Proposal for Requiring Save/Restore of Sets and Tags

[Jason Kraftcheck]

On 10/01/2010 02:59 PM, Carl Ollivier-Gooch wrote:
> On 09/29/2010 10:27 PM, Mark Miller wrote:

>>
>> In choosing a file format, there are several complicating factors to
>> consider:
>>    o Tags may be placed on entity sets as well as entities.
>>    o Entity sets may contain entity sets as well as entities.
>>    o The entity set containment graph may contain cycles.
>>    o The entity set parent/child graph may contain cycles.
> 
> I don't think it's particularly relevant to the way these things are
> likely to be stored, but I strongly disagree, semantically, with
> allowing cycles.  Also, it's harder to implement these the infinite
> depth retrieval with cycles (not super hard, but harder).
> 
> Containment can be interpreted as subset or member.  Subsets clearly
> can't be cyclical (okay, you could have improper subsets in a cycle, but
> why?).  Membership also doesn't make sense:  set A contains B contains C
> contains A?  Draw me that Venn diagram!
> 
> Parent-child semantics also don't make sense cyclically: A is the parent
> of B is the parent of C is the parent of A?
> 
> Finally, can anyone describe an actual use case for cycles?
> 

IIRC, the last time the group discussed this, it was decided that we should
allow cycles.  But even if we leave that as an implementation-defined
behavior, it is not relevant to this particular discussion.  When storing
iMesh data in a persistent format: if the implementation allows cycles then
it should be prepared to save/load sets with cyclical relations.

- jason