In a typical imperative application, business logic and side effects are inextricably linked. For example, when you write await db.checkInventory(…), the runtime immediately reaches out to the database. Our Effect System works differently. Instead of performing the action, our functions return a description of the action. When our code needs to check inventory, it doesn’t call the database; it returns a plain object instead, which will be executed later by an interpreter.
You can still test the functions individually or run the entire flow against a test database, but without an effect system like this, it’s very hard to test business logic in isolation.