I once read a textbook introducing the theory of algorithms that started with the Fibonacci series. The author gave a formal definition, and translated this directly into a naive, recursive algorithm. Then he showed how to improve the algorithm with memoization. This was a real revelation to me: using this one simple, generally applicable transformation, all kinds of algorithms can be improved.

Then the author introduced the iterative Fibonacci algorithm, and formally proved that the iterative and recursive algorithms were equivalent. But the author failed to explain where that iterative algorithm came from: there was no general purpose tool like memoization that could be used to derive the iterative form from the recursive form. I still remember the intellectual disappointment.

Now, with co-recursion and a couple of other techniques, that gap is finally bridged. Using co-recursion, I can derive the stream algorithm directly from the recursive definition. And then I can apply deforestation techniques and tail-call optimization to derive the iterative algorithm from the stream algorithm. That's a pretty powerful intellectual toolset, don't you think?