The default mode in Hack is partial: in partial mode, the code itself must be typed and must past the typechecker, but it can call untyped code (that's in a separate compilation unit).
Another mode (you specify the mode per file/compilation unit) is "strict". In strict mode, you can not code any un-typed code (note, the standard library is typed with hack).
(There is a bit more nuance here, but you can read that in the documentation.)
So the idea is to eventually migrate most of the code to strict, but code that relies on legacy can remain partial and you can write new code without waiting for a re-write to finish.
"Shapes" are also a neat feature specifically for parts where static typing can be frustrating for dealing with HTTP requests specifically: http://docs.hhvm.com/manual/en/hack.shapes.php
(FWIW I don't see myself using Hack, but I'm not a web developer. I'd say the ML family languages are my favourite, but for what I do day-to-day it's not really an option.)
Another mode (you specify the mode per file/compilation unit) is "strict". In strict mode, you can not code any un-typed code (note, the standard library is typed with hack).
(There is a bit more nuance here, but you can read that in the documentation.)
So the idea is to eventually migrate most of the code to strict, but code that relies on legacy can remain partial and you can write new code without waiting for a re-write to finish.
See http://docs.hhvm.com/manual/en/hack.modes.strict.php http://docs.hhvm.com/manual/en/hack.modes.partial.php
"Shapes" are also a neat feature specifically for parts where static typing can be frustrating for dealing with HTTP requests specifically: http://docs.hhvm.com/manual/en/hack.shapes.php
(FWIW I don't see myself using Hack, but I'm not a web developer. I'd say the ML family languages are my favourite, but for what I do day-to-day it's not really an option.)