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From Mark Hahn <>
Subject Re: Duplicate fields in documents
Date Thu, 20 Feb 2014 04:45:02 GMT
> The Erlang JSON parser is already being weird and nonstandard in
preserving the order of keys

Don't all javascript engines do the same?  The one exception I can think of
is in chrome when you spell out a number, i.e. "one".

On Wed, Feb 19, 2014 at 8:30 PM, Jens Alfke <> wrote:

> On Feb 19, 2014, at 6:07 AM, Dave Cottlehuber <> wrote:
> > TL;DR the appropriately named ECMA 404 JSON spec [1] is broken or more
> politely, insufficiently specific.
> This seems to fall into the category of "things so obvious that the people
> who wrote the spec didn't realize they had to mention them." I.e. "You
> can't have duplicate keys."
> > JSON is typically based on a dictionary or hash map, and there's no
> particular reason for that data structure to enforce uniqueness of keys.
> I disagree. Mathematically, a dictionary/map object is a function: it maps
> from a set of keys to a set of values, with each key mapping to exactly one
> value. (That's basically the definition of 'function'.) It's certainly
> possible to create implementations that map a key to _multiple_ values, but
> that's something different: it's a mapping from a key to a set. (For
> example, it's not from string-->int, it's now from string-->set<int>.) The
> JSON spec does not include this kind of mapping -- an object value in JSON
> can be a number, but not a set of numbers.
> There _are_ data formats out there that explicitly support multiple values
> for a key. The best-known one is probably MIME/HTTP headers. Parsers for
> this tend to use a representation that's a mapping from a string to a set
> or array of strings.
> IMHO the reasonable thing for a JSON parser to do if it encounters a
> duplicate key is to fail with a clear error. Failing that, the only other
> reasonable option is to discard one or the other  value (I don't have an
> opinion which.) But keeping both is unreasonable.
> (The Erlang JSON parser is already being weird and nonstandard in
> preserving the order of keys. This implicitly led to interoperability
> problems like the MIME multipart representation of a document not having a
> clear mapping of attachment names to MIME bodies, because whoever wrote it
> decided to put the MIME bodies in "the same order as" the attachment keys,
> not realizing that there isn't any order.)
> --Jens

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