[DFDL-WG] DFDL Statement Evaluation Timing (Assert, Discriminator, SetVariable, NewVariableInstance)

[Mike Beckerle]

>
>
> *
> Clarification:* At any single annotation point of the schema, there can
> be only one format annotation (dfdl:format, dfdl:element, dfdl:sequence,
> dfdl:choice, dfdl:group, dfdl:simpleType).
> *
> Glossary*: DFDL Statement annotations, or just DFDL Statements, are the
> annotation elements dfdl:assert, dfdl:discriminator, dfdl:setVariable, and
> dfdl:newVariableInstance.
> SMH: Nice idea. What about dfdl:defineVariable, is that a statement
> annotation too?  Where does that leave dfdl:defineFormat and
> dfdl:defineEscapeScheme - they are not format annotations (that's their
> content). Do we have 'global', 'statement' and 'format' annotations?
>
>
Yes, the idea is that there are "defining annotations", "format
annotations", and "statement annotations" as the 3 distinct kinds.

> *
> Glossary*: *Combined annotations*: When annotations are combined between
> a group reference and the sequence or choice of the referenced global
> group, or among an element reference, an element declaration, and its type
> definition, the combined set of is referred to as the *combined
> annotations*.
> *
> DFDL Statement Annotation Placement*
>
> dfdl:assert and dfdl:discriminator can be placed as annotations on
> sequence, choice, group references, local and global element declarations,
> element references, and simple type definitions.
>
> dfdl:setVariable may be placed as an annotation on sequence, choice, group
> references, local and global element declarations for elements of simple
> type, element references to elements of simple type, and simple type
> definitions.
>
> dfdl:newVariableInstance can be placed as an annotation on sequence,
> choice, and group references.
>
> The combined annotations for any schema component can contain only a
> single dfdl:discriminator, or any number of dfdl:assert statements, but not
> both asserts and a discriminator. It is a schema definition error otherwise.
>
> The combined annotations for any schema component can contain multiple
> dfdl:setVariable annotations, but they must each refer to a different
> variable. It is a schema definition error otherwise.
>
> The combined annotations for any schema component can contain multiple
> dfdl:newVariableInstance annotations, but they must each refer to a
> different variable. It is a schema definition error otherwise.
> *
> Evaluation Order for Statement Annotations*
> *
> Assertions Before*:
>
> dfdl:discriminator or dfdl:assert with testKind='pattern' are executed
> before parsing the annotated construct.
> SMH: Wording needs to cater for combined annotations.
>
>
Another problem is "the annotated construct". I want to say "the thing
we're talking about parsing here." What is the right term for that?

>
> Note that the pattern is used to match against the entire representation
> of the component; hence, the framing (including initiators, etc.) are all
> visible to the pattern. The dfdl:encoding property is used when decoding
> the data to characters before matching.
>
> It is a schema definition error if alignment is not 1 and a
> dfdl:discriminator or dfdl:assert with testKind='pattern' is used.
> (TBD: restrictions on lengthKind='prefixed' as well? Any other
> framing-based incompatibilities? where assertions with testKind='pattern'
> are really incompatible?)
>


> SMH: If  alignment <> 1 is schema definition error then so should
> leadingSkip <> 0.  I'd leave it there though.
> Also schema definition error if encoding not set.
>

Good. Those are improvements. I would like to just say cannot have
lengthKind="prefixed" also. (We can add it back, we can't take it away.)


>
> If there are multiple dfdl:assert statements with testKind='pattern' the
> order of execution among them is not specified.
> Schema authors can insert sequences to control the timing of evaluation of
> statements more precisely.
> *
> Assertions After:*
>
> dfdl:discriminator or dfdl:assert with testKind='expression' (the default)
> are executed after parsing the annotated construct.
> SMH: Wording needs to cater for combined annotations.
>
>
> Furthermore, an attempt to evaluate a discriminator must be made even if
> the parse of the annotated construct ended in a parse error. This is
> because a discriminator could evaluate to true thereby resolving a point of
> uncertainty even if the complete parsing of the construct ultimately caused
> a parse error. Such discriminator evaluation has access to the DFDL Infoset
> of the attempted parse as it existed immediately before detecting the parse
> failure.
>
> Implementations are free to optimize by recognizing and executing
> discriminators or assertions earlier so long as the resulting behavior is
> consistent with what results from the above description.
>
> If there are multiple dfdl:assert statements with testKind='expression',
> then the order of execution among them is not specified. Schema authors can
> insert sequences to control the timing of evaluation of statements more
> precisely.
> *
> The dfdl:newVariableInstance Statement*
>
> These statements are evaluated before the parsing of the annotated
> construct. When there is more than one newVariableInstance statement the
> order of execution among them is not specified.  Schema authors can insert
> sequences to control the timing of evaluation of statements more precisely.
>
> All dfdl:newVariableInstance statements are executed before any
> dfdl:setVariable statements on the same annotated construct.
> SMH:
>
> SMH: Wording needs to cater for combined annotations.
>
>
> *The dfdl:setVariable Statement*
>
> When a dfdl:setVariable annotation is found on an element reference,
> element declaration, or simple type definition, then it is executed after
> the parsing of the element, which implies after the evaluation of
> expressions corresponding to any computed format properties. That is, if an
> expression is used to provide the value of a format property such as
> dfdl:terminator, the evaluation of that expression occurs before any
> dfdl:setVariable annotation is executed; hence, the expression providing
> the value of the format property may not reference the variable.
>
> When a dfdl:setVariable annotation is found in the combined set of
> annotations for a sequence, choice, or group reference, then it is executed
> after any dfdl:newVariableInstance statements in that same combined set,
> but it is executed before the parsing of the sequence, choice, or group
> reference.
>
> If there are multiple dfdl:setVariable statements in one combined set of
> annotations, then the order of evaluation among them is not specified.
> Schema authors can insert sequences to control the timing of evaluation of
> statements more precisely.
>
> SMH: Wording needs to cater for combined annotations.
>
>
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-- 
Mike Beckerle | OGF DFDL WG Co-Chair
Tel:  781-330-0412
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