In our most recent episode on algebraic data types, we explored how we can use algebra to understand generics and recursive data structures in the Swift type system. Things got pretty abstract! We grounded the episode, though, in a useful type that doesn’t seem to get enough attention: the non-empty list—in fact, we gave it twice as much attention because math led us to two different formulas!

We deal with non-empty data regularly, but without a structure at hand it’s all too easy to reach for Array and inevitably we end up writing a bit of extra code to handle those empty cases. Today’s episode is all about embracing type-safe guarantees, avoiding those extra code paths, and building an ergonomic API over the non-empty list. Let’s build a low-friction, non-empty type suitable for your production code base!

Let’s start with what we came up with last time.