逐条解释一下，熟悉 “as”的用法，是“as”，不是“as？”和 “as!”：

for thing in things {
    switch thing {
    case 0 as Int:
        print("zero as an Int")

    case 0 as Double:
        print("zero as a Double")

    case let someInt as Int:
        print("an integer value of \(someInt)")

    case let someDouble as Double where someDouble > 0:
        print("a positive double value of \(someDouble)")

    case is Double:
        print("some other double value that I donot want to print")

    case let someString as String:
        print("a string value of \"\(someString)\"")

    case let (x,y) as (Double,Double):
        print("an (x,y) point at \(x),\(y)")

    case let movie as Movie:
        print("a movie called \(movie.name),dir.\(movie.director)")

    case let stringConverter as (String) -> String:
        print(stringConverter("Michael"))

    default:
        print("something else")

    }
}

(1) 0 as Int

(2) 0 as Double