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

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  • Tatu Saloranta
    On Fri, Feb 25, 2011 at 3:09 PM, johne_ganz wrote: ... This is true statement, although the more practical question seems to be what
    Message 1 of 35 , Feb 25, 2011
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      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 +-
    • 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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