Some thoughts since last week's call: 1) Expression language We've not thought much about how expressions will work on output. It's fine to say something like dfdl:length="..\count+1" when parsing, but what happens on output. I think we should not try to reverse engineer expressions, and rely on the user to set output fields correctly. So, taking my example, on output we would assume count had been set by the user, apply the expression to calculate the intended length of data, then apply padding etc rules as needed. Can we generalise that philosophy across all our uses of expressions? If we can't then perhaps that places a bound on the actual uses of expressions that we permit. 2) dfdl:length for sequences We have three cases here: a) Empty sequence - we agreed to disallow this b) Non-empty normal sequence - what does the length mean here? c) Non-empty sequence used as box array - the motivating scenario I think we should also disallow b). If we are disallowing a) on the grounds of not using sequence with a length to model opaque data then we should also disallow b). 3) Handling of comments in data Mentioning this as it was discussed on the call but not minuted. Example: A record is allowed to be followed by multiple free form text lines where the first such line contained //ADDINFOSTART, and the last such line contained //ADDINFOEND. This was put forward as a use case for regular expressions but it was noted that an explicit dfdl:commentScheme based on, or maybe an extension of, the existing dfdl:escapeSchema property would be a more natural solution for users Regards, Steve Steve Hanson WebSphere Message Brokers Hursley, UK Internet: smh@uk.ibm.com Phone (+44)/(0) 1962-815848 Mike Beckerle <beckerle@us.ibm.com> Sent by: dfdl-wg-bounces@ogf.org 12/09/2007 21:20 To dfdl-wg@ogf.org cc Subject [DFDL-WG] Notes from 2007-09-12 call Mike Beckerle, Alan Powell, Steve Hanson, Suman Kalia attended. Discussed these questions from Alan about expression language. 1. Accessing hidden values - it seems inconsistent to allow access to hidden values when xpath is used within the DFDL domain but not when used outside. 2. Where xpath is allowed in the schema - It is currently allowed in an arbitrary set of properties (initiator, terminator, separator, occurseparator, null, etc ). Why not allow it everywhere? Wr.t. (1) we decided this is correct. path expressions for dfdl properties can see hidden elements, path expressions in other places (e.g., schematron assertions) cannot. Wr.t (2) we decided that expressions should be allowed in principle everywhere for the value of any property; however, there may be exceptions for certain properties. Particularly, it seems some enum-valued properties are unlikely to ever want to be expressions. Example: dfdl:representation. However, it was also pointed out that once we put selectors back into the language you can interleave multiple formats in the same schema, and for any enumerated property you could just have one selector-chosen format for each possible value of the enumerated property. The reason we don't want a blanket statement that you can have expressions anywhere you need a property value is that there is some potential that this makes implementations unnecessarily complex due to the excess flexibility. Digression: (This added by MikeB - was not part of the call today.) Consider dfdl:byteOrder=" if (../../x = 'B') then 'bigEndian' else if (../../x='L') then 'littleEndian' else 'I don't know' }" DFDL implementations must be prepared to cope with recieving "I don't know" as the proposed value for the byteOrder. This is a schema definition error, but it is happening at run time so becomes a processing error. The only way to rule this out is to treat enumerated property values not as strings but as an enum type and force the expressions that compute them to return an enum type, not a string. This is a kind of type inference I had hoped implementations would not need. Selectors have the advantage of being statically verifiable. i.e., each selected format is known to use a value of the enum that is valid or a diagnostic could be issued by the DFDL processor. If we allow an arbitrary expression to return the value of an enumerated property then it presumably could also return a nonsense value: We discussed proposals circulated by MikeB: Here's an update to the first one. We decided sequences shouldn't be another way to carry opaque data. Easy and conservative way to fix this is to require the length of an empty sequence to be zero. Second proposal to eliminate hexBinary and base64Binary was discussed lightly. It was suggested that one could have both, and that would make it easy to explain what the hexBinary type is, because it is a shorthand for a string with encoding="hex", and similarly for base64Binary. We did not resolve this issue on the call. Finally, we discussed regular expression features for DFDL. There does appear to be need for regexp features to support parsing data which is delimited by changing data content. E.g. consider "12345Mike Beckerle". and a two-element sequence. One is a number which continues until the first non-digit character. The other is a string which begins with a non-digit character. Regexp length appears to be a good way to handle this kind of thing. Alan Powell has the action item to talk with the IBM internal TX product group. They have a speculative parser and so have fewer regular-expression features in their language. We want to understand how they deal with the header, body[], trailer use case. This case is where the data is lines of text, the header is the first line, the trailer is the last line, the body records are everything in between and there's no content that can be used to distinguish the record types. This is handled in some format-description systems with regexp features. In TX this is handled by speculative parsing and we want to understand how this comes out and if it is preferable to adding regexp features. Mike Beckerle STSM, Architect, Scalable Computing IBM Software Group Information Platform and Solutions Westborough, MA 01581 direct: voice and FAX 508-599-7148 assistant: Pam Riordan priordan@us.ibm.com 508-599-7046 -- dfdl-wg mailing list dfdl-wg@ogf.org http://www.ogf.org/mailman/listinfo/dfdl-wg Unless stated otherwise above: IBM United Kingdom Limited - Registered in England and Wales with number 741598. Registered office: PO Box 41, North Harbour, Portsmouth, Hampshire PO6 3AU