Make a protocol conform to another protocol

Question

I have two protocols: Pen and InstrumentForProfessional. I'd like to make any Pen to be an InstrumentForProfessional:

protocol Pen {   var title: String {get}   var color: UIColor {get} }  protocol Watch {} // Also Instrument for professional protocol Tiger {} // Not an instrument  protocol InstrumentForProfessional {   var title: String {get} }  class ApplePen: Pen {   var title: String = "CodePen"   var color: UIColor = .blue }  extension Pen: InstrumentForProfessional {} // Unable to make ApplePen an Instument for Professional: Extension of protocol Pen cannot have an inheritance clause  let pen = ApplePen() as InstrumentForProfessional 

Answer 1

Protocols can inherit each other:

Protocol Inheritance

A protocol can inherit one or more other protocols and can add further requirements on top of the requirements it inherits. The syntax for protocol inheritance is similar to the syntax for class inheritance, but with the option to list multiple inherited protocols, separated by commas:

protocol InheritingProtocol: SomeProtocol, AnotherProtocol {     // protocol definition goes here } 

So, you basically need to do this:

protocol InstrumentForProfessional {     var title: String {get} }  protocol Pen: InstrumentForProfessional {     var title: String {get} // You can even drop this requirement, because it's already required by `InstrumentForProfessional`     var color: UIColor {get} } 

Now everything that conforms to Pen conforms to InstrumentForProfessional too.