[bos]

Because this is tremendously important to the usability of the language.

One of the reasons that PHP has been such a success is that you can simply save a source file and reload a web page to see what's going on.

The server-based typechecker that runs instantaneously is what makes it possible for Hack to replicate this experience: you save a file and reload a web page, but you have the safety net of a typechecker that told you about your type errors as soon as you saved the file.

I can't overstate how important this instant feedback is: it's really the thing that distinguishes Hack from working in a more traditional compile-based static language.