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

Why isn't it also a @propertyWrapper?

Decimals are often relied upon to accurately represent monetary values in financial applications. Given this delicate use case, I decided the most responsible way to expose this API is as a first-class struct only, to be used wherever a Decimal type may be declared. This strict way of offering the library ensures no loss of precision leaks through one's codebase by otherwise forgetting to annotate a Decimal property with @PreciseDecimal.

Why doesn't it have [insert here] 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.