The proposal currently under consideration is:

• Schema objects inherit DFDL properties from a lexically enclosing xs:complexType or xs:group declaration
• DFDL properties on a referenced global schema object (except simpleTypes) cannot be overridden unless explicitly parameterized by the global object.

• DFDL properties explicity defined on an element and it's referenced simpeType are merged into a single set. It is an error is the same property is defined on both the element and simpleType.
• It must be possible to validate all global objects , except simpleTypes.

Validity can mean "is consistent", but should not require property specifications to be "complete".

This is an area of some confusion in DFDL. We have stated that a schema must have "all required properties" specified, and that there is no defaulting of property values by implementations. The purpose of this is to avoid implementation-specific or platform specific assumptions from creeping in so that DFDL schemas are more likely to be portable. This statement has been misinterpreted in the following sense. Some have interpreted this as meaning that all properties that are defined in the DFDL spec must have values set in order for a schema to be "valid". But when stating the "all required properties" rule (largely at my insistance), this was definitely not my intention. Consider for example if a format is all text, and uses a single-byte character set encoding, then I claim that dfdl:byteOrder need not be specified as it will never be needed to interpret the data. The point of saying there are no defaults for property values is NOT to require dfdl:byteOrder to always be specified, it is to say that if the format requires dfdl:byteOrder - because it has binary multi-byte representations in it, or wide characters which have endianness, then dfdl:byteOrder must be specified by the schema, either directly, by an included schema referenced by the schema, or must be specified explicitly via some external mechanism - section 21 of draft 035. The point is that the implementation cannot just say "there is an unstated default" in this implementation for dfdl:byteOrder based on the platform you are installed on. If an implementation were to do that, then the schemas usable with that implementation will not be portable for use with other implementations - something we are trying to avoid. 

The difference here is subtle but important. Section 22 of draft 035 is a place holder for some pre-defined include-files the inclusion of which will provide dfdl:defineFormat specifications for useful sets of properties. It is important for everyone to understand that including these in a DFDL schema is 100% optional, and is for convenience of obtaining consistent and meaningful sets of properties only. However, simple formats can be described without any inclusion of these at all. As another example: if a file contains only an array of binary floating point numbers, then no dfdl:encoding property is needed. Just a handful of properties are needed to parse/unparse such a file format, and those are the ones about binary floating point numbers, and in the case of an array, about multiple occurrences. 

Getting back to scoping and the validation of a global decl/def.... Upshot of all this: it means from the perspective of "validating" a global decl/def,  one can't have conflicting DFDL properties in a global type or element declaration, but properties can be unspecified/unstated also, to be provided by the way that global decl/def is used.

If a top-level element declaration is incomplete in this style, then it is unsuitable for use as the document element of a data file/stream unless augmented by external information - something possible and which we discuss in chapter 21 (version 035) of the spec without giving specific mechanism.

If a top-level element declaration is incomplete in this style, then it can be made complete by way of being used by reference from another point in the schema which surrounds it with a scope providing the needed properties, or which provides the needed properties directly at the point of reference. This preserves referential transparency, and makes the semantics of referential transparency be just plain textual substitution, which is the semantics in XML Schema in general.

I believe total validity (Consistency AND completeness) for global decls/defs is not worth trying to achieve for the sake of a tooling goal. Tooling may have to be more sophisticated, but discarding referential transparency is not something we should do for the sake of simplifying some goal for tooling that isn't even clearly a requirement.

A tooling "goal" might be to allow an interactive user to point at a schema anywhere and see a list of properties in effect at that point. Total validity (consistency and completeness) is required for a concrete answer to this. However, why do we think this tooling goal should be a requirement? The answer presented back to the user could be that some properties are "unspecified", while other properties have specific values. I don't see this as problematic. 

We carefully decided not to allow any lexical invocation of DFDL formats at top level in order to eliminate the issue of lexical closure for top level objects. This allows ordinary textual referential integrity to work. I.e., reference semantics is exactly that of textual substitution. This is very desirable, as it allows ordinary refactoring of DFDL schemas to share common decls/defs to work in the expected manner. 

To me this is very desirable, and is a primary composition principle which will allow creation of complex schemas from simpler parts.

On Mon, Sep 7, 2009 at 11:55 AM, Alan Powell <alan_powell@uk.ibm.com> wrote:

All

Attached is the description of the new DFDL scoping rules.

We did not discuss the rules for simpleType derivations so I have assumed that it uses the same rules as simpleType reference, namely that the properties are merged and there must not be any duplicate properties specified.

I have removed most of the complicated examples as they no longer apply.



Alan Powell

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