duck typing

ducktyping.php

<?php

// duck typing
final class Foo {
    public function bar ($object) {
        $object->quack(); 
    }
}
// Because we know nothing about $object, it's only by calling quack that we can 
// tell whether object supports quack()

// Same but with a language construct that statically declares the need for $object
// to support a quack method. It could now be verified at compile time. 
final class Foo {
    public function bar ($object :: quack()) {
        $object->quack();
    }
}

@mathiasverraes Do other languages have this?