I.e. more than an afternoon or probably a month to get good, vs. the few days it takes to learn another language with familiar paradigms.
Not sure from personal experience because I learned functional style Lisp/Scheme really early, more than a decade before OO, which made learning Clojure a relative snap. OO in the form of C++ was harder for me, but maybe functional is just easier for my mind to wrap around. The quality of the Lisp and Scheme texts I used, especially SICP, also probably made a difference (the OO texts I used were good, but not as great, with the exception of OOSE (http://www.amazon.com/Object-Oriented-Software-Engineering-A...), but that's as much about great design of your project as well as what the project is making).
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