Your version is certainly pretty, and it's based on an insight into the nature of the fibonacci series But the value of co-recursion is that you don't need tha

Your version is certainly pretty, and it's based on an insight into the nature of the fibonacci series But the value of co-recursion is that you don't need that insight. When you expand out the definitions of `tail`, `!!` and `zipWith`, your code turns out to be the same as mine[1], so I consider that a big win for co-recursion. [1]: At least I think it does.