순수 함수로 작성하는 비즈니스 규칙

A pure function returns the same output for the same input and touches nothing outside itself. That makes it perfect for shared code.

Pure means no logging, no network, no saving to disk. The function just takes values in and gives a result back.

Pure functions have no platform dependencies, so they compile and behave identically on Android, iOS and desktop. 🎯

With no hidden state, a test is just input in, expected output checked. No mocks, no setup, no flaky surprises.

Express each business rule as its own tiny function. Small pieces compose into bigger logic without tangling.

Combine small rules to make decisions. Each part stays pure, so the whole composition stays predictable and testable.

Reading the clock makes a function impure. Instead, pass the current time in so the rule stays deterministic.

Reading or changing a global variable breaks purity. Keep all the data a rule needs in its parameters.

Instead of editing the input, return a new value. This avoids surprises when the same object is used elsewhere.

Keep your rules pure at the core, and let the edges of the app handle effects like saving or showing results.

Because pure rules live once in shared code, a change updates both apps at once. Business logic can never drift apart again.

You wrote business rules as small, side-effect-free functions that test easily and run the same on every target. ✅