Objects model the world, and state is fundamental to a faithful modeling. Engineers use state machines to understand and reason about state transitions, but programming languages provide little support for reasoning about or implementing these state machines, causing software defects and lost productivity when objects are misused.We propose Typestate-Oriented Programming as a natural extension to the object paradigm, where objects are modeled not just in terms of classes, but in terms of changing states. Each state may have its own representation and methods which may transition the object into a new state. A flow-sensitive, permission-based type system helps developers track which state objects are in. First-class typestates are a powerful abstraction that will help developers model and reuse objects more efficiently and correctly.
Engineers in many disciplines use state machines to reason about system changes, and many object-oriented libraries require their clients to follow state machine protocols. No existing language, however, has native support for state machines, and programmers often lose productivity and introduce errors when trying to understand and follow interaction protocols. The Plaid language extends the object paradigm with explicit states and state transitions, in order to better model object state transitions. In this paper, we present P laidcore, a core calculus for Plaid, which uses states and permissions to statically guarantee that clients use object protocols correctly.
Linear permissions have been proposed as a lightweight way to specify how an object may be aliased, and whether those aliases allow mutation. Prior work has demonstrated the value of permissions for addressing many software engineering concerns, including information hiding, protocol checking, concurrency, security, and memory management.We propose the concept of a permission-based programming language-a language whose object model, type system, and runtime are all co-designed with permissions in mind. This approach supports an object model in which the structure of an object can change over time, a type system that tracks changing structure in addition to addressing the other concerns above, and a runtime system that can dynamically check permission assertions and leverage permissions to parallelize code. We sketch the design of the permissionbased programming language Plaid, and argue that the approach may provide significant software engineering benefits.
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