[DFDL-WG] Notes from 2007-09-12 call
Mike Beckerle
beckerle at us.ibm.com
Wed Sep 19 07:43:20 CDT 2007
Comments below in BLUE
Steve Hanson <smh at uk.ibm.com>
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09/19/2007 06:04 AM
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Re: [DFDL-WG] Notes from 2007-09-12 call
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.
Inverting will generally not be possible. Just make the example
dfdl:length="{ ../count * ../scale + 1 }" How do you split up the length
into count and scale?
In your example, I would expect the count field to have an
outputValueCalc="{ ../x.length() - 1 }" (I'm assuming the field with the
length calculation formula is named "x".)
In general, when something uses something else in it's calculation (length
or just the value - inputValueCalc), then the inverse is outputValueCalc
on the contributing parts.
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?
It means the box is potentially larger than the contents. If it isn't at
least as big it's an error. If these lengths are data dependent it could
be a processing error. Otherwise a schema definition error.
Draft 025 discusses this in the part on sequences with length.
We also could disallow this case if we want for now, knowing we could add
it back if we want.
One can always convert this case into the one below by wrapping the
sequence's child elements in an array element, with array occurrences
determined by "fillAvailableSpace" policy. If we allow this at all, I
think this should be the way we explain the semantics of it. (Though with
the inserted array the paths would all change which is undesirable. - so
we would say it works like this, but without the paths being changed...)
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
Whooops. Forgot this. We need a concrete proposal here.
Regards, Steve
Steve Hanson
WebSphere Message Brokers
Hursley, UK
Internet: smh at uk.ibm.com
Phone (+44)/(0) 1962-815848
Mike Beckerle <beckerle at us.ibm.com>
Sent by: dfdl-wg-bounces at ogf.org
12/09/2007 21:20
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[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 at us.ibm.com
508-599-7046
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