Extren可以用一个类型安全的方式来描述目标语言特定的交互。它们像普通类一样定义，除了： > Externs can be used to describe target-specific interaction in a type-safe manner. They are defined like normal classes, except that * class关键字被extern 关键字领先， * 方法（第4.3节）没有表达式， * 所有的参数和返回类型需要是显式的。 > * the class keyword is preceded by the extern keyword, > * methods (4.3) have no expressions and > * all argument and return types are explicit. 常见的例子在Haxe标准库（第10章），Math 类，摘录如下： > A common example from the Haxe Standard Library (10) is the Math class, as an excerpt shows: ~~~ extern class Math { static var PI(default,null) : Float; static function floor(v:Float):Int; } ~~~ 我们看到，externs 既可以定义方法也可以定义变量（时尚，PI是声明为一个只读属性（第4.2节））。一旦这个信息对于编译器可用，它启用相应的字段访问，也被称为类型： > We see that externs can define both methods and variables (actually, PI is declared as a readonly property (4.2)). Once this information is available to the compiler, it allows field access accordingly and also knows the types: ~~~ class Main { static public function main() { var pi = Math.floor(Math.PI); $type(pi); // Int } } ~~~ 这这可以运行，因为floor方法返回类型声明为Int。 > This works because the return type of method floor is declared to be Int. Haxe标准库带有许多外部类，对于Flash和JavaScript目标语言。它们使可以以类型安全的方式访问原生的APIs，当作设计高层APIs的工具。Externs也可以应用haxelib（第11章）中许多流行的原生库。 > The Haxe Standard Library comes with many externs for the Flash and JavaScript target. They allow accessing the native APIs in a type-safe manner and are instrumental for designing higher-level APIs. There are also externs for many popular native libraries on haxelib (11). Flash，Java和C#目标语言允许通过命令行（第7章）直接包括原生的库。目标特定的细节在每个目标语言细节（第12章）等各自的章节。 > The Flash, Java and C# targets allow direct inclusion of native libraries from command line (7). Target-specific details are explained in the respective sections of Target Details (Chapter 12). 一些目标语言如Python或者JavaScript可能需要产生附加的 import 代码，为原生模块加载一个extern 类。Haxe提供声明这样的依赖关系在各自的目标语言向细节（第12章）的方式。。 > Some targets such as Python or JavaScript may require generating additional ”import” code that loads an extern class from a native module. Haxe provides ways to declare such dependencies also described in respective sections Target Details (Chapter 12). **Haxe3.2.0以后剩余的参数和类型选择** > Rest arguments and type choices Since Haxe 3.2.0 haxe.extern 包提供两种类型帮助映射原生的语义到Haxe： > The haxe.extern package provides two types that help mapping native semantics to Haxe: **Rest<T>** 这个类型可以被实用为一个最终的函数参数，来使传递一个额外的调用参数任意的数值到函数。类型参数可以被使用来限制这些参数为某个特定类型。 > **Rest<T>**: This type can be used as a final function argument to allow passing an arbitrary number of additional call arguments. The type parameter can be used to constrain these arguments to a specific type. **EitherType<T1, T2>**：这个类型允许使用任何一个类型参数类型，因此表示了一个类型选择。它可以被嵌套来允许更多的类型。 > **EitherType<T1,T2>**: This type allows using either of its parameter types,thus representing a type choice. It can be nested to allow more than two different types. 我们在下面代码示例中演示了其用法： > We demonstrate the usage in this code sample: ~~~ import haxe.extern.Rest; import haxe.extern.EitherType; extern class MyExtern { static function f1(s:String, r:Rest<Int>):Void; static function f2(e:EitherType<Int, String>):Void; } class Main { static function main() { MyExtern.f1("foo", 1, 2, 3); // use 1, 2, 3 as rest argument MyExtern.f1("foo"); // no rest argument //MyExtern.f1("foo", "bar"); // String should be Int MyExtern.f2("foo"); MyExtern.f2(12); //MyExtern.f2(true); // Bool should be EitherType<Int, String> } } ~~~