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

Matthew Morley <matt@mpcm.com> Tue, 11 March 2014 15:31 UTC

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From: Matthew Morley <matt@mpcm.com>
To: Nico Williams <nico@cryptonector.com>
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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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This issue came up with json-rpc structures and streaming json, which while
not directly related in that we expect to encounter only objects at the top
level... having guidance on handling top-level streaming is important for
json.

https://groups.google.com/forum/#!msg/json-rpc/fd1_anYEnDs/nnvnU51bfCoJ
*https://groups.google.com/d/msg/json-rpc/cXlc8bD-a_M/cwfLYWSly54J
<https://groups.google.com/d/msg/json-rpc/cXlc8bD-a_M/cwfLYWSly54J>*
http://www.simple-is-better.org/json-rpc/transport_sockets.html


On Mon, Mar 10, 2014 at 6:11 PM, Nico Williams <nico@cryptonector.com>wrote;wrote:

> Consider jq (https://stedolan.github.io/jq) as an example.
>
> jq can and will read a sequence of JSON texts from stdin.  That
> sequence isn't an array.  jq applies its program to each input.  jq
> produces as its output all the values -encoded as JSON texts- output
> by the jq program.  This is exceedingly convenient, including for log
> files (for the reason that you give: log files are an indeterminate
> length, unending append-only sequence).
>
> 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.  While the sequence of true, "foo",
> false, "bar", 1, "foobar", and 2 doesn't.  An input that looks like
> "truefalsenull12" elicits a parse error.
>
> Note that the jq parser does NOT in fact parse multiple top-level
> values; it parses at most one top-level value.  Instead the program
> using the parser feeds input bytes to the parser until the parser
> finds the end of a top-level value and outputs it.  Remaining unparsed
> bytes are buffered, of course so that when the program adds more input
> bytes the parser can be restarted to parse the next top-level value.
>
> This does mean that the jq parser will not output null when fed 'null'
> until one more byte _or_ EOF are fed to it.  But if fed '[1,2]' then
> the parser emits the parsed array value immediately when the closing
> bracket is parsed, without waiting for further inputs.
>
> jq always outputs a newline (though it could be space or tab) after
> outputting any top-level value's JSON text encoding.  It does so
> precisely to avoid these ambiguities.
>
> (This is also why jq must continue to output a newline or other
> whitespace after every output text, except, perhaps, when in raw
> output mode, in which case the outputs aren't JSON texts.)
>
> Nico
> --
>
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-- 
Matthew P. C. Morley