I'm building a little Reactive library in C# for fun - in the same spirit as Conal's one - with the intension to convert that to F#, which I'm learning now. Right now I implemented Future, Event and Reactive, and functionality to merge/mappend two events. Although I'm not yet sure it fully works, using some tricks from Reactive, it was actually really easy to implement. I don't need unamb stuff since I just spawn one new thread for each merged event, and I use the WaitForMultipleObjects primitive to check which of the two event occurrences was first. The code is of course a zillion times more ugly than in Haskell, and since I'm not using lightweight threads, it might really be slow. I tried to make it as immutable as possible, so it's not really imperative :) But what is the minimal set of functions one must implement to build a Reactive library, in the sense that all others can be implemented using these? Merry Xmas!