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Generics: Type Erasure

Type Parameters are erased at runtime. This means that the type parameters are not available at runtime.


function foo(vec<int> $integers): void {

function bar(): void {
    foo(vec["foo", "bar"]);

The compiler will emit a type error for the call to foo() in bar() because the type of the argument is vec<string>, which is not a subtype of vec<int>.

If we ignore the error using --ignore flag, the code will compile and run, however, ignoring type errors is highly discouraged, as it can lead to runtime errors.


Due to the type erasure, the following features are not supported:

  • Type parameters in instanceof expressions ( if ($value instanceof Foo<T>) { ... } )
  • Type parameters in is expressions ( if ($value is Foo<T>) { ... } )
  • Type parameters in as expressions ( $value as Foo<T> )
  • Type parameters in new expressions ( new T() )
  • Type parameters in catch blocks ( try { ... } catch (T $e) { ... } )

For passing around class names, Ara has classname<T> type, which can be used to create instances of a specific class.


interface Foo<T> {
    public function __construct(T $value);

function foo<T>(classname<Foo<T>> $class, T $value): void {
    $instance = new $class($value);


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