We followed the evolution of fold and map, starting with lists and then generalizing to all Foldable and Traversable containers.

Following that, we saw how the Lens library (2012) places folding and traversing in an even broader context. Lenses give us a unified vocabulary to navigate through data structures, which explains why it has been described as a "query language for data structures".

In this chapter, as we moved up the layers of abstraction, the function type signatures became ever more generic.

Genericity through type parameterization represents only one kind of generic programming. In Chapter 6, Patterns of Generic Programming, we explore more such patterns. But before we can do that, we need to add a few more advanced language extensions to our toolset. This is the subject of the next chapter.