We identify three programming language abstractions for the construction of reusable components: abstract type members, explicit selftypes, and modular mixin composition. Together, these abstractions enable us to transform an arbitrary assembly of static program parts with hard references between them into a system of reusable components. The transformation maintains the structure of the original system. We demonstrate this approach in two case studies, a subject/observer framework and a compiler front-end.

Inheritance is well-known and accepted as a mechanism for reuse in object-oriented languages. Unfortunately, due to the coarse granularity of inheritance, it may be difficult to decompose an application into an optimal class hierarchy that maximizes software reuse. Existing schemes based on single inheritance, multiple inheritance, or mixins, all pose numerous problems for reuse. To overcome these problems we propose traits, pure units of reuse consisting only of methods. We develop a formal model of traits that establishes how traits can be composed, either to form other traits, or to form classes. We also outline an experimental validation in which we apply traits to refactor a non-trivial application into composable units.

Abstract. A software product-line is a family of related programs. Each program is defined by a unique combination of features, where a feature is an increment in program functionality. Modularizing features is difficult, as feature-specific code often cuts across class boundaries. New modularization technologies have been proposed in recent years, but their support for feature modules has not been thoroughly examined. In this paper, we propose a variant of the expression problem as a canonical problem in product-line design. The problem reveals a set of technology-independent properties that feature modules should exhibit. We use these properties to evaluate five technologies: model of feature composition that is technology-independent and that relates compositional reasoning with algebraic reasoning 1. 1

Abstract. Links is a programming language for web applications that generates code for all three tiers of a web application from a single source, compiling into JavaScript to run on the client and into SQL to run on the database. Links supports rich clients running in what has been dubbed ‘Ajax ’ style, and supports concurrent processes with statically-typed message passing. Links is scalable in the sense that session state is preserved in the client rather than the server, in contrast to other approaches such as Java Servlets or PLT Scheme. Client-side concurrency in JavaScript and transfer of computation between client and server are both supported by translation into continuation-passing style. 1

Abstract. Programs of a software product line can be synthesized by composing modules that implement features. Besides high-level domain constraints that govern the compatibility of features, there are also low-level implementation constraints: a feature module can reference classes that are defined in other feature modules. Safe composition is the guarantee that programs composed from feature modules are absent of references to undefined classes, methods, and variables. We show how safe composition can be

The expression problem is fundamental for the development of extensible software. Many (partial) solutions to this problem have been proposed in the past, but the question of how to use different, independent extensions jointly has received less attention so far. This paper proposes solutions to the expression problem that make it possible to combine independent extensions in a flexible, modular, and typesafe way. The solutions, formulated in the programming language Scala, are affected with only a small implementation overhead and are relatively easy to implement by hand. 1. The Expression Problem Since software evolves over time, it is essential for software systems to be extensible. But the development of extensible software poses many design and implementation problems, especially, if extensions cannot be anticipated. The expression problem is probably the most fundamental one among these problems. It arises when recursively defined datatypes and operations on these types have to be extended simultaneously. The term expression problem was originally coined by Phil Wadler in a post on the Java-Genericity mailing list [34], although it was Cook who first discussed this problem [9]. His work motivated several others to reason about variants of the problem in the following years [18, 27, 17, 12]. In his post to the Java-Genericity mailing list, Wadler also proposed a solution to the problem written in an extended version of GENERIC JAVA [3]. Only later it appeared that this solution could not be typed. For this paper, we paraphrase the problem in the following way: Suppose we have a datatype which is defined by a Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.

Permission to make digital or hard copies of all or part of this work for personal or classroom use is granted without fee provided that copies are not made or distributed for profit or commercial advantage and that copies bear this notice and the full citation on the first page. To copy otherwise, to republish, to post on servers or to redistribute to lists, requires prior specific permission and/or a fee.

Concurrent programming is indispensable. On the one hand, distributed and mobile environments naturally involve concurrency. On the other hand, there is a general trend towards multi-core processors that are capable of running multiple threads in parallel.

Traits are fine-grained components that can be used to compose classes, while avoiding many of the problems of multiple inheritance and mixin-based approaches. Since most implementations of traits have focused on dynamically-typed languages, the question naturally arises, how can one best introduce traits to statically-typed languages, like Java and C#? In this paper we argue that the flattening property of traits should be used as a guiding principle for any attempt to add traits to statically-typed languages. This property essentially states that, semantically, traits can be compiled away. We demonstrate how this principle applies to Featherweight-Trait Java, a conservative extension to Featherweight Java.

Generic programming has recently emerged as a paradigm for developing highly reusable software libraries, most notably in C++. We have designed and implemented a constrained generics extension for C++ to support modular type checking of generic algorithms and to address other issues associated with unconstrained generics. To be as broadly applicable as possible, generic algorithms are defined with minimal requirements on their inputs. At the same time, to achieve a high degree of efficiency, generic algorithms may have multiple implementations that exploit features of specific classes of inputs. This process of algorithm specialization relies on non-local type information and conflicts directly with the local nature of modular type checking. In this paper, we review the design and implementation of our extensions for generic programming in C++, describe the issues of algorithm specialization and modular type checking in detail, and discuss the important design tradeoffs in trying to accomplish both. We present the particular design that we chose for our implementation, with the goal of hitting the sweet spot in this interesting design space.