Swift 中的 undefined

/**
 * `undefined()` pretends to be able to produce a value of any type `T` which can
 * be very useful whilst writing a program. It happens that you need a value
 * (which can be a function as well) of a certain type but you can't produce it
 * just yet. However, you can always temporarily replace it by `undefined()`.
 *
 * Inspired by Haskell's
 * [undefined](http://hackage.haskell.org/package/base-4.7.0.2/docs/Prelude.html#v:undefined).
 *
 * Invoking `undefined()` will crash your program.
 *
 * Some examples:
 *
 *  - `let x : String = undefined()`
 *  - `let f : String -> Int? = undefined("string to optional int function")`
 *  - `return undefined() /* in any function */`
 *  - `let x : String = (undefined() as Int -> String)(42)`
 *  - ...
 *
 * What a crash looks like:
 *
 * `fatal error: undefined: main.swift, line 131`
 *
 */
public func undefined<T>(hint: String = "", file: StaticString = #file, line: UInt = #line) -> T {
    let message = hint == "" ? "" : ": \(hint)"
    fatalError("undefined \(T.self)\(message)", file:file, line:line)
}