[swift-users] Parsing Decimal values from JSON

Itai Ferber iferber at apple.com
Tue Oct 31 12:07:29 CDT 2017

Hi Evtim,

Just want to give some context for this.
This is due to the fact that `JSONEncoder` and `JSONDecoder` are 
currently based on `JSONSerialization`: when you go to decode some JSON 
data, the data is deserialized using `JSONSerialization`, and then 
decoded into your types by `JSONDecoder`. At the `JSONSerialization` 
level, however, there is no way to know whether a given numeric value is 
meant to be interpreted as a `Double` or as a `Decimal`.

There are subtle differences to decoding as either, so there is no 
behavior that could satisfy all use cases. `JSONSerialization` has to 
make a decision, so if the number could fit losslessly in a `Double`, it 
will prefer that to a `Decimal`. This allows guaranteed precise 
round-tripping of all `Double` values at the cost of different behavior 
when decoding a `Decimal`.

In practice, this might not really matter in the end based on how you 
use the number (e.g. the loss in precision can be so minute as to be 
insignificant) — what is your use case here? And can you give some 
numeric values for which this is problematic for you?

As others have mentioned, one way to guarantee decoding a numeric string 
in a specific way is to actually encode it and decode it as a `String`, 
then convert into a `Decimal` where you need it, e.g.

import Foundation

struct Foo : Codable {
     var number: Decimal

     public init(number: Decimal) {
         self.number = number

     private enum CodingKeys : String, CodingKey {
         case number

     public init(from decoder: Decoder) throws {
         let container = try decoder.container(keyedBy: CodingKeys.self)
         let stringValue = try container.decode(String.self, forKey: 
         guard let decimal = Decimal(string: stringValue) else {
             throw DecodingError.dataCorruptedError(forKey: .number, in: 
container, debugDescription: "Invalid numeric value.")

         self.number = decimal

     public func encode(to encoder: Encoder) throws {
         var container = encoder.container(keyedBy: CodingKeys.self)
         try container.encode(self.number.description, forKey: .number)

let foo = Foo(number: Decimal(string: 
print(foo) // => Foo(number: 2.71828182845904523536028747135266249775)

let encoder = JSONEncoder()
let data = try encoder.encode(foo)
print(String(data: data, encoding: .utf8)!) // => 

let decoder = JSONDecoder()
let decoded = try decoder.decode(Foo.self, from: data)
print(decoded) // => Foo(number: 

print(decoded.number == foo.number) // => true

— Itai

On 28 Oct 2017, at 11:23, Evtim Papushev via swift-users wrote:

> Hello :)
> I am trying to find a way to parse a number as Decimal without losing 
> the number's precision.
> It seems that the JSON decoder parses it as Double then converts it to 
> Decimal which introduces errors in the parsing. That behavior is in 
> fact incorrect.
> Does anyone know if there is a way to obtain the raw data for this 
> specific field so I can write the conversion code?
> Thanks,
> Evtim
> _______________________________________________
> swift-users mailing list
> swift-users at swift.org
> https://lists.swift.org/mailman/listinfo/swift-users
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