An E cient Implementation of Plaid

State is central to understanding objects in the real world a moving car is very di erent from a parked car. Yet most object-oriented languages provide no native support for state or state change. The Plaid language introduces a new object model in which objects are not only instances of a class, but can also be in mutable states. In Plaid, a car object can have one abstract state for driving and another for not driving. Transitioning the object from the driving to the not driving state with Plaid's state change operator removes members that are available only in the driving state for instance, speed and adds members associated with the not driving state. Traditionally, efcient implementations of object-oriented languages have required that objects have stable members in order to facilitate fast member access. State change alters members at runtime, presenting a new challenge for e cient compilation. To address this challenge, we developed a novel representation for state at runtime that enables state change without sacri cing fast member access. We implemented our representation in a new code generator and runtime system for compiling Plaid to JavaScript. Our implementation improves program execution time by a factor of 48 over a Plaid implementation with an alternative state representation that slows member access.