[DFDL-WG] Nested Delimited Constructs - For discussion Wed 3/28

[Mike Beckerle]

I believe the language below, while needing examples perhaps to clarify, 
solves the problem of how to deal with nested delimited constructs where 
outer delimiters are terminating inner nested constructs.
I'd like to see if people understand it on our DFDL call on wednesday.
...mikeb


Nested Delimited Constructs
There are two kinds of terminating delimiters:
postfix separators (TBD: currently only on arrays)
terminators 
Both of these can be optional. (TBD: can postfix separators be optional? I 
believe there is no finalSeparatorCanBeMissing though there is a 
terminatorCanBeMissing property) 
This means the parser can encounter a terminating delimiter from an 
enclosing array or group to indicate the termination of a nested array or 
group. 
The behavior of parsing when it encounters one of these terminating 
delimiters, that is, one that is defined on an enclosing construct, is to 
indicate that it found a terminating delimiter of some sort, but to not 
consume that terminating delimiter when computing the new pos result. That 
is, the parse function succeeds and returns a new pos, value, etc., but 
the pos reflects the position where the terminating delimiter begins.
The invariant that this insures is that a parser for any construct 
consumes its own delimiters and only its own delimiiters. 
For example, the parse function for a sequence group which has separators 
specified will recursively parse the elements it contains; however, if one 
of those elements' representation is terminated by finding the enclosing 
sequence group's separator, then that separator will not be consumed, and 
when the recursive parse unwinds back to the parse function of the 
enclosing sequence group, the separator will then be consumed by the 
sequence group's parse function which is prepared to recognize it and 
advance past it.
This principle works regardless of how deeply nested the constructs are. 
Parsing must, however, take into account the complete set of terminating 
delimiters that it might encounter, along with the escape/quoting schemes 
that can be specified for them which allow them to appear as content 
rather than as delimiters.


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