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Re: [json] Re: JSON and the Unicode Standard

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  • David Graham
    I had the normalization question while writing json-stream in Ruby as well. I decided the parser shouldn t do Unicode normalization for the following reasons:
    Message 1 of 35 , Feb 25, 2011
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      I had the normalization question while writing json-stream in Ruby as well.
      I decided the parser shouldn't do Unicode normalization for the following
      reasons:


      1. The json and yajl-json Ruby parsers and the popular org.json Java parser
      do not do normalization.


      2. CouchDB does not do normalization. I wrote json-stream to handle CouchDB
      documents so this was my primary use case.


      3. Ruby and Java consider combined characters to be unequal to their single
      codepoint counterparts. The � character, for example, can be a 2 byte
      single codepoint form of \u00e9 or a 3 byte two codepoint form of
      \u0065\u0301.


      In Ruby, "\u00e9" == "\u0065\u0301" => false.


      So, given a Ruby Hash (or Java Map) like this:


      {"\u00e9" => 1, "\u0065\u0301" => 2}

      => {"�"=>1, "�"=>2}


      A JSON serializer that performed Unicode normalization on this Hash object
      would corrupt the data in some way. The two keys would become equal, so
      which value gets serialized: 1 or 2?


      In my opinion, this means JSON parsers and generators must not perform
      normalization. They must respect the data stored in the JSON byte stream as
      is.


      It's easy for the application to normalize data before handing it to the
      JSON library for serialization, though. In Ruby, we can do:


      ActiveSupport::Multibyte::Chars.new("\u0065\u0301").normalize(:c)


      I hope that helps.


      David

      On Fri, Feb 25, 2011 at 4:45 PM, Tatu Saloranta <tsaloranta@...>wrote:

      >
      >
      > On Fri, Feb 25, 2011 at 3:09 PM, johne_ganz <john.engelhart@...>
      > wrote:
      > ...
      >
      >
      > > Unicode is not just a simple set of code points.
      >
      > This is true statement, although the more practical question seems to
      > be what is the practical relationship of JSON with Unicode
      > specification.
      > I think your suggestions for clarifying some parts do make sense,
      > although it may be hard to reconcile basic diffences between full
      > Unicode support, and goals of simplicity for JSON.
      >
      >
      > >
      > > Another issue is normalization. In particular, the way normalization is
      > handled for the "key" portion of an "object" (i.e., {"key": "value"}) can
      > dramatically alter the meaning and contents of the object. For example:
      > >
      > > {
      > > "\u212b": "one",
      > > "\u0041\u030a": "two",
      > > "\u00c5": "three"
      > > }
      > >
      > > Are these three keys distinct? Should there be a requirement that they
      > MUST be handled and interpreted such that they are distinct? Does that
      > requirement extend past the "channel" demarcation point (i.e., not a JSON
      > library or communication channel used to interchange the JSON between two
      > hosts) to the "host language"?
      > >
      > > In case it is not obvious, under the rules of Unicode NFC (Normalization
      > Form C), all three of the keys above will become "\u00c5" after NFC
      > processing.
      >
      > For what it is worth, I have not seen a single JSON parser that would
      > do such normalization; and the only XML parser I recall even trying
      > proper Unicode code point normalization was XOM. This is not an
      > argument against proper handling, but rather an observation regarding
      > how much of a practical issue it seems to be.
      > Nor have I seen feature requests to support normalization (XOM
      > implements it because its author is very ambitious wrt supporting
      > standards, it is very respectable achievement), during time I have
      > spend maintaining XML and JSON parser/generator implementations.
      > Do others have difference experiences?
      >
      > So to me it seems that most likely high-level clarifications regarding
      > normalization aspects would be:
      >
      > (a) Whether to do normalization or not is up to implementation
      > (normalization is left out of scope, on purpose), or
      > (b) Say that with JSON no normalization would be done (which would be
      > more at odds with unicode spec)
      >
      > Why? Just because I see very little chance of anything more ambitious
      > having effect on implementations (beyond small number that are willing
      > to tackle such complexity). While it would seem wrong to punt the
      > issue, there is the practical question of whether full solution would
      > matter.
      > My guess is that about last thing I implements would want was a
      > mandate to support full Unicode 4.0 (and above) normalization rules.
      > It would just mean that there would be the specification in one
      > corner; and implementations, practically none of which would be
      > compliant.
      >
      > ...
      >
      > > Your snarky comment ignores the real world complexities that one faces
      > when attempting to create a "RFC 4627 compliant" JSON implementation, at
      > least if one is trying to do so "the right way" as opposed to a quick hack
      > JSON implementation.
      >
      > For better or worse, most JSON implementations fall in quick hack
      > category; which is just to say that chances of getting significant
      > number of implementations to do much more than decoding code points
      > correctly is vanishingly small. Or that even getting them to do basic
      > decoding is quite a challenge in itself.
      >
      >
      > > For someone who is creating a JSON library or some other form of a JSON
      > implementation, the corner cases are usually far more important than the
      > obvious, common case.
      >
      > True.
      >
      > I think your suggestions of how this could be clarified make sense.
      >
      > -+ Tatu +-
      >
      >
      >


      [Non-text portions of this message have been removed]
    • johne_ganz
      ... There is another relevant section (ECMA-262, 8.4 The String Type, pg 28) When a String contains actual textual data, each element is considered to be a
      Message 35 of 35 , Mar 3, 2011
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        --- In json@yahoogroups.com, Dave Gamble <davegamble@...> wrote:
        >
        > To save people looking it up:
        >
        > ECMA-262, section 7.6:
        >
        > Two IdentifierName that are canonically equivalent according to the
        > Unicode standard are not equal unless they are represented by the
        > exact same sequence of code units (in other words, conforming
        > ECMAScript implementations are only required to do bitwise comparison
        > on IdentifierName values). The intent is that the incoming source text
        > has been converted to normalised form C before it reaches the
        > compiler.
        >
        > ECMAScript implementations may recognize identifier characters defined
        > in later editions of the Unicode Standard. If portability is a
        > concern, programmers should only employ identifier characters defined
        > in Unicode 3.0.

        There is another relevant section (ECMA-262, 8.4 The String Type, pg 28)

        When a String contains actual textual data, each element is considered to be a single UTF-16 code unit. Whether or not this is the actual storage format of a String, the characters within a String are numbered by their initial code unit element position as though they were represented using UTF-16. All operations on Strings (except as otherwise stated) treat them as sequences of undifferentiated 16-bit unsigned integers; they do not ensure the resulting String is in normalised form, nor do they ensure language-sensitive results.

        NOTE The rationale behind this design was to keep the implementation of Strings as simple and high-performing as possible. The intent is that textual data coming into the execution environment from outside (e.g., user input, text read from a file or received over the network, etc.) be converted to Unicode Normalised Form C before the running program sees it. Usually this would occur at the same time incoming text is converted from its original character encoding to Unicode (and would impose no additional overhead). Since it is recommended that ECMAScript source code be in Normalised Form C, string literals are guaranteed to be normalised (if source text is guaranteed to be normalised), as long as they do not contain any Unicode escape sequences.

        > I think it's fairly clear that a JSON parser has ABSOLUTELY NO
        > BUSINESS poking around with actual data strings; Douglas has been very
        > clear that you are to pass them bit-identical to the recipient. On the
        > other hand, there's an argument for some kind of sanitation when it
        > comes to object member names.
        > I'm really tempted by the idea of a JSON-secure spec, which clamps
        > down on these details.

        I disagree with your first statement. The ECMA-262 standard, at least in my opinion, tries to side step a lot of these issues. It makes a fairly clear distinction between "what happens inside the ECMA-262 environment (which it obviously has near total control over)" and "what happens outside the ECMA-262 environment".

        IMHO, the ECMA-262 standard advocates that "stuff that happens outside the ECMA-262 environment should be treated as if it is NFC".

        Since the sine qua non of JSON is the interchange of information between different environments and implementations, it must address any issues that can and will cause difficulties. Like it or not, the fact that it's Unicode means these things can and will happen, and it's simply not practical to expect or insist that every implementation treat JSON Strings as "just a simple array of Unicode Code Points".

        > Arguing the Unicode details is decidedly NOT compatible with the
        > "spirit" of JSON, which Douglas has been very clear about; a
        > lightweight, simple, modern data representation.

        I completely agree that these details are NOT compatible with the "spirit" of JSON.

        But.... so what? Unicode is not simple. I'm not the one who made it that way, but the way that RFC 4627 is written, you must deal with it. There are ways RFC 4627 could have been written such that the JSON to be parsed is considered a stream of 8 bit bytes, and therefore stripped of its Unicode semantics (if any). However, it very clearly and plainly says "JSON text SHALL be encoded in Unicode.", which pretty much kills the idea that you can just treat it as raw bytes.

        There's a saying about formalized standards: The standard is right. Even it's mistakes.

        As an aside, there is a RFC for "Unicode Format for Network Interchange", RFC 5198 (http://tools.ietf.org/html/rfc5198). It is 18 pages long. RFC 4627 is just 9 pages.

        Actually, I would encourage people to read RFC 5198. I'm not sure I agree with all of it, but it goes over a lot of the issues I think are very relevant to this conversation. It's great background info if you're not familiar with the details.
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