Section: Software and Platforms

EScala and JEScala

Participants : Jacques Noyé [correspondent] , Jurgen Van Ham.

AOP, inheritance, event-based programming, events, declarative events, asynchronous events, join operator, Scala

EScala is an extension of the programming language Scala with support for events as object members. EScala combines ideas of event-driven, aspect-oriented and functional reactive programming.

Events are natural abstractions for describing interactive behavior as part of an object interface. In conventional object-oriented languages, events are implemented indirectly, typically using the Observer pattern. C# eliminates the corresponding glue code and directly supports events as object members. However, events are still explicitly triggered at specific locations within the program.

EScala goes much further. First, it also supports implicit events. Akin to join points in aspect-oriented languages, these events are implicitly produced at specific execution points, such as the beginning or the end of the execution of a method. Second, declarative events make it possible to compose events using logical operators as well as to filter them and alter their content.

EScala events are fully integrated with object-oriented features. An event is defined in the context of its owner object. Event definitions are inherited in subclasses and event uses are late-bound. Unlike typical aspect-oriented languages, EScala preserves object-oriented encapsulation and modular reasoning.

JEScala extends EScala with support for concurrent programming (see Sec.  6.2 ). Events can be declared as asynchronous so that their handling takes place concurrently. A new composition operator, the join operator, inspired by the join calculus, can also be used to synchronize the concurrent activities created by asynchronous events and communicate between them.

This is joint work with the Software Technology Group at TU Darmstadt.

Prototype implementations of these languages are available through http://www.stg.tu-darmstadt.de/research .