Handling of infinity and NaN

[vinniefalco]

Specifically, if the user puts an inf or NaN into a value, what happens on serialization? We need to specify what should happen on serialization. And if we output "inf" and "nan" then we need to specify what happens on parsing. And we might need to provide a configuration option for allowing it / disallowing it (on parse).

[accelerated]

I have also bumped into this while testing. NaN is serialized as such but then it actually throws as it's not recognized by the parser.

[klemens-morgenstern]

Currently it produces invalid json (https://gcc.godbolt.org/z/31ebz6b9q).

I personally serialize to reuse it in javascript (and I imagine that would be quite common) so just putting in a string would work out alright for that. "Infinity" * 42 == Infinity, because javascript.

Alternatively, 1e800 is Infinity (-1e800) in JS, plus NaN could be null. I don't know how well that works for other languages though.

[grisumbras]

@pdimov suggested using a struct like this:

struct serialize_options {
  string_view pinf;
  string_view ninf;
  string_view nan;
};

to control serializer's behaviour.

Usage would be something like

os << serialize(jv, { .pinf = "null", .ninf = "null", .nan = "null" });

If a string_views is empty and the corresponding special value is encountered the serializer would error-out.

@vinniefalco has added that sometimes users would want to know if a special value has been encountered even when they do provide a string to serialize it as. So, maybe serializer should always produce an error_code when a special value is encountered.

Last thing to consider: what should be the default behaviour, i.e. when serialize_options is not used?

[pdimov]

The default should be the same as passing {}, that is, throw/error.

This may not technically be the most correct default, but it's the most intuitive given the interface. The rule is, if you provide a representation for a special value, this representation is used, otherwise, there's an error on encountering it.

No, I don't think the serializer should error out even when a representation is supplied.

[madmongo1]

I agree with Klemens here. JSON is closely linked with the value models of JavaScript and to a lesser extent, python. If you’re going to handle non-numbers at all, the treatment should model that of JavaScript, otherwise interoperability becomes a headache - you’ll have to remember two rule sets and code up the conversions between the two.

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[pdimov]

and to a lesser extent, python.

That's exactly what the current behavior is.

[vinniefalco]

@vinniefalco has added that sometimes users would want to know if a special value has been encountered even when they do provide a string to serialize it as. So, maybe serializer should always produce an error_code when a special value is encountered.

But this is not the same thing. A non-successful error code is non-recoverable. You can't "always produce an error code when a special value is encountered." If we are going to aggregate statistics on the number of non-compliant numbers encountered, they need to be communicated out of band. For example by calling a separate function which returns the statistics, when serialization is complete. Since this can always be added later as a feature if/when needed, without affecting the design of the original topic of this issue, we don't have to worry about it.

[x10000year]

what about deserialize?

[grisumbras]

You mean parse? JSON spec doesn't support infinities and NaNs. If it did, this issue wouldn't have existed.

[grisumbras]

An approach that is very simple to implement that will allow us to support infinity (but not NaN) is to serialize infinity as 1e999. Many implementations (including ours) parse that as infinity.

[pdimov]

1e999 is a valid long double, so we should avoid it and use, e.g. 1e99999.