"When developers talk about pure functions, they often recite the textbook definition: same inputs, same outputs, no side effects. It's correct, but it undersells the point. Purity is not an aesthetic choice or an academic curiosity. It is a way of reclaiming control over the semantics of your system. It is how you establish that a piece of business logic actually behaves like business logic, rather than a negotiation with global state, shared caches, volatile time checks,"
"Shoppers add and remove items, the system reevaluates promotions, seasonal rules apply or expire, and flash sales may activate in the middle of a session. A change to the cart or its context prompts a recalculation. The mechanics sound straightforward; the reality rarely is. Many pricing engines embed side effects throughout their logic: they reach into global registries of promotions, pull loyalty tiers from shared caches, write audit logs during discount evaluation, and update counters in observability systems."
Mutable state, especially data once treated as factual, tends to drift over time and destabilize large systems. Immutability preserves facts and reduces one dimension of instability by preventing such drift. A second dimension of stability concerns behavior: whether logic performs only its intended computation or also triggers external interactions. Pure functions provide behavioral stability by ensuring deterministic outputs for given inputs and eliminating side effects. In commerce systems, cart repricing illustrates how embedded side effects—reading global promotion registries, consulting shared caches, writing audit logs, and updating observability counters—entangle pricing with surrounding systems and produce nondeterministic outcomes under concurrency.
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