Re: Improving JSON pretty printing, how to represent floats?

["Mattias Engdegård" <mattias.engdegard@gmail.com>]

5 apr. 2024 kl. 14.35 skrev Eli Zaretskii <eliz@gnu.org>:

>> --8<---------------cut here---------------start------------->8---
>> {
>>    "a": 3.333333333333000000000000000001
>> }
>> --8<---------------cut here---------------end--------------->8---
>> 
>> then the float gets rounded.  I think pretty printing should be 
>> lossless (keeping the exact format, exp notation, etc.).
> 
> The digits in 3.333333333333000000000000000001 after 16th digit are
> meaningless: they aren't supported by IEEE floating-point standard, so
> they are just numerical noise.

The float printer we use tries to produce the shortest output that, when read back in using IEEE-754 64-bit floats and the default rounding mode, results in the same number. There is no loss of information.

The JSON specs mandate little about range and precision for numbers and implementations indeed vary a lot; see [1] for a recent overview. However given its Javascript origins and the prevalence of 64-bit floats, we are pretty safe here for anything that doesn't have very special requirements (such as decimal FP). Our support for arbitrary big integers is more than many other implementations do.

[1] https://blog.trl.sn/blog/what-is-a-json-number/