My 2 cents: dynamic typing works okay for library consumers. For libraries themselves though, or platform code, the disadvantages are real. It is harder to fix

My 2 cents: dynamic typing works okay for library consumers. For libraries themselves though, or platform code, the disadvantages are real. It is harder to fix and extend code when you don't know who calls it, how they call it, what they get in return. Complex code becomes littered with 'black holes'. That is a big part of why facebook implemented Hack. I heard a talk by one of the developers. Even now there are PHP blackholes in the Facebook code base that they can't migrate to Hack.