Lykn

What About Mutation?

You are, at this point, entirely within your rights to ask: “If everything is immutable, how do I do anything stateful?”

The Short Answer

The tools exist. They’re in Chapter 13.

• For state that changes over time: use cell — a controlled, explicit mutable container with swap! and reset!
• For “updating” objects and arrays: use assoc, dissoc, and conj — surface macros that produce new values via shallow copy instead of mutating the original
• For JS interop with mutable APIs: use the js: namespace, which provides access to kernel forms like let and direct property assignment

Why Not Here?

bind and cell are separated into different chapters because they’re different concepts. bind is about naming values. cell is about managing state. Conflating them — as JavaScript does by making let both a naming mechanism and a state mechanism — is precisely the source of the bugs that motivated Lykn’s design.

When you reach Chapter 13, you’ll find that cells are simple, explicit, and hard to misuse. The ! suffix on every mutating operation (swap!, reset!) makes state changes visible at a glance. But the important thing for now is: immutability is the default, not the only option. The vast majority of bindings in a typical program never need to change, and bind handles all of them.

A Promise

If at any point in the next eight chapters you find yourself thinking “but I need a mutable variable for this,” make a note and keep reading. By Chapter 13, you’ll either have found a functional alternative that’s cleaner, or you’ll learn the cell pattern that handles the case explicitly. Either way, the language has you covered.