[swift-users] Parsing Decimal values from JSON

[Jon Shier]

	The appropriate solution here would be for Swift to have its own native JSON parser that allows direct decoding into generic types without the intermediary of JSONSerialization. For whatever reason there seems to be resistance to this from the Swift team, but until we have that ability, these types of issues will keep coming up, and the performance overhead of JSONSerialization with JSONDecoder on top of it will continue to leave Swift without a very performant JSON solution. 
	That said, I appreciate the support given Codable on this list.



Jon

> On Oct 31, 2017, at 1:07 PM, Itai Ferber via swift-users <swift-users at swift.org> wrote:
> 
> 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: .number)
>         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: "2.71828182845904523536028747135266249775")!)
> print(foo) // => Foo(number: 2.71828182845904523536028747135266249775)
> 
> let encoder = JSONEncoder()
> let data = try encoder.encode(foo)
> print(String(data: data, encoding: .utf8)!) // => {"number":"2.71828182845904523536028747135266249775"}
> 
> let decoder = JSONDecoder()
> let decoded = try decoder.decode(Foo.self, from: data)
> print(decoded) // => Foo(number: 2.71828182845904523536028747135266249775)
> 
> 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
> 
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