We’ve now spent a couple episodes exploring “functional setters”: functions that allow us to build up expressive, immutable data transformations from small units. We’ve explored how they compose together in surprising ways to let us make changes to deeply-nested values: changes that are generally cumbersome to make. And we’ve leveraged a wonderful and unique Swift feature, key paths, to pluck setters out of thin air for properties on our classes and structs.

Setters are an incredibly powerful and broadly useful tool, but the current functions we’ve written have some rough edges when it comes to using them. They’re also not the most performant things in the world: because setters are immutable, they create copies of their values every step of the way. Today we’ll smooth out those rough edges and explore how we can use Swift’s value mutation semantics to make things more performant.