Should I learn oCaml instead ? Looks appealing because companies like Facebook are using it create tools like Flow ( https://github.com/facebook/flow ).

https://github.com/facebook/Haxl

My 2 cents: definitely _also_ learn OCaml down the line. But:

- my pet peeve with OCaml is the lack of a builtin type for Unicode strings (the only other language still commonly used with this shortcoming is PHP), and this could be an issue if you plan to write some web app code that needs to be internationalized

- OCaml is strict, Haskell is one of the very few languages that is non-strict: aside from the didactic purpose of exposing yoursefl to something different, that's a feature that can help reasoning about what your code is doing:

http://augustss.blogspot.co.uk/2011/05/more-points-for-lazy-...

(then again, there're obviously downsides to non-strictness, and you might decide down the line that you prefer to write strict code in production... but it's good to know the alternatives)