Philosophy

We have developed this jukebox for a couple of reasons, first being that it is fun and useful, and it is a good demonstration of the capabilites of the Ensemble group communications system.

We also have a strong desire to demonstrate the capabilites of higher level language principles, like those used to design ML, and OCaml in particular. There are many of these principles, but the foremost are probably the following:

• A strong type system and a safe language

  Static type safety is the guarantee that type violations (like dereferencing a NULL pointer or calling a function with an argument of the wrong type) are detected and disallowed at compile time. This does require that the programmer understand the type system. But it also makes programs much easier to modify. Many program errors are caught by the type system.

• Type polymorphism

  Type polymorphism encourages code reuse. This shares the goals of the template mechanism of C++, but it provides a cleaner formulation.

• Module system

  Module systems are another mechanism that encourage code reuse. Code to be used can be collected in modules, and even parameterized by other modules. All this is performed in cooperation with the type system, and the same safety guarantees hold for the module system.

The jukebox is a fairly large application on its own (some 30,000 lines of code outside of Ensemble). The development time for EJB was rather short (about 2 months with mostly one programmer), and changes to the code are reasonably easy. There is little run time penalty for running in a safe language which is in large part due to the quality of the OCaml compiler.

The priciple we use for developing this jukebox are the same principle that we promote in our research. We believe that by using powerful formal techniques it is possible to produce programming languages that are easy to use and efficient, and make programs easier to develop and maintain.