partial.swift

@dynamicMemberLookup
struct Partial<Wrapped> {
    private struct AnyProperty {
        let keyPath: PartialKeyPath<Wrapped>
        let value: Any
        let writer: (inout Wrapped, Any) -> Void
        
        init<T>(keyPath: WritableKeyPath<Wrapped, T>, value: T) {
            self.keyPath = keyPath
            self.value = value
            self.writer = { instance, value in
                instance[keyPath: keyPath] = value as! T
            }
        }
        
        func update(_ wrapped: inout Wrapped) {
            self.writer(&wrapped, value)
        }
    }
    
    private var values: [PartialKeyPath<Wrapped>: AnyProperty] = [:]
    
    subscript<T>(dynamicMember keyPath: WritableKeyPath<Wrapped, T>) -> T? {
        get { self.values[keyPath]?.value as? T }
        set {
            if let value = newValue {
                self.values[keyPath] = AnyProperty(keyPath: keyPath, value: value)
            } else {
                self.values.removeValue(forKey: keyPath)
            }
        }
    }
    
    func apply(to original: Wrapped) -> Wrapped {
        var copy = original
        for (_, property) in values {
            property.update(&copy)
        }
        return copy
    }
}