&gt; The compiler statically checks your type annotations, but then generates untyped PHP code. If Hack-generated PHP interacts with PHP from 5 years ago, &quot;type errors&quot; are possible.<p>That&#x27;s not entirely true. Though some parts of Hack&#x27;s annotations are not enforced by the runtime, if you say a function takes an int and pass it a float at runtime, it does throw an error from what I remember.<p>It&#x27;s correct that Hack doesn&#x27;t guarantee runtime soundness, though.