I can’t speak to any resistance you’ve seen from the Swift team (nor
to any performance issues you’ve encountered with
`JSONSerialization`), but just keep in mind that
a) `JSONSerialization` is maintained by the Foundation team, and
b) maintaining a separate JSON parsing implementation just for Swift is
a great way to introduce new, separate, likely incompatible bugs
That being said, we’ve considered it, and continue to consider it —
there is just a cost-benefit analysis that goes into prioritizing
developer time.
On 31 Oct 2017, at 10:18, Jon Shier wrote:
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@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
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
<https://lists.swift.org/mailman/listinfo/swift-users>
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
_______________________________________________
swift-users mailing list
swift-users@swift.org
https://lists.swift.org/mailman/listinfo/swift-users
