After writing a couple small programs in Hack I've realized that it's not types themselves that make writing Hack enjoyable: it's the tight feedback loop Hack creates between the machine and myself. Integrating the Hack type checker into my editor means that my entire codebase is analyzed in a split second as soon as I save a file. This immediately surfaces any dumb, or subtle mistakes I made. I find myself writing code fearlessly: when I forget what a function returns, I just write code that calls it with what I think it returns. If I'm wrong, the type checker will tell me immediately. I can fix it quickly, and move on.

http://www.dmiller.io/blog/2015/4/26/comparing-the-php7-and-hack-type-systems