3.2.1 Constraints

Type parameters can be constrained to multiple types:

typedef Measurable = {
  public var length(default, null):Int;

class Main {
  static public function main() {
    trace(test(["bar", "foo"]));
    // String should be Iterable<String>

  static function test<T:(Iterable<String>, Measurable)>(a:T) {
    if (a.length == 0) return "empty";
    return a.iterator().next();

Type parameter T of method test is constrained to the types Iterable<String> and Measurable. The latter is defined using a typedef for convenience and requires compatible types to have a read-only property named length of type Int. The constraints then say that a type is compatible if

  • it is compatible with Iterable<String> and
  • has a length-property of type Int.

We can see that invoking test with an empty array in line 7 and an Array<String> in line 8 works fine. 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>.