Swift 2 add protocol conformance to protocols

Question

Can I add protocol conformance to a protocol via a swift extension?

//Plain old protocol here protocol MyData {     var myDataID: Int { get } } 

I want to make the MyData protocol equatable by default (just compare the ID)

extension MyData : Equatable { } 

But I get this lovely error:

"Extension of protocol 'MyData' cannot have an inheritance clause"

The behavior i'm looking is BananaData conforming to Equatable (a protocol) because it implements the MyData protocol which can provide a default implementation of Equatable

//This is the method to implement Equatable func ==(lhs: MyData, rhs: MyData) -> Bool {     return lhs.myDataID == rhs.myDataID }  struct BananaData: MyData {     var myDataID: Int = 1 }  func checkEquatable(bananaOne: BananaData, bananaTwo: BananaData) {     //This compiles, verifying that BananaData can be compared     if bananaOne == bananaTwo { }     //But BananaData is not convertible to Equatable, which is what I want     let equatableBanana = bananaOne as Equatable     //I don't get the additional operations added to Equatable (!=)     if bananaOne != bananaTwo { } //Error } 

Answer 1

As the error message says: an extension of a protocol cannot have an inheritance clause. Instead, you could make MyData protocol inherit from Equatable in the original declaration.

protocol MyData: Equatable {     var myDataID: Int { get } } 

You could then extend add an implementation of == for types that conform to MyData:

func == <T: MyData>(lhs: T, rhs: T) -> Bool {     return lhs.myDataID == rhs.myDataID } 

However, I would highly not recommend this! If you add more properties to conforming types, their properties won't be checked for equality. Take the example below:

struct SomeData: MyData {     var myDataID: Int     var myOtherData: String }  let b1 = SomeData(myDataID: 1, myOtherData: "String1") let b2 = SomeData(myDataID: 1, myOtherData: "String2")  b1 == b2 // true, although `myOtherData` properties aren't equal. 

In the case above you'd need to override == for SomeData for the correct result, thus making the == that accepts MyData redundant.