[rejschaap]

I have never seen anyone successfully bolt type-checking onto an existing language. It is always painful to use. It usually requires ugly added syntax and you need a special compiler if the language isn't flexible enough.

Why not just use a different language altogether? One that was designed with static type checking from the ground up?

Maybe Facebook likes the pain, they also pushed PHP further than anyone else was willing to. Burning insane amounts of resources with a somewhat disappointing result.