PreciseDecimal

Introduction

Swift has long suffered a problem with its Decimal type: unapparent loss of precision.

This happens with all common ways of initializing:

let badDecimal = Decimal(3.133) // 3.132999999999999488

let badDecimal: Decimal = 3.133 // 3.132999999999999488

But not these ones:

let goodDecimal = Decimal(string: "3.133") // 3.133

let goodDecimal = Decimal(sign: .plus, exponent: -3, significand: 3133) // 3.133

Furthermore, this also applies to JSON decoding since it uses NSJSONSerialization under the hood, which is presumed to parse decimal numbers as Double and then initializing a Decimal via its lossy Double initializer as exemplified above. A common workaround for this is to receive sensitive Decimal values as strings and parsing into Decimal with the working string initializer, however oftentimes the format of a JSON payload is out of one's control.

This is something that Apple will most likely fix at some point. In the meantime, PreciseDecimal has your back.

Usage

This library declares a lightweight PreciseDecimal type as a wrapper around Decimal, with precise init and Decodable implementations.

let goodDecimal = PreciseDecimal(3.133) // 3.133

let goodDecimal: PreciseDecimal = 3.133 // 3.133

struct Price: Decodable {
    let amount: PreciseDecimal
}

let json = #"{ "amount": 3.133 }"#.data(using: .utf8)!
let goodDecimal = try JSONDecoder().decode(Price.self, from: json).amount // 3.133

Try it out!

PreciseDecimal supports Arena to effortlessly take it for a spin in a playground before you decide to add it to your codebase.

Simply install Arena and run arena davdroman/PreciseDecimal --platform macos in your terminal.

FAQ

Is this solution bullet-proof?

No.

PreciseDecimal falls short for very high precision numbers. Case in point:

let a = PreciseDecimal(  1234567890.0123456789 )
let b = Decimal(string: "1234567890.0123456789")!
print(a) // 1234567890.0123458
print(b) // 1234567890.0123456789

So if you're going to be dealing with more than 6 decimal places, this library is not for you. Instead, the best solution as it currently stands is to represent decimals as strings, especially when it comes to JSON serialization.

It's up to Apple and only Apple to introduce real Decimal literals into the language, as well as fixing the JSON serialization mechanisms in Foundation.

Why not make it a @propertyWrapper?

Because it's very easy to forget to annotate properties, especially since there aren't any compiler checks or tests to ensure the slight change in behavior it provides, leading to sneaky bugs down the road.

Why doesn't it have [x] feature?

In order to keep the library's scope and implementation as lightweight as possible, optimistic for a painless obsolescence once Apple fixes Decimal.

Do feel free to suggest otherwise if I missed a vital part of functionality that should definitely be in this library.

Will Apple ever fix this?

I don't know.

However, you're not alone in your discontent. Here are two relevant issues you can upvote to improve the chances for Apple to see them:

• SR-920
• SR-7054