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Re: JSON and the Unicode Standard

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  • johne_ganz
    ... Restricting keys to
    Message 1 of 35 , Mar 3, 2011
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      --- In json@yahoogroups.com, Dave Gamble <davegamble@...> wrote:
      > Would it be too much to specify that key names are to be ASCII top-bit-unset
      > strings?
      > i.e. in the definition of an object, designate that the "string" there is a
      > "simplestring" which uses a restricted definition of char?
      > As far as I can see, this is the only case where the Unicode interpretation
      > is potentially dangerous.
      > In usage of strings as data, I believe they are to be delivered unprocessed
      > to the user of the data.

      Restricting keys to < U+0080 (i.e., ASCII) is one way. Personally, I'm kinda partial to something along the lines of this:

      A JSON generator SHOULD only emit key names that are (some word smithed language along the lines of possibly NFC, precomposed, possibly even NFKC, etc...)

      A JSON generator SHOULD NOT emit keys that are (Unicode Equivalent) (http://en.wikipedia.org/wiki/Unicode_equivalence) (... or some word smithed language to this effect.)

      A user/application MUST NOT depend on behavior that requires two (Unicode Equivalent) keys that are not (word smithed language for concept of 'bit identical').

      The behavior is UNDEFINED for two keys that are (Unicode Equivalent) but not (word smithed language for concept of 'bit identical').

      Two keys compared to each other are equal if they are (some word smithed language that incorporates the concept of Unicode Equivalence). A JSON implementation MAY perform normalization on parsed keys, but is not required to. A JSON parser implementation MAY treat two keys that are (Unicode Equivalent) but not (word smithed language for concept of 'bit identical') as different, but a parser SHOULD strive for (just Unicode Equivalent).

      When "round-tripping", a user/application MUST NOT depend on behavior where two (Unicode Equivalent) keys that are not (word smithed language for concept of 'bit identical') to remain unchanged. (the intent is that a parser may perform some form of normalization on the keys, so when they are round tripped, an unnormalized key may become normalized in the process).

      In other words..... this is probably fairly close to exactly how things really are right now, it just spells it out. It also places the responsibility for the "problem" squarely on the user/application that's generating the keys- it should only generate, use, and manipulate keys that respect the fact that some unicode strings may be slightly modified from their original form, but still considered equal.

      And most of these issues can be avoided if you stick to just plain ASCII code points, or use code points that can not be mutilated by Unicode (i.e., don't use Å, or U+212B, which can be transformed in to either U+00C5 or U+0041 U+030A).
    • 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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