Lykn

When to Use Cells

Cells are intentional friction. The ceremony of cell/express/swap! compared to direct variable reassignment is by design. If mutation feels slightly inconvenient, good — it should make you consider whether you actually need it.

Good Use Cases

• Counters and accumulators — loop indices, request counts, retry trackers
• Caches — memoization, lazy initialization, computed values
• Event-driven state — UI state, handler coordination, observable values
• Interop with mutable JS APIs — DOM manipulation, timer state

When Cells Aren’t the Answer

• Data transformation — use map/filter/reduce, threading macros, or assoc/dissoc/conj
• Accumulating results — use reduce or recursion
• Branching on state — use match on an immutable value

The Rule of Thumb

If you find yourself wanting more than two or three cells in a function, you’re probably solving a data transformation problem imperatively. Step back and think functionally. Threading macros and immutable updates (later in this chapter) handle most of what people reach for mutation to do.