Learning F# with "Pex for fun"

Microsoft Research has published Pex for fun. It is a puzzle game. The only thing you are initially given is a signature of a function and you have to work out the implementation. There is an "Ask Pex!" button that will provide you some hints about how close you are. Wonderful!

There are couple of drawbacks though, such as that sometimes you are not given negative answers (inputs for which your implementation does not provide expected outputs), which is not very helpful. The most ugly thing I have found is how ChallengeFactorial puzzle behaves. Even the most stupid implementation (returning 0) is said to exceed the time-limit, thus effectively preventing you from succeeding.

Nevertheless, I like Pex for fun very much! And I find it especially useful when learning F# as it makes you explore the possibilities of the language.