We now know that flatMap is an important operation that solves a common problem. Important enough for Swift to provide this operation for arrays and optionals in the standard library.

However, all of the types we define for our own problems and applications, the ones that the Swift standard library knows nothing about, all have this nesting problem. The Result, Validated, Func, Parallel type and more. Can we define flatMap on them?

The answer is “yes we can and should be!” And as you define flatMap it uncovers interesting semantics of your types. It also will show interesting connections with zip, and that is what clarifies the purpose of the type. For example, why would you use Result over Validated? Why would you use Func over Parallel? It turns out that the way flatMap and zip relate to each other uncovers all of that.

So let’s define flatMap on our own types and see what it teaches us!