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

[Mike Beckerle]

To add a thought. Same issue arises with lengthKind='pattern'. Doesn't
interact well with initiator/terminator.

On Sat, Oct 22, 2011 at 2:12 PM, Mike Beckerle <mbeckerle.dfdl at gmail.com>wrote:

> 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
>
>


-- 
Mike Beckerle | OGF DFDL WG Co-Chair
Tel:  781-330-0412
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