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

Nico Williams <nico@cryptonector.com> Tue, 11 March 2014 18:59 UTC

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Date: Tue, 11 Mar 2014 13:59:22 -0500
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From: Nico Williams <nico@cryptonector.com>
To: Tim Bray <tbray@textuality.com>
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Cc: Paul Hoffman <paul.hoffman@vpnc.org>, Phillip Hallam-Baker <hallam@gmail.com>, Matthew Morley <matt@mpcm.com>, "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 Tue, Mar 11, 2014 at 1:35 PM, Tim Bray <tbray@textuality.com> wrote:
> My assumption is that many (most? all?) JSON parsers can be altered to, when
> they encounter a JSON text containing an object, just stop reading when they
> hit the trailing “}”.  So my notion is that using such a parser you have a
> loop like

That's true of arrays as well.

It's also true of strings for any parser that permits them at the top-level.

It's only true, false, null, and numeric top-level values that require
a disambiguation separator.

Using a newline (or any other separator character or string) permits
the parsing of any JSON text sequences with unmodified parsers.  This
is especially true when the JSON texts are encoded with no newlines
and the separator is or has a newline.

> while !eof
>    obj = parser.ReadObjectAndStop() // parse error handling left as an
> exercise for the reader

The key is being able to parse one text, then the next.  If the texts
self-delimit (e.g., they contain no embedded newlines and are
terminated by a newline) then any JSON parser can be used to build a
JSON text sequence parser.

Even if a text contains a new line (e.g., after every element of an
array or after every name or value in an object), one could parse a
JSON text sequence even with the simplest JSON parsers (ones that have
no incremental parsing capability).

Thus I'd RECOMMEND that JSON text sequences consist of sequences of
JSON texts encoded with no embedded newlines, and I'd REQUIRE that
each text followed by a newline.

Then the general case (any parser, incremental or not) is as simple as:

    while read line; do
        parse "$line" ...
        ...
    done

> So yeah, requiring a SINGLE NEWLINE AND NOTHING ELSE would be simpler.  But

Indeed, it's the simplest: because every language/runtime has a "read
a line at a time" primitive.  If your texts have embedded newlines and
you have an incremental parser, you're still OK.

Heck, even if you don't have an incremental parser, you'd be OK,
though the result would be inefficient:

    buffer=
    while read line; do
        last_is_partial=true
        if [[ -z $buffer ]] && ! parse "$line"...; then
            buffer=$line
            continue
        elif [[ -n $buffer ]]; then
            buffer+=$line
            if ! parse "$buffer"...; then
                continue
            fi
        fi
        last_is_partial=false
        ...
    done
    if $last_is_partial; then
        # Report error
        ...
    fi

> if you’re going to allow any other non-significant white space characters,
> then why bother requiring that one of them be a newline?

Because reading lines is a common (universal?) primitive.  Sure, if
you have an evented app and an incremental JSON parser then you'll be
reading bytes till you get a complete entry, newline or not.

Nico
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