Hack is more than just a type system. At the type level, it would be trivial to erase the types and run the code on vanilla PHP. Dealing with everything else is

Hack is more than just a type system. At the type level, it would be trivial to erase the types and run the code on vanilla PHP. Dealing with everything else is much harder (xhp, containers, lambda, etc.), at which point it just makes more sense to use HHVM.