In this chapter, we started with the category theory concepts such as Functor, Applicative Functor, and Monad and explored higher-order functions such as map, filter, flatMap, flatten, and reduce. Then, we examined Swift-provided versions of higher-order functions and implemented a simple version ourselves. Also, we developed map, filter, flatMap, and flatten functions in terms of the reduce function.

Then, we continued with the apply, join, and zip functions and were introduced to chaining higher-order functions.

Finally, we explored some practical examples of higher-order functions such as removing nil values from an array, removing duplicates, and partitioning arrays.

These functions are going to be great tools in our day-to-day development toolkit to use and solve a lot of different kinds of problems.

In the following chapter, we will get familiar with optional types and discuss non-functional and functional ways to deal with them.