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Re: JSON-LD Telecon Minutes for 2013-05-14 / RDF-ISSUE-128 and RDF-ISSUE-129

From: Gregg Kellogg <gregg@greggkellogg.net>
Date: Sun, 19 May 2013 18:20:48 -0700
Cc: "'Linked JSON'" <public-linked-json@w3.org>, <public-rdf-comments@w3.org>
Message-Id: <7080B157-21D7-45CF-933B-DF50409D6AD5@greggkellogg.net>
To: Markus Lanthaler <markus.lanthaler@gmx.net>
On May 19, 2013, at 5:10 PM, Markus Lanthaler <markus.lanthaler@gmx.net> wrote:

> On Sunday, May 19, 2013 5:21 PM, Gregg Kellogg wrote:
>>> We did that quite some time ago but decided to move the conversion
>> between native types and strings to to/from RDF because that's where
>> the complexity has to live. If you are staying within JSON-LD you
>> shouldn't have to worry (or even know) about that.
>> 
>> Yes, we did discuss it before, but I think that the RDF round-tripping
>> loss, and general need for developers to be able to use native types,
>> have come into perspective. By always doing a full value-object based
>> transformation of typed literals, we eliminate the data loss issue, but
>> make it less convenient for developers to just use the native types.
> 
> You don't eliminate it, you just move it somewhere else. 

The point is that for transport issues, such as getting a JSON-LD representation of an RDF dataset, we retain full fidelity. To do that now, would require that the _use native types_ flag be set as an type parameter in the HTTP header, something we don't provide for AFAIKR. By having transport format use string representations of literals, we don't incur any data loss.

The idea behind putting it in the Expansion algorithm is to defer any data-loss to the end application which chooses to work with the data using native JSON values. Then it becomes a chose of the consuming application, rather than an assumption of the providing service which is serializing RDF as JSON-LD.

>> The problem is, that as a publisher, you really don't know what the
>> intention of the consumer is, so imposing some behavior, through
>> defaults in the core transformation algorithms, can lead to a bad
>> experience. If it's really for the convenience of the developers, then
>> doing it through algorithms run on the consumer side seems to really
>> fit this target, and allows us to be more comprehensive.
> 
> Not sure I buy that argument. You seem to assume that consumers will always
> expand/compact but never convert from or to RDF. I think that oversimplifies
> it too much. I do assume (and hope) that users of JSON-LD will use native
> types in 99.9% of the cases. Only for very special use cases (money) they
> might use typed strings instead.

I don't make any such assumptions. Data is transmitted in full fidelity, and the application decides if it wants to use native values.

>> My thought was that the fromRdf algorithm would always use full value
>> objects for typed literals (e.g., like _use native types_=false). An
>> added flag to expansion, passed through from compaction, flatten and
>> framing, would allow value objects with numeric or boolean XSD types,
>> to be transformed to the native JSON representation, or vis-versa. This
>> means that there could be data loss, but this would happen only in the
>> client application, where the application is in full control of the use
>> of the data.
> 
> Why remove it from fromRdf? A "client application" could call that algorithm
> just as well. 

If the client were taking another RDF representation and converting to JSON-LD, then sure. In any case, the client could string together a series of transformations to get what they want. I'm presuming that a JSON-LD client performs content negotiation with a servce to get what they want. If they prefer JSON-LD, they may get that and then transform to native types if so desired. If they get some other format (presumably triples or quads), they could proceed this by first using the fromRdf algorithm.

>> More specifically, in the Expansion Algorithm after step 8.4:
>> 
>> [[[
>> If the _use native types_ flag is true, the value of result's @value
>> member is a string, and the value of result's @type string is
>> xsd:boolean, xsd:float, xsd:decimal, xsd:double or a type derived from
>> any of these datatypes (as defined in section 3 of [XSD]), transform
>> the string into a native representation (using language currently in
>> section 10.6).
>> 
>> Otherwise, if the _use native types_ flag is false and the value of
>> result's @value member is a native value using canonical representation
>> for the datatype in result's @type member, defaulting to xsd:boolean or
>> xsd:double, depending on if value is a boolean or number.
>> ]]]
>> 
>> At one time, all this was encapsulated in the Value Expansion
>> Algorithm, but now it doesn't seem to be. Does this step capture all
>> the places where a value might need to be transformed? If not, then we
>> should consider re-writing Value Expansion and using that in the
>> different locations.
> 
> It does except if your value is already expanded, i.e., it is using @value
> already.
> 
> 
>> Alternatively, we could consider doing this transformation as part of
>> the Compaction or Value Compaction Algorithms, but Expansion seems
>> better to me.
> 
> That's exactly the problem is see. If we move these kind of transformations
> to compaction/expansion you will probably lose even more information.
> Currently, I can mix native types and typed strings in JSON-LD, e.g. I could
> use native booleans but literal doubles. I don't want that to be messed up
> when expanding/compacting because I'm staying within JSON-LD. Since RDF does
> not have any native types, I obviously have to accept the fact that
> everything will become a typed literal when converting to RDF and I may not
> be able to round-trip it cleanly in such cases.
> 
> I just don't think it makes sense to move this complexity away from the RDF
> conversion algorithms. The "problem" lies there and should thus be handled
> there. JSON itself has no range or precision restrictions at all. If this
> turns out to be a problem in the future parsers will handle it more smartly.
> I just happens that JSON works just fine for 99% of the use cases and such
> problems never arise. We already have an elegant solution for the other
> cases as well. You express your data as a string and add a type to it.
> 
> As it currently stands, I would -1 the proposal to move the transformation
> to expansion/compaction.

Did you listen to where this was discussed on the telecon? I think that imposing data-loss by default is a problem, and allowing the client to perform this operation makes sense to me, rather than have it imposed on them. If you really do object to this, then to satisfy Sandro's concern, perhaps the thing to do is to change the default for _use native types_ to false, and add it as a mime parameter.

Perhaps we should defer a resolution on this matter until you can join the telecon, presumably next week?

Gregg

>>>> Sandro Hawke:  we many need to refer to a different spec
>>>>  regarding futures - the DOM WHATWG one might change.
>>>> Sandro Hawke:  hard requirement is to refer to stable things. it
>>>>  is hard to argue that the "living spec" is stable
>>>>  ... not saying we change the reference, but change how we use
>>>>  the reference
>>>> Manu Sporny:  We don't actually reference the Futures spec
>>>>  directly. We only use the Future concept in our spec, not the API
>>>>  itself.
>>>> Sandro Hawke:  if they change Futures, then every piece of
>>>>  software using futures would be broken and have to change
>>>> Manu Sporny:  Being pedantic, but the spec wouldn't change, just
>>>>  the implementation.
>>>> Sandro Hawke:  The director probably won't be okay with that. You
>>>>  shouldn't build on specs that are not stable
>>>>  ... We have to hard-code it with the current view of futures
>>>>  so that if it changes, we use the old version of futures
>>> 
>>> Hmm... that kind of surprises me. JSON itself e.g. is not a IETF
>> standard but just an informational note. HTML5 is referencing a large
>> number of living standards:
>>> 
>>> http://www.w3.org/html/wg/drafts/html/master/iana.html#references
>>> 
>>> In this case I think it makes no sense to hardcode the reference to a
>> specific version because as Manu says, we just use the concept of a
>> Future. The JSON-LD API should be based on what browser vendors
>> implement - and that will be the WHATWG living standard.
>> 
>> The difference between DOM and JSON is that JSON quite stable, whereas
>> who knows how much change might come to DOM?
> 
> Fair enough, but does that really affect us?
> 
> Promises/Futures have been known since the 70ies so the concept is
> definitely not new. Our dependency is so loose that only a name change
> ("Future" to something else, e.g. "Promise") would affect us. It is true
> that implementations may have to be updated if the DOM spec changes but the
> idea behind that spec is to describe what browsers vendors implement, so it
> is actually the other way round.
> 
> 
> 
> --
> Markus Lanthaler
> @markuslanthaler
> 
> 
Received on Monday, 20 May 2013 01:21:17 UTC

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