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 building software based on state abstractions. We propose Plaid, a language in which objects are modeled not just in terms of classes, but in terms of changing abstract states. Each state may have its own representation, as well as methods that may transition the object into a new state. A formal model precisely defines the semantics of core Plaid constructs such as state transition and trait-like state composition. We evaluate Plaid through a series of examples taken from the Plaid compiler and the standard libraries of Smalltalk and Java. These examples show how Plaid can more closely model state-based designs, enhancing understandability, enhancing dynamic error checking, and providing reuse benefits.
In object-oriented programming, unique permissions to object references are useful for checking correctness properties such as consistency of typestate and noninterference of concurrency. To be usable, unique permissions must be borrowed -for example, one must be able to read a unique reference out of a field, use it for something, and put it back. While one can null out the field and later reassign it, this paradigm is ungainly and requires unnecessary writes, potentially hurting cache performance. Therefore, in practice borrowing must occur in the type system, without requiring memory updates. Previous systems support borrowing with external alias analysis and/or explicit programmer management of fractional permissions. While these approaches are powerful, they are also awkward and difficult for programmers to understand. We present an integrated language and type system with unique, immutable, and shared permissions, together with new local permissions that say that a reference may not be stored to the heap. Our system also includes change permissions such as unique>>unique and unique>>none that describe how permissions flow in and out of method formal parameters. Together, these features support common patterns of borrowing, including borrowing multiple local permissions from a unique reference and recovering the unique reference when the local permissions go out of scope, without any explicit management of fractions in the source language. All accounting of fractional permissions is done by the type system "under the hood." We present the syntax and static and dynamic semantics of a formal core language and state soundness results. We also illustrate the utility and practicality of our design by using it to express several realistic examples.
In object-oriented programming, unique permissions to object references are useful for checking correctness properties such as consistency of typestate and noninterference of concurrency. To be usable, unique permissions must be borrowed -for example, one must be able to read a unique reference out of a field, use it for something, and put it back. While one can null out the field and later reassign it, this paradigm is ungainly and requires unnecessary writes, potentially hurting cache performance. Therefore, in practice borrowing must occur in the type system, without requiring memory updates. Previous systems support borrowing with external alias analysis and/or explicit programmer management of fractional permissions. While these approaches are powerful, they are also awkward and difficult for programmers to understand. We present an integrated language and type system with unique, immutable, and shared permissions, together with new local permissions that say that a reference may not be stored to the heap. Our system also includes change permissions such as unique>>unique and unique>>none that describe how permissions flow in and out of method formal parameters. Together, these features support common patterns of borrowing, including borrowing multiple local permissions from a unique reference and recovering the unique reference when the local permissions go out of scope, without any explicit management of fractions in the source language. All accounting of fractional permissions is done by the type system "under the hood." We present the syntax and static and dynamic semantics of a formal core language and state soundness results. We also illustrate the utility and practicality of our design by using it to express several realistic examples.
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At the core of the Plaid typestate-oriented programming language is the ability to change the representation of an object at run-time. As such, the semantics of the state change operation impact how Plaid programs are structured and how objects are composed in Plaid's trait-based composition system. We describe the challenges with respect to designing a modular state change operation and suggest two options. Secondly, we explore the issues both designs create for explicit conflict resolution and sketch a potential solution which eliminates the flattening property to allow conflicts to be resolved only when they come into scope.
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