Migrating to Hack from PHP was huge undertaking requiring years to conclude, and that almost kept the same syntax and semantics where possible. What would the benefit of spending thousands of engineer years (read: billions of dollars) on a rewrite be? Especially when it would nearly certainly come with a huge performance regression compared to continuing to use HHVM which has come with years of optimization for worki…
Do you know if Facebook uses Composer to manage PHP dependencies?
Not 100% accurate, but if a package's composer.json does not require HHVM, it will most likely install into a hack project, but be unusable.