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RE: CIF-JSON draft 2017-05-08

On Monday, May 08, 2017 2:29 PM, Marcin Wojdyr wrote:
>> The other issue is indeed the one you describe, that CIF overall does
>> not draw an inherent distinction between unlooped items and items
>> presented in a single-packet loop.  The mmCIF and other DDL2
>> dictionaries in fact explicitly disclaim any semantic distinction
>> between those alternatives
>So DDL2 cannot ensure that, for example, _entry.id is a string, not a list of strings?
>I didn't realize that. It looks to me as a step back from DDL1 which can define _list yes/no/both.

You understand correctly: DDL2 does not require _entry.id values to be presented unlooped when, as is usual, that item takes only one value in a given data block.  The same applies to all other items, on a category-by-category basis.  From a DDL2 perspective, categories are intentionally analogous to relational tables, so it doesn't make any sense to forbid certain ones to be presented in loop form.  I am uncertain why DDL2 didn't simply insist that all data be presented looped (which is effectively what I am proposing CIF-JSON should do), but with it having allowed unlooped presentation at all, I guess it seemed needless make a distinguishing feature of whether categories could be presented unlooped.

Interpreting that as a backward step depends on asserting some kind of inherent significance to whether an item or category is presented looped.  There isn't any.  The important characteristic is not the *presentation* of the category but its *multiplicity* and key structure.

Anyway, that is all water under the bridge.  The fact is that whether a given CIF item is presented in a loop or not simply has no inherent meaning.  It may be that there is a reason to preserve that strictly-syntactic distinction in CIF-JSON, but it is not worth preserving purely for its own sake.



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