Phantom types in Swift

Not every language with a static type system is as strong as Swift. This is made possible by Swift features such as phantom types, generic type extensions, and enumerations with associated types. This week we will learn how to use phantom types to create type-safe APIs.

The basics

 — , , , . API. .

struct Identifier<Holder> {
    let value: Int
}
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

Identifier Holder. , Holder Identifier. . .

struct User {
    let id: Identifier<Self>
}

struct Product {
    let id: Identifier<Self>
}

let product = Product(id: .init(value: 1))
let user = User(id: .init(value: 1))

user.id == product.id
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

User () Product (), Identifier. 1 user product. , Swift :

«==» Identifier-User Identifier-Product.

, «» «». . Swift . , Swift .

func fetch(_ product: Identifier<Product>) -> Product? {
    // return product by id
}

fetch(user.id)
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

HealthKit

. . , HealthKit Apple Watch. , Apple Health.

import HealthKit

let store = HKHealthStore()
let bodyMass = HKQuantityType.quantityType(
    forIdentifier: HKQuantityTypeIdentifier.bodyMass
)!
let query = HKStatisticsQuery(
    quantityType: bodyMass,
    quantitySamplePredicate: nil,
    options: .discreteAverage
) { _, statistics, _ in
    let average = statistics?.averageQuantity()
    let mass = average?.doubleValue(for: .meter())
}

store.execute(query)
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

Apple Health. . , , . , API.

enum Distance {
    case mile
    case meter
}

enum Mass {
    case pound
    case gram
    case ounce
}

struct Statistics<Unit> {
    let value: Double
}


extension Statistics where Unit == Mass {
    func convert(to unit: Mass) -> Double {

    }
}

extension Statistics where Unit == Distance {
    func convert(to unit: Distance) -> Double {

    }
}

let weight = Statistics<Mass>(value: 75)
weight.convert(to: Distance.meter)
      
      

        
        
        
      

    
        
        
        
      
      

        
        
        
      

    
    

HealthKit, API . Mass () Distance (), . , Swift , :

Distance Mass.

, Swift. , . API . , . Twitter . !

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