Response to Ghost of Swift Bugs Future

static dispatch in protocol extensions.playground

/*:
# response to [__Ghost of Swift Bugs Future__ by ALEXANDROS SALAZAR](http://nomothetis.svbtle.com/the-ghost-of-swift-bugs-future)
*/

protocol A {
    func m1() -> String
}

extension A {
    func m1() -> String {
        return "hello"
    }
    
    func m2() -> String {
        return "planet"
    }
}

struct B : A {
    func m1() -> String{
        return "greetings"
    }
    
    func m2() -> String {
        return "earthling"
    }
}

struct C : A {
    func m2() -> String {
        return "darkness, my old friend"
    }
}

/*:
# first error
type inferences can't (arguably shouldn't) infer this as an array with Element type A
*/
//let e = [B(), C()]

/*:
# second error
if the array has an Element type of Any, an incomprehensible error is shown caused by m1 and m2 not being visible to the compiler's static type checking
*/
//let e2:[Any] = [B(), C()]
//print(e2.map{$0.m1()}) // ["greetings", "hello"]
//print(e2.map{$0.m2()}) // ["earthling", "darkness, my old friend"]

/*:
# array of A
an array of A shows that static dispatch is used for m2, calling the protocol extension method
*/
let a:[A] = [B(), C()]
print(a.map{$0.m1()}) // ["greetings", "hello"]
print(a.map{$0.m2()}) // ["planet", "planet"]

/*:
# direct objects
calling the methods on values of the concrete types B and C shows static dispatch calling the implementations of those types
*/
B().m1() // "greetings"
C().m1() // "hello"
B().m2() // "earthling"
C().m2() // "darkness, my old friend"