[Json] Regarding JSON text sequence ambiguities (Re: serializing sequences of JSON values)

Nico Williams <nico@cryptonector.com> Mon, 10 March 2014 22:06 UTC

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Date: Mon, 10 Mar 2014 17:06:43 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Paul Hoffman <paul.hoffman@vpnc.org>
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Subject: [Json] Regarding JSON text sequence ambiguities (Re: serializing sequences of JSON values)
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On Mon, Mar 10, 2014 at 3:19 PM, Paul Hoffman <paul.hoffman@vpnc.org> wrote:
> [ No hat ]
>
> On Mar 10, 2014, at 7:35 PM, Paul E. Jones <paulej@packetizer.com> wrote:
>> Why would sequences of objects not be preferred for logging.
>
> Because the log is always open for appending, so the sequence never terminates.

This.

Other use cases relate to transferring large sequences of texts.  For
example, serializing a database: it may be easier to arrange for the
encoding of such a serialization as multiple top-level values than as
one large array (because, e.g., the encoder may not support online
functionality).  Yes, yes, the application could emit a '[' to start,
',' between top-level values, and a ']' at the end.  But if the
sequence is indeterminate in length (because it is an event log
including future events as they happen) then the parser on the other
end had better be an online (at least as to top-level array elements)
parser!  Sure the application could specially handle top-level arrays
to "explode" them in an online manner, but this still requires the
parser to identify exactly where in the input stream each array
element ends.

IOW, some use cases are easier to code as sequences of top-level values.

> This, and many other related topics, were discussed at length earlier on the mailing list and the rough consensus was that the current wording was sufficient.

Yes, though it's worth going into this a bit further.

There are two ambiguities then.  The values true, false, null, and
numeric values are all ambiguous unless separated by whitespace or
other unambiguous values (strings, arrays, or objects).  The second
ambiguity is between true/false/null/numeric values and EOF: a parser
fed 'true' cannot produce that value (or a parse error) until one more
byte is read _or_ EOF is detected.

This means that top-level non-array/object/string values should be
immediately _followed_ by whitespace when sequences of of such
top-level values are emitted.  Note: _followed_, not _preceded_ --
it's OK to also precede them with whitespace, but it is necessary to
follow them with whitespace in order to avoid the EOF/error ambiguity.

Nico
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