[Swift4] String's flatMap

string_flatmap_sample.swift

var message: String = "test"
// Returns Array<Character> 
let conv = message.flatMap { (chr) /*Character*/ in
    return String(chr) + "/"
}
print(type(of: conv)) //Array<Character>
print(conv) //["t", "/", "e", "/", "s", "/", "t", "/"] 

//https://developer.apple.com/documentation/swift/string/2903352-flatmap
let conv2 = message.flatMap { (chr) /*Character*/ -> String? in
    return String(chr) + "/"
}
print(type(of: conv2)) //Array<String>
print(conv2) //["t/", "e/", "s/", "t/"]