The natural recursive definition of a Fibonacci function is a different beast. The big issue with the naive and simple Fibonacci implementation is that it duplicates work, a ton of it. The 'map' above doesn't have this issue. To make Fibonacci efficient, it can be implemented as an iterative algorithm which isn't too bad to read. But there's better news! The recursive algorithm can be kept as-is, provided that the function is memoized, so that already computed values are cached.
