Unsafe casts are useful to subvert the type system. The compiler types expr as usual and then wraps it in a monomorph (2.9). This allows the expression to be assigned to anything. 不安全的類型轉換不使用任何 dynamic（第2.7節）類型，如下面例子展示的： > Unsafe casts do not introduce any dynamic (2.7) types, as the following example shows: ~~~ class Main { public static function main() { var i = 1; $type(i); // Int var s = cast i; $type(s); // Unknown<0> Std.parseInt(s); $type(s); // String } } ~~~ 變量 i 類型化為 Int，然后經過不安全轉換 cast i 之后賦值到變量。這使 s 成為一個 unknown 類型，一個單形。根據合一（第3.5節）的一般規則，它可以之后被綁定為任何類型，例如例子中的 String 。 > Variable i is typed as Int and then assigned to variables using the unsafe cast cast i. This causes s to be of an unknown type,amonomorph. Following the usual rules of uni?cation(3.5), it can then be bound to any type, such as String in this example. 這些轉換被稱為不安全的，因為無效轉換的運行時行為沒有被定義。而多數 動態目標語言（第2.2節）可能可以工作，但是在靜態目標語言（第2.2節）可能導致未定義的錯誤。 > These casts are called ”unsafe” because the runtime behavior for invalid casts is not de?ned. While most dynamic targets (2.2) are likely to work, it might lead to unde?ned errors on static targets (2.2). 不安全轉換幾乎沒有運行時的開銷。 > Unsafe casts have little to no runtime overhead.