A sample to demonstrate how to simulate multimethods in Swift

multimethod.swift

// Simulating multimethods in Swift

// Consider these two hierarchies:

//       FamilyA
//      /       \
//    A1         A2
//      \
//       SubA1

public class FamilyA { }
public class A1: FamilyA { }
public class SubA1: A1 { }
public class A2: FamilyA {}

//       FamilyB
//      /       \
//    B1         B2
//      \
//       SubB1

public class FamilyB { }
public class B1: FamilyB { }
public class SubB1: B1 { }
public class B2: FamilyB {}

public func multimethod( a: FamilyA, b: FamilyB ) {
    
    // Dispatch based on the dynamic type of a and b:
    // Be careful about case order: They should be from the most specific to less
    // specific and you may have a hard time deciding on some cases.
    
    switch (a, b) {
        
    case let (a as SubA1, b as B1): multimethod(subA1: a, b1: b)
    case let (a as A2, b as B1):    multimethod(a2:    a, b1: b)
    case let (a as A2, b as B2):    multimethod(a2:    a ,b2: b)
        
    default:                        _multimethod(a,b)
    
    }
}

// Default implementation for the general parameter types:
private func _multimethod(a: FamilyA, b: FamilyB) {
    println("Default: a is FamilyA, b is FamilyB")
}

// As you see, there is no point in function overloading here. Selection is done at runtime:

private func multimethod(#subA1: SubA1, #b1: B1) { println("Case: a as SubA1, b as B1") }
private func multimethod(#a2: A2, #b1: B1)       { println("Case: a as A2, b as B1") }
private func multimethod(#a2: A2, #b2: B2)       { println("Case: a as A2, b as B2") }

// Now lets try it out:

var a1 = A1(), a2 = A2(), a3 = SubA1()
var b1 = B1(), b2 = B2(), b3 = SubB1()

println("1. a is A1, b is B1")
multimethod(a1,b1)

println("2. a is A1, b is B2")
multimethod(a1,b2)

println("3. a is A1, b is SubB1")
multimethod(a1,b3)

println("4. a is A2, b is B1")
multimethod(a2,b1)

println("5. a is A2, b is B2")
multimethod(a2,b2)

println("6. a is A2, b is SubB1")
multimethod(a2,b3)

println("7. a is SubA1, b is B1")
multimethod(a3,b1)

println("8. a is SubA1, b is B2")
multimethod(a3,b2)

println("9. a is SubA1, b is SubB1")
multimethod(a3,b3)