Record from: https://itnext.io/single-line-persistence-definitions-in-swift-31f32a1dd2aa

PersistentStore.swift

import UIKit

protocol StoredObject {
    associatedtype Primitive
    func toPrimitive() -> Primitive?
    static func from(primitive: Primitive) -> Self?
}

protocol StoredAsSelf: StoredObject where Primitive == Self { }

extension StoredAsSelf {
    
    func toPrimitive() -> Primitive? {
        return self
    }
    
    static func from(primitive: Primitive) -> Primitive? {
        return primitive
    }
}

extension String: StoredAsSelf {
    typealias Primitive = String
}

extension Int: StoredAsSelf {
    typealias Primitive = Int
}

extension Float: StoredAsSelf {
    typealias Primitive = Float
}

extension Date: StoredObject {
    
    typealias Primitive = TimeInterval
    
    func toPrimitive() -> Primitive? {
        return timeIntervalSince1970
    }
    
    static func from(primitive: Primitive) -> Date? {
        return Date(timeIntervalSince1970: primitive)
    }
}

protocol StringLiteralBoilerplate {
    init(stringLiteral value: String)
}

extension StringLiteralBoilerplate {
    typealias StringLiteralType = String
    
    init(unicodeScalarLiteral value: String) {
        self.init(stringLiteral: value)
    }
    
    init(extendedGraphemeClusterLiteral value: String) {
        self.init(stringLiteral: value)
    }
}

struct Storable<Object>: ExpressibleByStringLiteral, StringLiteralBoilerplate where Object: StoredObject {
    
    private let key: String
    private let store = UserDefaults.standard
    
    init(stringLiteral value: String) {
        self.key = value
    }
    
    var value: Object? {
        set {
            store.set(newValue?.toPrimitive(), forKey: key)
        }
        get {
            guard let p = store.value(forKey: key) as? Object.Primitive else { return nil }
            return Object.from(primitive: p)
        }
    }
}