[DFDL-WG] lengthKind='prefixed' clarification needed

[Mike Beckerle]

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]]

<element name='x' type='int' dfdl:initiator="[[" dfdl:terminator="]]"
dfdl:lengthKind='prefixed' .../>

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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