Although C# has a feature to iterate a sequence using the for or while loop, it's better for us to approach functional programming using recursion to iterate the sequence. We already discussed how the recursion routine works and refactored the iteration into recursion. We know that in the recursion, we have a base case that will define the end of the recursion chain.

In the traditional recursion model, the recursive call performs first then returns the value and then calculates the result. The result won't be displayed until the recursive call is finished. Whereas, the tail recursion doesn't do anything after recursion at all. There are two styles in tail recursion; they are APS and CPS.

Besides direct recursion, we also discussed indirect recursion. Indirect recursion involves at least two functions. Then, we applied recursion into the functional approach using the Aggregrate LINQ operator. We also delved into the Aggregate operator and discussed how it works.

In the next chapter, we will discuss optimizing techniques to make our code more efficient. We will use laziness thinking so the code will be executed at the perfect time and also caching techniques so the code doesn't need to execute every time it's required.