A JSON document can contain numbers, written in decimal notation. The standard does not specify any type for these numbers – a number is just defined syntactically as a sequence of digits, optionally followed by a dot and some more digits. (There may also be a minus sign in the front and an exponent in the end – see the standard for the exact specification.)

A JSON parser should do the best job it can to preserve this information in the types available to the programmer. In this blog post I am taking a look at the tools available on the Apple platforms, and explore a problem that occurs when using the Decimal type.

JSONSerialization

The JSONSerialization class, known as NSJSONSerialization in Objective-C, was the standard method of deserializing JSON up until the Swift 4 release. How does it handle numbers? Let’s take a look at a few examples. You may follow along in a playground.

func parseJsonArray(_ jsonString: String) -> [Any] {
    let data = jsonString.data(using: .utf8)!
    return try! JSONSerialization.jsonObject(with: data, options: []) as! [Any]
}
func printInfo(_ items: [Any]) {
    for item in items {
        print("\(type(of: item)) - \(item)")
    }
}
printInfo(parseJsonArray("[5, 3.133]")
// Prints:
// __NSCFNumber - 5
// __NSCFNumber - 3.133

This being a typical Objective-C API, we’re not specifying what types we expect; the framework parses into types it sees fitting, and in this case we get a private subclass of NSNumber called __NSCFNumber. Exactly how that type stores the number, we are not told. So, do we always get this type for numbers? No:

printInfo(parseJsonArray("[1.23456789123456789]"))
// Prints:
// NSDecimalNumber - 1.23456789123456789

It has now switched to an NSDecimalNumber. In all cases, whether we get an __NSCFNumber or an NSDecimalNumber, we can get a string representation of the parsed number that is exactly as how we wrote it in the JSON – no precision was lost. It presumably chooses an NSDecimalNumber where it can no longer fit the decimals in the mantissa of a Double.

JSONDecoder

With Swift 4, the Codable API arrived. This gives us a new, more Swifty way of decoding JSON. Now the programmer specifies the types to expect. For most use cases, if you expect non-integer numbers, you will use the Double type.

func parseJsonDoubleArray(_ jsonString: String) -> [Double] {
    let data = jsonString.data(using: .utf8)!
    return try! JSONDecoder().decode([Double].self, from: data)
}
parseJsonDoubleArray("[5, 3.133]") // 5, 3.133
parseJsonDoubleArray("[1.23456789123456789]") // 1.2345678912345678

Note the truncation in the latter example – there are more digits than can fit. If you would need that many decimals, you might then want to choose the Decimal type instead.

There are also other, probably more typical, situations where you may prefer Decimal. Decimal math is generelly preferred for example when dealing with currency, and you may want to decode directly into such a type.

But alas:

func parseJsonDecimalArray(_ jsonString: String) -> [Decimal] {
    let data = jsonString.data(using: .utf8)!
    return try! JSONDecoder().decode([Decimal].self, from: data)
}
let decimals = parseJsonDecimalArray("[5, 3.133]")
decimals[1] // 3.132999999999999488

Hmmm.

What’s going on here?

Before we get to this problem that appears to happen when we use decimal floating point math, we have to talk a bit about binary floating point. Binary floating point is the kind that we generally use, with types that programming languages typically call float and double.

These can be kind of difficult to really wrap your head around. We can write things like let foo: Double = 0.1 all day long, and the compiler will be happy, and the number will print as “0.1” – so it iss easy to forget that we can’t actually represent that value as a binary floating point number. Just think about how the number 1/3 can’t be represented in decimal notation – no finite sequence of 0.33333… will ever reach the value – and you can imagine how not all decimal quotients are representable in binary.

Now, the whole point of a type like Decimal is to overcome this problem. With a Decimal, you can precisely represent both 0.1 and 3.133.

let myDecimal = Decimal(string: "3.133")! // 3.133

There is no reason why a JSON decoder shouldn’t be able to do this. Unfortunately, JSONDecoder doesn’t do JSON decoding of its own – it wraps JSONSerialization. (One way to conclude that this is the case is to give it some invalid JSON as input, and you may recognize the errors as JSONSerialization errors, but we can also take a look at the source code – here is the call to JSONSerialization.)

So, we are then not simply reading a decimal number string into a decimal number type, but converting via a binary floating point number. In the method with signature func unbox(_ value: Any, as type: Decimal.Type) throws -> Decimal? we find how this conversion happens.

        // Attempt to bridge from NSDecimalNumber.
        if let decimal = value as? Decimal {
            return decimal
        } else {
            let doubleValue = try self.unbox(value, as: Double.self)!
            return Decimal(doubleValue)
        }

The question is, why is this initializer doing such a poor job of converting the Double to the Decimal? This is no longer a JSON issue – we can easily reproduce this in a playground:

let myDecimal2 = Decimal(Double(3.133)) // 3.132999999999999488

Or even just:

let myDecimal3: Decimal = 3.133 // 3.132999999999999488

That last one may be particularly surprising – the thing to remember here is that there are no Decimal literals; the Decimal types just conforms to the ExpressibleByFloatLiteral. I don’t think this is a great design decision; anyone who uses the Decimal type will do that for the particular reason of staying within decimal floating point logic; that person does not want a convenient way to initialize a Decimal that passes through a binary floating point conversion.

Especially not when that conversion doesn’t seem to be very good! It seems to me that, for any string representation of a decimal floating point number I can come up with (that does not overflow), this two-step conversion:

func decimalGood(from double: Double) -> Decimal {
    return Decimal(string: "\(double)")!
}

Gives better results than:

func decimalBad(from double: Double) -> Decimal {
    return Decimal(double)
}

I might very well be missing something here. This isn’t new behavior, [[NSDecimalNumber alloc] initWithDouble:3.133] works the same way. If anyone can tell me why the initializer of decimal numbers from binary floating point doesn’t use the same kind of decimalization algorithm as the string representation does, I’d be curious to know.

Conclusion

A few people who read the initial version of this text pointed out that the most reliable way of passing a decimal number via JSON is in a string. That is probably the most pragmatic thing to do, if you have the possibility. In some cases, however, the API is already a fact. If you are using JSONDecoder, you might prefer to use a Double rather than a Decimal as the decoded type even if you want the end result to be a Decimal, and do the conversion yourself, possibly via a String. Unless someone fixes JSONDecoder.

Thanks to Tim Vermeulen for bringing up the issue, Helge Heß and Roland Persson for discussions on the topic and Emre Berge Ergenekon for proof-reading.