[whatwg] Recursion and loops of Microdata items

[Philip Jägenstedt]

On Tue, 28 Jun 2011 16:51:25 +0200, Tomasz Jamroszczak <toja at opera.com>  
wrote:

> Dnia 08-06-2011 o 21:51:57 Ian Hickson <ian at hixie.ch> napisał(a):
>
>> The goal of itemref="" was just to have a way to handle cases where you
>> have an item's properties scattered around a document.
>>
>> It's caused us more difficulties than helped anything, as far as I can
>> tell. Has anyone implemented it or used it and liked it? I'd be fine  
>> with
>> removing it if it's not a lot of trouble...
>>
>> I haven't fixed the algorithm to be written more simply, nor fixed the
>> loops in the JSON stuff, because if we remove itemref="" then those
>> problems just go away. If we want to keep itemref="", though, I will fix
>> them. Any opinions one way or the other?
>
> Summary:
> 1. itemRef attribute must stay.
> 2. Loops are allowed in our implementation.
> 3. Infinite recursion prevention should be simple.
>
> 	For sure itemRef attribute of Microdata have to stay, because it makes  
> possible separation of data (the Microdata item properties, the  
> semantics) and view (where contents of those properties should be laid  
> out for browser user).  Without itemRef, Microdata becomes "Picodata".
>
> 	After some internal discussion we've came to conclusion that loop  
> removal is possible to implement using DFS Gabow's algorithm of finding  
> strongly connected components of a graph.  But it doesn't seems worth  
> the implementation complexity.  So instead of Microdata item loop  
> removal, we're more liberal and allow loops.
>
> 	But then, what to do when translating Microdata to other format, such  
> as stringification to JSON in Drag'n'drop?  The JSON itself is quite  
> primitive when it comes to stringification loops - it just throws an  
> exception.  We thought we'll be more flexible.  We'll make  
> stringification "as best as possible", and cutting only the last  
> offending link of a cycle.  See  
> http://people.opera.com/tjamroszczak/microdata/microdata-loops.png .   
> Unfortunately it means that items which belong to Microdata item loops  
> sometimes will lose properties, and it depends on from where the cycle  
> was reached (see point A1 and A2 in the image).
>

Another option would be to let stringification of microdata with itemref  
loops throw and exception and fail, similar to what happens when you try  
to JSON.stringify a JavaScript object with cyclic references. The exact  
approach taken here probably isn't very important, since itemref loops  
aren't valid in the first place.

Note also that other algorithms defined in terms of items and their  
properties need to handle loopiness in some way. That's currently RDF,  
vCard and iCal conversion. Perhaps something like "loopy item" could be  
defined and those algorithms could skip loopy items wherever they occur?  
Simply failing is also an acceptable solution, IMO.

-- 
Philip Jägenstedt
Core Developer
Opera Software