/setVariable are always executed after the
component on which they are positioned has been fully parsed. With the
exception of asserts/discriminators with test=pattern.
- Asserts/discriminators/setVariable have a 'timing' flag that defaults to
'after'. If it is set to 'before' and the expression does not evaluate
successfully then it is a schema definition error.
This would allow earlier rejection of the wrong branch in the model ( and
therefore more efficient parsing ) in cases where the expression only
refers to items that have already been parsed.
regards,
Tim Kimber, DFDL Team,
Hursley, UK
Internet: kimbert at uk.ibm.com
Tel. 01962-816742
Internal tel. 37246742
From: Steve Hanson/UK/IBM at IBMGB
To: Mike Beckerle <mbeckerle.dfdl at gmail.com>,
Cc: dfdl-wg at ogf.org
Date: 30/10/2012 13:12
Subject: Re: [DFDL-WG] DFDL Statement Evaluation Timing (Assert,
Discriminator, SetVariable, NewVariableInstance)
Sent by: dfdl-wg-bounces at ogf.org
Thanks for writing this up, I have a couple of comments from thinking
about this in parallel, and reading the write-up.
- In 2), replace element with object as applies to sequence/choice as well
- newVariableInstance should be evaluated before the object is parsed
(note: self-axis is not allowed)
- setVariable should be evaluated before the object is parsed unless it
uses self-axis
- setVariable should be evaluated after the object is parsed if it uses
self-axis
- consider only allowing self-axis in setVariable for simple
elements/types
- add statement about early evaluation of non-pattern
asserts/discriminators if that can be done (as per IBM implementation)
Regards
Steve Hanson
Architect, Data Format Description Language (DFDL)
Co-Chair, OGF DFDL Working Group
IBM SWG, Hursley, UK
smh at uk.ibm.com
tel:+44-1962-815848
From: Mike Beckerle <mbeckerle.dfdl at gmail.com>
To: dfdl-wg at ogf.org,
Date: 29/10/2012 22:54
Subject: [DFDL-WG] DFDL Statement Evaluation Timing (Assert,
Discriminator, SetVariable, NewVariableInstance)
Sent by: dfdl-wg-bounces at ogf.org
I'll write this up like an errata, but this is for discussion of whether
we believe this is clear and complete.
-------------------------------------
Glossary: DFDL Statements are the annotation elements dfdl:assert,
dfdl:discriminator, dfdl:setVariable, and dfdl:newVariableInstance.
Errata: Locations where DFDL Statements are allowed to appear are extended
to also include Global Element Declarations, and on Simple Type
Definitions.
Errata: Clarification about discriminators: Discriminators exclude
Assertions even when combining across references.
Beyond the stipulation that there can be only one dfdl:discriminator at
any annotation point of the DFDL schema, there are further constraints.
A single dfdl:discriminator annotation may appear on an element reference,
or on the global element declaration it refers to, or on the simple type
appearing immediately within or referenced from the global element
declaration. But only one of those places. In addition, if a discriminator
occupies one of those places, then no dfdl:assert annotations may appear
in any of those locations.
A dfdl:discriminator annotation may appear on a group reference or on the
model group within the global group definition it refers to. But only one
of those places, and similarly, if a discriminator appears in any of those
places, then no dfdl:assert annotations may appear in any of those
locations.
Errata: Clarification about the execution order of DFDL Statements when
they appear on an element reference or element declaration.
DFDL Statement annotations for a given schema component are executed as
follows:
1) all relevant DFDL statement annotations are gathered to form a single
list which preserves schema-definition order.
For a simpleType definition, the DFDL statement annotations found
immediately on it are kept in schema-definition order, and are appended to
the end of a list of those from any base simpleType definition.
For an element declaration having simple type, the DFDL statement
annotations found immediately on the declaration are appended to the end
of the list of those from its simple type.
For an element reference, DFDL statement annotations found immediately on
the element reference are appended to the end of the list of those from
the global element declaration it references.
2) given the combined list, the annotations are executed as follows:
1. before any parsing of the element, a dfdl:discriminator with
testKind="pattern" is executed.
2. if there is no discriminator, then before any parsing of the
element, all dfdl:asserts (there could be several) with testKind="pattern"
are executed in the order they appear in the list of DFDL statements.
3. The element itself is parsed, or its inputValueCalc property is
evaluated to create its value.
4. all newVariableInstance annotations are executed and new
variables are placed into scope for the duration of these remaining steps.
The statements are executed in the order they appear in the list of DFDL
statements.
5. all setVariable annotations are executed. The statements are
executed in the order they appear in the list of DFDL statements.
6. if a discriminator is present it is executed
7. if no discriminator is present, then assert annotations can be
present, and they are executed. If there are multiple assert annotations
the statements are executed in the order they appear in the list of DFDL
statements.
If the element reference or local element declaration is an array, then
this evaluation is repeated for each occurrence of the array.
Discussion:
The above allows the default expressions associated with any statement to
refer to the value of the element itself as "."
However, there's this anomaly of syntax where things don't seem right:
<dfdl:newVariableInstance ref="foo:bar" default="{....some expression
...}"/> creates a new variable for the scope of the entity it annotates.
It's clear what this means if this annotation is placed on a sequence or
choice. For the children of that sequence/choice the new variable instance
is in effect.
On a simpleType or element having simple type, similarly it is clear (in
that case it's a very local variable, just for the expressions in other
newVariableInstance statemsnts, setVariableStatements, and discriminators
and assertions).
The rub: on an element declaration (or reference) when there is a complex
type, its not clear.
<element name="foo">
<annotation><appinfo ....>
<dfdl:newVariableInstance......> <!-- what can refer to this? -->
</appinfo></annotation>
<complexType>
<sequence>
<annotation><appinfo ....>
<dfdl:assert>... according to rules above this cannot refer to
the newVariableInstance...</dfdl:assert>
</appinfo></annotation>
....
</sequence>
</complexType>
</element>
The above rules say the newVariableInstance isn't evaluated until AFTER
the element is parsed, so the assert down inside on the sequence will NOT
see this new variable even tho textually, the newVariableInstance
annotation looks like it would be in scope over the sequence.
Possible ways to avoid this oddity without messing up evaluation order:
allow newVariableInstance only on simpleType, element declarations and
element references having simpleType, group references, and
sequence/choice. Disallow them on element declarations of complex type or
on element references to those. The schema author can always introduce an
extra tier of sequence to provide the exact behavior they need, and this
otherwise error-prone issue can be avoided.
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