// Swift 5.0
var json = "{ \"value\": 3.58, \"amount\": 3.58 }"

struct Model: Codable {
    var value: Decimal?
    var amount: Double?
}

let data = json.data(using: .utf8)
let jsonDecoder = JSONDecoder()
let model = try? jsonDecoder.decode(Model.self, from: data!)
let value: Decimal = model?.value ?? 0
let amount: Double = model?.amount ?? 0
print(value)
print(amount)
print(String(describing: model))

let encoder = JSONEncoder()
encoder.outputFormatting = .prettyPrinted
let newData = try? encoder.encode(model)
print(String(data: newData!, encoding: .utf8)!)