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

Nico Williams <nico@cryptonector.com> Tue, 11 March 2014 00:15 UTC

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Date: Mon, 10 Mar 2014 19:15:06 -0500
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From: Nico Williams <nico@cryptonector.com>
To: John Cowan <cowan@mercury.ccil.org>
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Cc: Paul Hoffman <paul.hoffman@vpnc.org>, "json@ietf.org" <json@ietf.org>
Subject: Re: [Json] Regarding JSON text sequence ambiguities (Re: serializing sequences of JSON values)
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On Mon, Mar 10, 2014 at 6:59 PM, John Cowan <cowan@mercury.ccil.org> wrote:
> Nico Williams scripsit:
>
>> jq handles the ambiguity by requiring top-level values to be
>> "terminated" by any of: unambiguous top-level values, whitespace, or
>> EOF.  Thus the sequence of null, true, false, 1, and 2, requires
>> whitespace between every value.
>
> Why is that?  The input "nulltruefalse1 2" would be unambiguous.

The input "[nulltrue]" is clearly an error.  The input "nulltrue"
could either be: an error, or null then true.  Since there's two
possible interpretations of "nulltrue" it is ambiguous.  But you're
right that one could take the position that the "e" in "true" and
"false" terminates those values, and the second "l" in "null
terminates that one, and that resolves that ambiguity.  But there's
still an ambiguity in an online parser: is online parsing of
"[nulltrue]" to produce an array element value of null followed by an
error, or is it to produce just the one error?

Given that we have to separate top-level numbers with whitespace
anyways, I think the safe position is that "nulltrue" and so on are
invalid and that when emitting top-level values other than
arrays/objects/strings (and even then anyways) the encoding
application should follow the encoded value with whitespace (a
newline, preferably).

(I say encoding application because I believe an encoder is not
defined in such a way that permits it to emit a sequence of more than
one top-level values in a single octet stream output.)

Nico
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