Hi Mike,
I have always assumed that it works like this:
The Prefix region includes leading alignment, leading skip and initiator
The Content region contains the data, and the lengthKind property
describes how to determine the content length
The Suffix region includes Terminator and trailing alignment.
The lengthKind property describes the content region, and is not examined
until the Content region is reached. So the element's iniitator, if
defined, is not included in the length described by the prefix length.
If you view the prefixed length as describing the length of the *element*
(i..e its entire representation ) then this definition is not intuitive.
But I have always viewed lengthKind='prefixed' as being like the other
lengthKinds - it describes the length of the element's *content*.
So it's a consistent definition, but is it useful? I think so. In my
experience, prefixed lengths tend to be applied to complex elements (
structures ) rather than simple values. In such cases, the content of the
complex element will always be either a sequence group or a choice group,
and any initiator/terminator can be located on that group..
regards,
Tim Kimber, Common Transformation Team,
Hursley, UK
Internet: kimbert@uk.ibm.com
Tel. 01962-816742
Internal tel. 246742
From: Mike Beckerle
To: dfdl-wg@ogf.org
Date: 22/10/2011 19:12
Subject: [DFDL-WG] lengthKind='prefixed' clarification needed
Sent by: dfdl-wg-bounces@ogf.org
For agenda/issues list
With respect to lengthKind='prefixed'. I'm concerned that there's a
complex interaction with initiator/terminator.
Can we have a prefix length and an initiator and terminator as well? If so
which comes first, and if it's the prefix does the prefix length include
the length of the initiator and terminator?
The grammar as written in current draft of spec has the initiator first,
then the prefix, then the content, and then the terminator. I think this
is wrong. I mean we can make it work, but it's not a useful, nor intuitive
behavior.
If we're going to fix this, I think we should make prefixed an alternative
to initiator and terminator, so that you can't have both on the element.
The alternative is to change the order around. Because initiator and
terminator can each be lists of alternative choices, the only sensible
composition of prefixed with these has prefix length providing the length
of a syntax which includes static initiator and terminator fields, which
are sort of like static padding to be trimmed off the string before
extracting the value.
E.g., prefix length of 10 preceeding these characters: [[123456]]
But,....this is obscure enough that I'd rather make prefix length
exclusive of initiator/terminator. I.e. Schema Def Error if both are
specified.
Rationale: Even if such formats are possible, and even if they do exist
somewhere, it's possible to model this format differently with hidden
fields, lengthKind='explicit' etc., so it's not like removing this complex
interaction of prefix with initiator/terminator reduces DFDL's expressive
power in any way.
Summary: To reduce complexity, suggest that lengthKind='prefixed' is
exclusive of both initiator and terminator properties directly on the same
element. Schema Definition Error if both are specified.
--
Mike Beckerle | OGF DFDL WG Co-Chair
Tel: 781-330-0412
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