Generic Types And Functions: Type Arguments

A generic type argument can be a type-specifier or a name that is a type parameter. Either way, at runtime, it is used in place of the corresponding type parameter. A generic type argument can either be open or closed ($$).

final class Pair<Tv1, Tv2> implements ConstVector<mixed> {
  public function getIterator(): KeyedIterator<int, mixed> { ... }
  public function keys(): Vector<int> { ... }
  public function toMap(): Map<int, mixed> { ... }
  public function zip<Tu>(Traversable<Tu> $iter): Vector<Pair<mixed, Tu>> {...}

In this case, the type specifiers ConstVector<mixed>, KeyedIterator<int, mixed>, Vector<int>, Map<int, mixed>, Traversable<Tu>, Vector<Pair<mixed, Tu>>, and Tu, and are type arguments.