Request for I-JSON (RFC 7493) Parsing Mode

[jayeshbadwaik]

I was wondering if there is any interest in implementing the I-JSON parsing mode for boost.json. I-JSON is an interoperable version of JSON which aims to remove some ambiguities in the parsing. The changes required for such a compliance are listed in Section 2 of the document.

[vinniefalco]

It is very likely that we already follow these rules. If so, we should document it. If not, we should consider whether to follow it.

[sdkrystian]

The only normative requirement we don't comply with is checking for duplicate object key. There is some normative encouragements in there that we don't strictly follow, such as limiting precision.

[vinniefalco]

We never emit JSON with duplicates, as the DOM (via json::object) does not allow it. When we parse JSON, I believe we always take the first value if there are duplicate keys

[jayeshbadwaik]

The restriction of allowing only a single object key is during the parsing, not emission.

[vinniefalco]

When an object is constructed during parsing, we check its elements for duplicates here:
https://github.com/CPPAlliance/json/blob/ec29dfa768a16672534f758005dfe4841a8ea93d/include/boost/json/detail/impl/object_impl.ipp#L79
The duplicates are removed by replacing the duplicate with the last element in the object array, and then shrinking the array size by 1.

[sdkrystian]

@vinniefalco To conform to I-JSON we would have the check for duplicate keys within basic_parser, not the handler.

[vinniefalco]

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[vinniefalco]

basic_parser can never do such a thing, so I don't think we can claim I-JSON support for it. But that's okay, because we can claim I-JSON support in parser which is what 99% of users care about. For the other 1% of users who are implementing their own parser by using basic_parser directly, I think it is reasonable to expect that if they want I-JSON compliance, that can be responsible for it. How do you feel about that @jayeshbadwaik ?

[jayeshbadwaik]

One way to implement that in your codebase I see is to throw in case of duplicate in here: https://github.com/CPPAlliance/json/blob/ec29dfa768a16672534f758005dfe4841a8ea93d/include/boost/json/detail/impl/object_impl.ipp#L79

And handle that exception upward to report that the provided I-JSON document is ill-formed. This will require adding an option to parse_options to allow the user to parse in "I-JSON" mode and require you to remove noexcept qualifer for object_impl.build(). Since I am not aware of how the rest of the code works, I wonder if that is palatable.

[vinniefalco]

Actually, when we detect a duplicate we can set a bit somewhere in the object and then in parser we can query new objects to see if there's a duplicate and throw as needed.

[jayeshbadwaik]

That can work too, yeah.