[swift-evolution] [Accepted] SE-0167: Swift Encoders

Johannes Weiss johannesweiss at apple.com
Tue May 9 11:27:59 CDT 2017


Sorry, I'm very late to the party but have one quick question that I think should be resolved/documented before the patch is landed:

What do we do with integers outside the range [-(2**53)+1, (2**53)-1])? Those are which are the integers that are precisely representable by doubles (IEEE 754-2008 binary64 (double precision). And the problem is that at least JavaScript treats all numbers as doubles, which leads to this problem:

[9851624185071827, 9851624185071829] as [Double] makes them
[9851624185071828, 9851624185071828]

(so two different numbers get both mapped to a third number which sometimes causes problems in the real world [4])

The I(nternet)-JSON RFC [1] states that

   Implementations that generate I-JSON messages cannot assume that
   receiving implementations can process numeric values with greater
   magnitude or precision than provided by those numbers.

Now since Swift isn't JavaScript we fortunately don't store all numbers as doubles so I'm sure a roundtrip of the number 9851624185071827 (which is outside that range) will just work. Nevertheless the RFC [1] says

   For applications that require the exact interchange of numbers with
   greater magnitude or precision, it is RECOMMENDED to encode them in
   JSON string values.

I'm not sure if following that recommendation is a good idea but in any case I think it would be worth documenting it. Other encoders sometimes allow you to specify 'numbers as strings' as an option [2] or outright refuse to encode it.

Twitter also covers the subject [3] for its API.


[1]: https://tools.ietf.org/html/rfc7493#section-2.2
[2]: https://fasterxml.github.io/jackson-core/javadoc/2.4/com/fasterxml/jackson/core/JsonGenerator.Feature.html#WRITE_NUMBERS_AS_STRINGS
[3]: https://dev.twitter.com/overview/api/twitter-ids-json-and-snowflake
[4]: https://github.com/nodejs/node/issues/12115
> On 26 Apr 2017, at 12:10 am, Douglas Gregor via swift-evolution <swift-evolution at swift.org> wrote:
> Proposal Link: https://github.com/apple/swift-evolution/blob/master/proposals/0167-swift-encoders.md
> The review of SE-0167 "SE-0167: Swift Encoders” ran from April 6...12, 2017. The proposal is accepted. Thanks to everyone who participated in the review!
> 	- Doug
> 	Review Manager
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