I have learned a "Gray Magic" of the Swift.

GrayShark.swift

import Foundation

class GrayShark {
    var name:String
    var howSwim:String
    
    init(name:String, howSwim:String) {
        self.name = name
        self.howSwim = howSwim;
    }
}

func letSharkSwim(shark:GrayShark) -> String {
    return "\(shark.name) swims like '\(shark.howSwim)'."
}

// OK
letSharkSwim(GrayShark(name:"The Average Shark", howSwim:"Swim Swim Swim Lurk"))

// Error
// Could not find an overload for '__conversion' that accepts the supplied arguments
//letSharkSwim(nil)

// Here I cast a couple of "Gray Magics" of the swift language!
// http://vperi.com/2014/06/06/opt-in-type-casting-in-swift/
// http://swift.sh/topic/80/swift/
// 
// There are several Gray Magics available in the article above, like:
// - nil is a singleton instance of the NilType (and we can make a extension of NilType)
// - @conversion func __conversion() -> Type for custom type conversions (also works for as operators as well)
//   I believe we can omit @conversion attribute
// - Optional<T> is actually a enum which has 2 values, None and Some<T> (it's written in swift standard header so not that graymagic-ish)
//
// Here are some additional Gray Magics I've learned after some investigation:
// - ImplicitlyUnwrappedOptional<T> is, on the other hand, struct!
// - implement LogicValue protocol to be used in logical statement (e.g. if)
// - Reflectable protocol found... hmm looks like this could be really interesting magical source... XD
extension NilType {
    @conversion func __conversion() -> (GrayShark) {
        return GrayShark(name:"Gray Shark", howSwim:"UNBELIEVABLE POWERRRRRRRR")
    }
}

// Enough long stories. Lets try it out.
// Oh my goodness. It worked.
letSharkSwim(nil)

let eof = "EOF"