It was very important to us to have a gradual and smooth upgrade path, such that you can start using Hack by just tweaking the "<?php" header at the top of a so

It was very important to us to have a gradual and smooth upgrade path, such that you can start using Hack by just tweaking the "<?php" header at the top of a source file. Obviously switching to a syntactically incompatible programming language would be at odds with that goal.