Decimal comparisons

Brendan Eich brendan at mozilla.org
Fri Sep 19 09:38:01 PDT 2008


On Sep 19, 2008, at 10:25 AM, Kris Zyp wrote:

>>> Consider 1.1m instead  of 1m.
>
> Would be:
> '{"foo":1.1}'

Ok, how about serializing the object you get by evaluating {"foo":0.1 
+0.2}?


>> JSON does not provide for decimal, and receiver-makes-it-wrong is  
>> a  bug.
>> JSON would need to be extended to handle decimal reliably.
>
> No, JSON only provides for decimal, JSON uses radix 10 to encode  
> numbers,

Of course it uses base 10 for encoding and parsing, but the RFC says  
nothing about the required precise value of a number in any  
representation -- including in base 10.


> it
> has no support for binary. JSON doesn't need to be extended (unless  
> you want
> to see binary added, but I don't that would be popular).

All JSON codecs today (AFAIK -- all the ones written in JS) use 64- 
bit double precision, not any kind of decimal, to represent numbers.


> JSON makes no
> assertions about what format a receiver must use to store the  
> numbers it
> receives, it transfers numbers in decimal format, and  
> implementations can
> and should be able to use any format they desire to internally  
> represent it
> based on their needs and expectations.

Conversion plus rounding means that a JSON peer may not be able to  
ignore storage format, which is double. It's possible that real JSON- 
based apps depend on this de-facto standard. At any rate, under- 
specification in the RFC allows it, and via Postel's Law it is likely  
that apps *do* depend on double being the storage format.


> Today implementations do vary in
> which format they choose to use,

Do you know of any that use decimal or any format other than IEEE 754  
double?


> but there is no restriction preventing
> implementations from using decimal formats for de-serialization of  
> numbers,
> and it seems like it would make sense for JavaScript to do so when  
> decimals
> become available.

Could be. Best case, there's no significantly-used web app or page  
that depends on JSON peers storing using double. We just do not know.

/be



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