Protocol inheritance in Swift

protocols.swift

//: Playground - noun: a place where people can play

import UIKit

protocol A { }
protocol B: A { }
class MyClass: B { }

let b: [B] = (1...3).map { MyClass(indx: $0) }

// Cannot convert value of type '[B]' to specified type '[A]'
let a: [A] = b

Solution: let a: [A] = b.map { $0 as A }
Explanation: http://stackoverflow.com/questions/33090626/swift-upcasting-array-of-protocol-to-array-of-super-protocol-causes-error