[swift-users] Swift 3 Decimal NSDecimal NSDecimalNumber

[Brent Royal-Gordon]

> On Dec 20, 2016, at 8:32 AM, Alex Thurston via swift-users <swift-users at swift.org> wrote:
> 
> Swift 3 introduced the Decimal class, which at first I thought was supposed to 'replace' NSDecimalNumber in Swift, like Error is the new NSError, etc.  However, upon further investigation, it's not clear to me that this is actually the case. It appears that Swift 3's Decimal is more just an NSDecimal, but with some bridging so it can use NSDecimalNumber's methods when needed. What I'm wondering is, should I be replacing NSDecimalNumber in my app with Decimal? Is Decimal the successor to NSDecimalNumber in Swift?

`Decimal` is actually a struct, not a class, and in fact it's just the Swift name for the `NSDecimal` struct in Objective-C. In Swift, functions like `NSDecimalAdd` are exposed as operators on `Decimal` and are used to satisfy the requirements of various numeric protocols like `SignedNumber`.

The end result is that `Decimal` is a lot like `Int` or `Double`—it's a value type which can be conveniently used to do all sorts of math. `NSDecimalNumber`, then, is equivalent to `NSNumber`.

> Since my app is doing a lot of formatting, it would require much casting between Decimal and NSDecimalNumber, or adding of extensions to use, so I don't think it would be natural/convenient without many extensions. Is Swift planning to improve Decimal to be more usable with NumberFormatter, or other object-oriented number APIs? Hopefully I have given enough information to answer my question. 

Objective-C APIs which use `NSDecimalNumber` should be bridged to `Decimal`, just as `NSString` is bridged to `String`. That bridging may not work for category methods, though; you could expose those to Swift by writing an extension on `Decimal` which casts `self` to `NSDecimalNumber` and then calls through to your Objective-C methods.

Similarly, you should be able to use `Decimal` with `NumberFormatter` by casting to `NSDecimalNumber`, just as you would cast `Int` or `Double` to `NSNumber` to use them with `NumberFormatter`.

-- 
Brent Royal-Gordon
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