Multitudes of Objects: First Implementation and Case Study for Java

In object-oriented programs, the relationship of an object to many objects is usually implemented using indirection through a collection. This is in contrast to a relationship to one object, which is usually implemented directly. However, using collections for relationships to many objects does not only mean that accessing the related objects always requires accessing the collection first, it also presents a lurking maintenance problem that manifests itself when a relationship needs to be changed from to-one to to-many or vice versa. Continuing our prior work on fixing this problem, we show how we have extended the Java 7 programming language with multiplicities, that is, with expressions that evaluate to a number of objects not wrapped in a container, and report on the experience we have gathered using these multiplicities in a case study

A Generic Approach to Flow-Sensitive Polymorphic Effects

Effect systems are lightweight extensions to type systems that can verify a wide range of important properties with modest developer burden. But our general understanding of effect systems is limited primarily to systems where the order of effects is irrelevant. Understanding such systems in terms of a lattice of effects grounds understanding of the essential issues, and provides guidance when designing new effect systems. By contrast, sequential effect systems --- where the order of effects is important --- lack a clear algebraic characterization. We derive an algebraic characterization from the shape of prior concrete sequential effect systems. We present an abstract polymorphic effect system with singleton effects parameterized by an effect quantale --- an algebraic structure with well-defined properties that can model a range of existing order-sensitive effect systems. We define effect quantales, derive useful properties, and show how they cleanly model a variety of known sequential effect systems. We show that effect quantales provide a free, general notion of iterating a sequential effect, and that for systems we consider the derived iteration agrees with the manually designed iteration operators in prior work. Identifying and applying the right algebraic structure led us to subtle insights into the design of order-sensitive effect systems, which provides guidance on non-obvious points of designing order-sensitive effect systems. Effect quantales have clear relationships to the recent category theoretic work on order-sensitive effect systems, but are explained without recourse to category theory. In addition, our derived iteration construct should generalize to these semantic structures, addressing limitations of that work

Locally Static, Globally Dynamic Session Types for Active Objects

Active object languages offer an attractive trade-off between low-level, preemptive concurrency and fully distributed actors: syntactically identifiable atomic code segments and asynchronous calls are the basis of cooperative concurrency, still permitting interleaving, but nevertheless being mechanically analyzable. The challenge is to reconcile local static analysis of atomic segments with the global scheduling constraints it depends on. Here, we propose an approximate, hybrid approach; At compile-time we perform a local static analysis: later, any run not complying to a global specification is excluded via runtime checks. That specification is expressed in a type-theoretic language inspired by session types. The approach reverses the usual (first global, then local) order of analysis and, thereby, supports analysis of open distributed systems

μ-DSU:A Micro-Language Based Approach to Dynamic Software Updating

Today software systems play a critical role in society’s infrastructures and many are required to provide uninterrupted services in their constantly changing environments. As the problem domain and the operational context of such software changes, the software itself must be updated accordingly. In this paper we propose to support dynamic software updating through language semantic adaptation; this is done through use of micro-languages that confine the effect of the introduced change to specific application features. Micro-languages provide a logical layer over a programming language and associate an application feature with the portion of the programming language used to implement it. Thus, they permit to update the application feature by updating the underlying programming constructs without affecting the behaviour of the other application features. Such a linguistic approach provides the benefit of easy addition/removal of application features (with a special focus on non-functional features) to/from a running application by separating the implementation of the new feature from the original application, allowing for the application to remain unaware of any extensions. The feasibility of this approach is demonstrated with two studies; its benefits and drawbacks are also analysed

Gradual Certified Programming in Coq

Expressive static typing disciplines are a powerful way to achieve high-quality software. However, the adoption cost of such techniques should not be under-estimated. Just like gradual typing allows for a smooth transition from dynamically-typed to statically-typed programs, it seems desirable to support a gradual path to certified programming. We explore gradual certified programming in Coq, providing the possibility to postpone the proofs of selected properties, and to check "at runtime" whether the properties actually hold. Casts can be integrated with the implicit coercion mechanism of Coq to support implicit cast insertion a la gradual typing. Additionally, when extracting Coq functions to mainstream languages, our encoding of casts supports lifting assumed properties into runtime checks. Much to our surprise, it is not necessary to extend Coq in any way to support gradual certified programming. A simple mix of type classes and axioms makes it possible to bring gradual certified programming to Coq in a straightforward manner.Comment: DLS'15 final version, Proceedings of the ACM Dynamic Languages Symposium (DLS 2015

Safe Transferable Regions

There is an increasing interest in alternative memory management schemes that seek to combine the convenience of garbage collection and the performance of manual memory management in a single language framework. Unfortunately, ensuring safety in presence of manual memory management remains as great a challenge as ever. In this paper, we present a C#-like object-oriented language called Broom that uses a combination of region type system and lightweight runtime checks to enforce safety in presence of user-managed memory regions called transferable regions. Unsafe transferable regions have been previously used to contain the latency due to unbounded GC pauses. Our approach shows that it is possible to restore safety without compromising on the benefits of transferable regions. We prove the type safety of Broom in a formal framework that includes its C#-inspired features, such as higher-order functions and generics. We complement our type system with a type inference algorithm, which eliminates the need for programmers to write region annotations on types. The inference algorithm has been proven sound and relatively complete. We describe a prototype implementation of the inference algorithm, and our experience of using it to enforce memory safety in dataflow programs