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Re: Review: JSON-LD syntax

From: Gregg Kellogg <gregg@greggkellogg.net>
Date: Sun, 24 Jun 2012 18:10:08 -0400
To: Gavin Carothers <gavin@carothers.name>
CC: Manu Sporny <msporny@digitalbazaar.com>, "public-rdf-wg@w3.org" <public-rdf-wg@w3.org>, Franšoise Conil <fconil@liris.cnrs.fr>
Message-ID: <42A460EE-4CE6-4A65-9D17-69C1C9A25DC2@greggkellogg.net>
On Jun 24, 2012, at 6:01 AM, Gavin Carothers wrote:

> On Sat, Jun 23, 2012 at 12:47 PM, Manu Sporny <msporny@digitalbazaar.com> wrote:
>> On 06/19/2012 12:51 PM, Pierre-Antoine Champin wrote:
>>> ...
>> 
>>> S3.1.1 one-pass parsing sounds good, but how can I know in advance
>>> that the JSON-LD I get will respect the good practices that allow me
>>> to do so? As those good practices are SHOULDs and not MUSTs, I will
>>> never know for sure if I can use a one-pass processor or if it will
>>> fail. So I would recommend to add a parameter to the content-type
>>> (annex C) to indicate whether the JSON-LD guarantees that @context's
>>> and @id's are in first positions, so that I can chose the most
>>> appropriate parser.
>> 
>> I don't think we should tell folks that there are two types of JSON-LD
>> processors. Maybe the terminology needs to be changed because all
>> parsing is one-pass... it just can't happen until the @context is known.
>> We could re-name this to "efficient memory footprint processing", as
>> that's what it really is. It's a bit heavy-handed to say that input MUST
>> be ordered in a certain way. I think we'd rather let this be a social
>> contract between Web Services and clients of those services. That is, if
>> the Web Service wants to use a "efficient memory footprint" processor,
>> it can reject any input that doesn't have @context listed as the first key.
> 
> I'm confused how this works with JSON. "An object is an unordered set
> of name/value pairs", there isn't really a way to say that @context
> comes first is there? I mean in all the examples it does, but those
> are produced by hand. Nothing in JSON provides for any ordering of
> name/value pairs.

Gavin's correct, although a specific character stream may have an order, the JSON model [1] says that keys without objects are not ordered:

[[[
An object is an unordered set of name/value pairs.
]]]

When possible, it is a good idea to perform lexicographical ordering of keys, which leads to @context, @id, and @type coming out in order prior to keys that begin with an alphanumeric character, but implementations can't count on this.

In effect, this means that to implement a streaming parser, the processing of keys and values in an object needs to be deferred until an @context is seen. This certainly makes doing a streaming processor more difficult, and potentially to have little advantage over an in-memory processor.

Gregg


[1] http://www.json.org/

>> ...
>> 
>> -- manu
>> 
>> --
>> Manu Sporny (skype: msporny, twitter: manusporny)
>> Founder/CEO - Digital Bazaar, Inc.
>> blog: PaySwarm Website for Developers Launched
>> http://digitalbazaar.com/2012/02/22/new-payswarm-alpha/
>> 
> 
Received on Sunday, 24 June 2012 22:11:06 GMT

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