ReferenceKeyPathEditable Complete Source code including consumption

ModifyClassPropertyUsingKeyPath.swift

enum KeyPathError: Error {
    case unableToCast(String)
}

protocol ReferenceKeyPathEditable {
    associatedtype Root
    func update<KeyPathType>(type: Root, path: PartialKeyPath<Root>, to value: KeyPathType) throws
}

extension ReferenceKeyPathEditable {
    func update<KeyPathType>(type: Root, path: PartialKeyPath<Root>, to value: KeyPathType) throws {
        guard let writableKeyPath = path as? ReferenceWritableKeyPath<Root, KeyPathType> else {
            throw KeyPathError.unableToCast("because of passed \(value) value data type")
        }
        type[keyPath: writableKeyPath] = value
    }
}

class Merchant: ReferenceKeyPathEditable {
    typealias Root = Merchant
    private(set) var name: String
    private(set) var occupation: String
    
    init(name: String, occupation: String) {
        self.name = name
        self.occupation = occupation
    }
}

let merchant = Merchant(name: "Ryan", occupation: "Grocery Business")
print("** Initial: \(merchant.occupation) **")
try? merchant.update(type: merchant, path: \Merchant.occupation, to: "Furniture Business")
print("** Updated: \(merchant.occupation) **")