類型參數可以被約束為多種類型： > Type parameters can be constrained to multiple types: ~~~ typedef Measurable = { public var length(default, null):Int; } class Main { static public function main() { trace(test([])); trace(test(["bar", "foo"])); // String should be Iterable<String> //test("foo"); } static function test<T:(Iterable<String>, Measurable)>(a:T) { if (a.length == 0) return "empty"; return a.iterator().next(); } } ~~~ 方法test的類型參數T被約束為Iterable<String>和Measurable。后者方便起見被使用一個 typedef（第3.1節）定義，需要兼容有一個只讀（第4.2節）的Int類型length屬性的類型。約束然后說一個類型是兼容的，如果： > Type parameterT of method test is constrained to the types Iterable<String> and Measurable. The latter is de?ned using a typedef (3.1) for convenience and requires compatible types to have a read-only property (4.2) named length of type Int. The constraints then say that a type is compatible if * 和 Iterable<String>兼容 * 并且有一個Int類型的length屬性 > * it is compatible with Iterable<String> and > * has a length-property of type Int. 我們可以看到，在第7行調用test并傳遞一個空的數組，和第8行一個Array<String>是沒有問題的。因為數組都有length屬性和一個 iterable 方法。然而，傳遞一個字符串作為參數，如第9行則會導致約束失敗，因為字符串不兼容 Iterable<T>。 > We can see that invoking test with an empty array in line 7 and an Array<String> in line 8 works ?ne. This is because Array has both a length-property and an iterator-method. However, passing a String as argument in line 9 fails the constraint check because String is not compatible with Iterable<T>.