JSONEncoder floating point behavior differs between Darwin and non-Darwin implementation

[fumoboy007]

The floating point behavior of JSONEncoder differs between the Darwin and non-Darwin implementation of Foundation.

Test Case

import Foundation

let array = [
  1.1
]

let jsonEncoder = JSONEncoder()

let jsonBytes = try! jsonEncoder.encode(array)
let jsonString = String(decoding: jsonBytes, as: UTF8.self)

print(jsonString)

Darwin Output

[1.1000000000000001]

Non-Darwin Output

[1.1]

[fumoboy007]

If I had a vote, I would vote for the non-Darwin behavior, which is neater.

One could argue the Darwin behavior is more precise but I think that argument misses the fact that floating point is imprecise to begin with… one cannot tell whether the floating point value actually represents 1.1 or 1.1000000000000001.

So, in the absence of precision, I would suggest choosing the neater one.

[fumoboy007]

This Swift commit changed Swift’s floating point formatting logic, mentioning the following:

This format is also ideal for logging, debugging, and other general display. In particular, the shortness guarantee avoids the confusion of unnecessary additional digits, so that the result of 1.0 / 10.0 consistently displays as 0.1 instead of 0.100000000000000000001.

[fumoboy007]

Filed FB11996256 requesting that the Darwin implementation be updated to match the non-Darwin implementation.

[21DCS059]

i want to contribute in this project please assign it to me.

[fumoboy007]

@21DCS059 See above comment. There shouldn’t be any action needed in this repository. Instead, I filed FB11996256, which requests that Apple modify their closed-source version of Foundation.

[parkera]

This produces [1.1] using swift-foundation's JSONEncoder. See also #5001.