Perhaps the most important criteria to be used in decomposing modules is to identify secrets that modules should hide from the rest of the system [Parnas]. Data structures are the most common secrets, and I can hide data structures by encapsulating them with Encapsulate Record (162) and Encapsulate Collection (170). Even primitive data values can be encapsulated with Replace Primitive with Object (174)—the magnitude of second-order benefits from doing this often surprises people.
