Add proposal doc about standard profile for review
- Updated the version number and the revision date.
- Enhanced the explanation of certain concepts and restrictions to improve clarity.
- Revised some sections for better readability and improved grammar.
- Included new sections explaining how to request or enable the Standard Profile.
- Expanded the section on use cases and mechanisms for requesting the Standard Profile.
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+# Proposal: DFDL Standard Profile
+
+#### Version 0.3 2023-12-08
+
+## Introduction
+
+In attempting to integrate Apache Daffodil with other data processing software, the need to make
+DFDL schemas interoperate properly in conjunction with other data models has arisen.
+
+Other tools such as Apache NiFi, Apache Drill, Apache Spark, etc. have data models which are
+powerful, but not as expressive as DFDL.
+
+DFDL's data model is a simplification of
+[XML Schemas's Post Schema Validation Infoset (PSVI)](https://www.w3.org/XML/2002/05/psvi-use-cases);
+however, even this causes problems.
+Most other data processing systems were not designed with markup languages in mind, but rather for
+structured data.
+
+The following things are allowed in DFDL v1.0, but are difficult to map into most data models:
+
+- anonymous choices
+- duplicate element child names
+- namespaces that are different, but where the prefixes are not unique
+- global names for element children
+
+A more restrictive subset of DFDL, a _standard profile_, is needed which can be enforced (on
+request) to ensure that DFDL schemas will be usable with a variety of data processing systems.
+Creating DFDL schemas that adhere to this standard profile ensures maximal interoperability,
+including the ability to convert into JSON without name/namespace collisions.
+
+This is a proposal for a switch/option to be added to Daffodil which turns on enforcement of this
+standard profile (which is a subset of DFDL).
+
+## Standard Profile Restrictions
+
+### No Anonymous Choices
+
+Choices must be the model groups of complex type definitions and are not allowed in any other
+context.
+
+Each choice branch must begin with a different element. (This is already a XML Schema requirement -
+Unique Particle Attribution.)
+
+### Group References Cannot Carry DFDL Properties
+
+Group references are allowed, but DFDL format properties cannot be expressed on group references; hence,
+combining those properties with those of the group definition is not required.
+
+Note that dfdl:assert, dfdl:discriminator and other DFDL *statements*, not format properties.
+These may be expressed in group definitions, but not on group references.
+
+While most data structure systems do not have this notion of reusable groups, when restricted as
+described, reusable groups are something users could implement by way of a simple macro
+pre-processor, so having this in the standard profile really does not create any particular
+challenge when mapping from DFDL standard profile schemas into any data structure system.
+
+Groups and group references are used heavily in DFDL schemas to push down complexity like
+discriminators that are reused in many places.
+Allowing groups and group references reduces the difficulty of converting many large DFDL
+schemas to conform to the standard profile, and makes this possible without introducing many
+otherwise unneeded element and type definitions.
+
+### No Element References
+
+There is no corresponding form of sharing in most data structure systems.
+
+### No Namespace-Qualified Names
+
+Only elementFormDefault 'unqualified' is allowed.
+Note that this is the default for XML Schema and DFDL.
+
+### Unique Namespace Prefixes
+
+All namespace prefixes must be unique in the entire schema.
+
+This enables one to create unique identifiers by concatenating prefix_local to create global names.
+
+### All Element Children Have Unique Names
+
+All children element declarations must have unique names within their enclosing parent element.
+
+#### Discussion
+
+Note that this causes issues in a number of large DFDL schemas which attempt to implement a
+single DFDL schema that is capable of handling multiple versions of the data format.
+
+In this case, the schema uses a construct like:
+```xml
+<choice dfdl:choiceDispatchKey="{ ... }">
+ <sequence dfdl:choiceBranchKey="C">
+ <element name="VersionC" type="zString"/><!-- zero-length string -->
+ <element name="hdr" type="hdr_version_C_type"/>
+ </sequence>
+ <sequence dfdl:choiceBranchKey="D">
+ <element name="VersionD" type="zString"/>
+ <element name="hdr" type="hdr_version_D_type"/>
+ </sequence>
+</choice>
+```
+In the above, you can see that there are two separate element declarations named hdr, of different types.
+This allows common sub-structure that is the same in versions C and D to be addressed by path expressions that are
+polymorphic. They do not have a path step component that identifies the version.
+
+However, if we require all children to have unique names, then this would have to be elements with distinct names on each
+branch such as hdrC and hdrD, and then paths, even those reaching sub-fields that are common to both versions,
+have path steps that are specifically requesting a particular version.
+
+This is a bit painful particularly if there are few differences between versions, and many expressions that
+need to reference into the common fields, because
+all such expressions would need to be duplicated for version C and version D.
+
+An alternative solution is that this could be overcome by a way of creating path expressions with wildcards in
+them eg., ".../hdr*/...".
+An extension of this kind in DFDL has already been proposed/discussed some time ago by the DFDL workgroup, but has
+not yet turned into a formal proposal. (The DFDL4Space implementation by the ESA has a kind of wildcard feature like
+this as a DFDL extension.)
+
+Another way of addressing this is to refactor the schema to put common fields outside the choice, and
+have a version-driven choice only for points where the different versions diverge.
+This has a downside in that one point in the schema which is version sensitive becomes perhaps many such points, so
+the places where version-distinctions are required are less predictable to a schema user.
+
+This also requires a more sophisticated DFDL schema generator that takes into account multiple versions, computing their
+deltas, and generating common schema when it can.
+This is, however, not the way some large schemas were created, as these schemas are machine generated from the
+individual format specifications, without performing an analysis of what is common and what is unique.
+
+### Nillable Simple Types Only (TBD: May not be necessary)
+
+Nillable is allowed only for simple type elements.
+
+Note that in DFDL, element declarations express nillability, not type definitions.
+
+#### Rationale
+
+DFDL does not allow the same representation for nilled complex elements as it does for nilled simple elements.
+The representation DFDL allows for nillable complex types is only empty string whereas for simple types a variety of
+dfdl:nilValue values can be expressed.
+
+Hence, omitting this feature removes an awkward corner case.
+
+### Element Name/Identifier Restrictions
+
+Element names must consist of all non-whitespace characters from the
+Unicode basic multilingual plane (no surrogate pairs in element names).
+
+They may not contain any control characters (Unicode class Cc) and may not contain various punctuation
+characters (Unicode classes Ps, Pe, Pd, Pc, Pf, Pi, Po, nor $).
+
+This is a lowest-common denominator of identifier rules intended to allow
+DFDL schema identifiers to be mapped into ANY programming language or
+structure declaration language, while at the same time allowing use of
+Unicode characters.
+
+Element names may not begin with a digit.
+
+Users are encouraged to use "A-Za-Z0-9" only as some systems may not allow use of Unicode characters in
+identifiers.
+
+Element names may not begin with any prefix defined as part of the schema
+followed by an "_" as this could be ambiguous with names being made globally
+unique by appending prefix, "_" and local name.
+
+### String Content Restrictions
+
+Schemas may only be written in UTF-8 encoding.
+
+#### Rationale
+
+Simplify implementations.
+
+### Import `schemaLocation`
+
+Imported files - a namespace must be imported from a single unique imported file.
+A single schema may not contain different import statements for the same
+namespace which specify different files.
+
+This is a practical requirement in XML Schema and in Apache Daffodil today, but should be made explicit.
+
+If the `schemaLocation` begins with a "/" it is interpreted as an absolute path, otherwise a
+relative path. Both may be interpreted relative to a classpath.
+
+## Existing DFDL Restrictions
+
+Just as a reminder, the above standard-profile restrictions go on top of DFDL
+existing limitations on XML Schema such as:
+
+- arrays/optional - only for elements
+- no mixed content
+- no complex type derivation
+- no attributes
+- limited set of simple types
+- pattern facets only for xs:string elements
+- other facet restrictions by type
+
+## Possible additional restrictions
+
+### Troublesome Placement of dfdl:assert and dfdl:discriminator on Sequences & Choices
+
+The DFDL v1.0 rules about sequences/choices and statement annotations on them are confusing.
+In particular, a dfdl:assert or dfdl:discriminator with testKind 'expression' appears lexically at
+the top of the sequence/choice, but is executed after the sequence/choice content has been parsed.
+
+This is sufficiently error-prone that the standard profile should disallow
+it, requiring instead that an inner sequence carrying the assertion or
+discriminator with NO child content, be inserted in the sequence at the
+point where the evaluation is required to occur.
+
+# Requesting/Enabling the Standard Profile
+
+If the standard profile is requested, then use of constructs outside of the standard profile is a
+Schema Definition Error.
+
+## Use Cases
+
+A DFDL schema author may want to, by default, state that they want to conform to the standard profile
+for new DFDL schemas they create.
+The purpose of this is to maximize the utility of these new schemas when used with a variety of data systems.
+
+However, a DFDL schema author may also have a mixture of DFDL schema projects they use or maintain, and some of them
+may pre-date or intentionally not use the standard profile; hence, individual schema projects must be able to
+carry a declared intention of whether they do, or do not, expect to adhere to the standard profile.
+
+Finally, an individual schema project may contain a variety of subsections, some of which use the standard profile, and
+some of which do not; hence, individual DFDL schema files must be able to declare their use of the standard profile, or
+intention to not use it.
+
+### Mechanisms for Requesting the Standard Profile
+
+The following ways should be available for a schema author to tell Daffodil they want enforcement of
+the standard profile (or not).
+
+- *Home Directory Properties File* - a file such as ~/.daffodil or daffodil.properties should
+ contain a default value for choosing the standard profile. This default value can be overridden by
+ other mechanisms which have higher priority.
+- *Schema Project Properties File* - a Daffodil config file at the root of the Daffodil schema
+ project directory tree, should be able to specify a default value for use by the schema project.
+
+ If the XML config file system is superceded by some other properties file per-schema, then that
+ same file should enable specifying the standard profile is (or is not) to be used.
+- *Per Schema-File property* - A property expessed at the top level of a DFDL schema file should
+ indicate that the standard profile is requested.
+
+ All schemas imported or included by a schema that requests the standard profile would be assumed
+ to also be required to obey the standard profile.
+
+ However, the inverse is not true. Including/importing a schema file that requests the standard profile does not
+ change the profile behavior (using or not) of the schema doing the include/import.
+- *Command Line Interface (CLI) Option* - The daffodil CLI should take an option (perhaps
+ --standard-profile yes/no) telling it the
+ schema is expected to obey the standard profile. The CLI should also obey the other mechanisms
+ listed here. The CLI flag should override properties files (homedir, or schema project
+ expressed), but not things specified directly in the schema files themselves.
+
+An opposite expression - that a schema explicitly is known to require more than the
+standard profile, should also be allowed to be placed in any of these locations.
+
+Including such an explicitly non-standard-profile schema into a schema that requests the standard
+profile should cause a Schema Definition Error.
+The inverse however, is not true.
+A schema that explicitly obeys the standard profile can be included/imported into any schema.
+
