Based on experience creating DFDL schemas and running them using Daffodil, there are a number of things not currently described in the DFDL v1.0 standards documents that would be helpful in modeling.
They are described here in no particular order, along with some discussion of them.
DFDL v1.0 was designed to standardize the behaviors of many ad-hoc product-specific data format description capabilities.
One set of goals for DFDL v2.0 will be to advance the state-of-the-art, and describe data formats that none of the prior-generation ad-hoc product-specific data tools have been able to address effectively.
The DFDL Workgroup of the Open Grid Forum also has been saving some issues targeted at DFDL 2.0.
One of the first things people want to model in DFDL always seems to be a binary legacy document formats like RTF or older MS Word documents. These have recursive structures where a section can contain text and other sections. DFDL v1.0 was not designed with document formats in mind, but rather with more traditional "data sets" or files of data in mind.
One advantage of DFDL without recursion is that it is not a 'Turing complete' language. This is helpful from a security perspective. Adding recursion may break this boundary. Of course since DFDL has a rich regular expression capability, and its own backtracking, one can still create schemas that take absurdly long to execute even without recursion, so maybe this is not an issue.
Layering - Data Source/Target Indirection
Often one needs multiple passes. The value of some element, which might be a string, a hexBinary, or an array of bytes, wants to be used as the input for more parsing.
Some data must be decoded - lines may be folded to a max length, but logically must be reassembled. Data can contain compressed regions.
In addition, one needs the ability to verify (parsing) or compute (unparsing) checksums, CRCs, or parity elements based on the contents of a layer.
Complex Representations for Simple Types
XML Schema's simple vs. complex type distinction is quite painful. Often you want the logical result to be some computed element (using dfdl:inputValueCalc) of simple type, but one must have a hidden sequence group of several elements that are the more complicated representation details. In DFDL v1.0, one must of necessity model such a thing as a complex type, so that you have a place where both a hidden group and the 'value' element of simple type can live side by side.
A means is needed to embed a hidden group within the definition of a simple type, so that the hidden group is implicitly laid down next to the element having that simple type.
The ability to have data of simple type become XML attribute values would go a long ways to making DFDL-created-XML more human-friendly, and more efficient - attributes don't repeat the attribute name twice. E.g., <bigLongName>0</bigLongName> becomes bigLongName="0". This is very close to the same level of density as JSON representation.
More XML Schema Constructs
Several things in XML Schema seem to be missing. Unless there is a clear reason not to support them, it would be helpful. This list includes at least:
- repeating sequence and choice groups (minOccurs and maxOccurs)
- complex type derivations
- attributes (already mentioned above)
- substitution groups - to enable separate compilation of multi-part DFDL schemas that are very large. (might be overkill - unclear if this is truly needed.)
XML Schema 1.1
This new standard supports richer validation rules. They are useful since XML Schema 1.0's validation capabilities are so limited. Alternatively, embedding schematron rules directly in a DFDL schema is an option.
Delimited by Next Item
The ability to say that an element or group is delimited, but that it is delimited by the boundary of finding the initiator of the next element or group would simplify the description of many formats.
Another formulation would be to specify that an element/group is delimited, but that the terminating markup is not consumed, and hence, must be consumed by whatever comes next in the model.
Character Class Entities
We badly need an entity that means 'any whitespace that is not a line ending'. This avoids the specification of separators like:
which is sometimes needed when %NL; is the terminator and you want to distinguish the separator and terminator. The %WSP+; entity encompasses all whitespace.
A possible good name is %LSP*; where the "L" is for "Linear" as in "within a line".
For both parsing and particularly for unparsing, one often must measure something. Fixed length formats often have tabular layouts in them, and the widths of the columns need to be computed from the longest string in the data.
This is a form of multi-pass (aka Layers), but for the unparsing case, it's really just about computing the length of something from values in the infoset, a capability DFDL already has. The need is to just generalize the calculation capability with some sort of map/reduce on arrays.
Extensions with User-defined Functions
Some computations are too complex to render directly in DFDL expressions. The ability to add functions in an orderly way is necessary.
Examples of this are computing the CRC for a network packet or checksums/hashes for other data structures, or encrypt/decrypt and compress/decompress.
These functions need to be able to examine the Daffodil processor state (Infoset and data streams).
Note that checksums would need to work in conjunction with layering, as a layer would supply the 'raw' data for the checksum, but also allow the structure of that data to be expressed so that it can be parsed.
No Network Mode: This is less a DFDL language feature than a characteristic desirable for all implementations of DFDL. Applications using DFDL must be able to execute both in an environment which has no access to the internet, and even on machines that do have such access, in a mode where they make no attempt to access anything remotely.
Regular Expression Enhancements
DFDL schemas involve some large and complex regular expressions. Even the most advanced regular expression languages lack convenient ability to define a given construct once and name it, and then reuse it by somehow referencing that name. This would dramatically ease construction of regular expressions, and it is simply basic software engineering that large and complex things need to be named and reused, not duplicated.
A coherent proposal here would be very useful outside of DFDL/Daffodil.
Graph of Nodes and Edges Data
Users have requested a way to describe data structures with pointers linking objects to other objects. E.g. the arbitrary link structure a typical *NIX file system image can contain, with its hard links. This is related to the next topic about offsets to data.
Offsets to Data
Some data formats contain header information including a table of offsets in the file to later parts of the data. The ability to directly express offsets within the data (absolute, or relative to some anchor, such as the end of the table of offsets) would make describing these kinds of data files much more direct.
A good example of such a format is TIFF.
Expression Language - Let - Return Construct
In many cases expressions in DFDL's expression language become massively redundant - things that should be expressed once have to be copied and pasted repeatedly (See IPSrcGrp in pcap.dfdl.xsd for an example.)
To fix this, we should enable at least one construct from XQuery to be used in addition to just the XPath-like language specified in DFDL v1.0, and that's the LET-RETURN construct.
A trivial example:
It is simply basic software engineering that large and complex things need to be named and reused, not duplicated. DFDL as a language should have features to allow good basic practices to be followed.
Complex Type Derivation - Allowing Properties on ComplexType Definitions
Complex type elements often want to have different default properties than simple types. E.g., dfdl:lengthKind might be explicit for simple types, but implicit for complex types. This makes putting a mixture of simple and complex type elements in the same file painful. If you have a type that you reuse many times, every time you reference the type, you have to add some properties (like dfdl:lengthKind="implicit").
This would be ok, except that you cannot move these added properties down onto the complex type definition. You can with simple types, as the properties on the simple type are combined with those of the element. But complex types don't offer this in DFDL v1.0. This makes you want to use element references a lot in order to hide this noise, but those require global element names, which interferes with namespace management and the very desirable elementFormDefault='unqualified'.
For DFDL 2.0 we should fix these annoyances. We should allow complex type derivations, combining properties on them in the exact same manner as we do for simple types. We should allow properties to be annotated on a complex type definition and for those to be combined with those on an element referencing that type.
Separate LengthKind for Simple and Complex Types
Very often one wants dfdl:lengthKind='delimited' or dfdl:lengthKind='explicit' for simple types, but dfdl:lengthKind="implicit" for complex types. Separating the dfdl:lengthKind into two properties, or having the ability to specify either way, would simplify many schemas that otherwise have a error-prone need to have a dfdl:ref='complex' format reference on every element of complex type to override the default dfdl:lengthKind. That or you have to split the schema and put all simple types in one file (and use only those simple types), and all complex types in another.
Table and Range Lookups
(see concrete Proposal)
Often one has a representation containing enumerations - integer values - which have symbolic meanings. The parsed result from such data wants to contain strings so the logical infoset is readable and understandable. A means is needed to specify a table of integer constants and their corresponding strings, to be used for parsing, and unparsing. Ranges are a generalization where a symbolic string is used to name all the integers that fall in a range.