Lambda the Ultimate

Morphing: Safely Shaping a Class in the Image of Others, Shan Shan Huang, David Zook, and Yannis Smaragdakis. ECOOP 2007.

We present MJ: a language for specifying general classes whose members are produced by iterating over members of other classes. We call this technique “class morphing” or just “morphing”. Morphing extends the notion of genericity so that not only types of methods and fields, but also the structure of a class can vary according to type variables. This offers the ability to express common programming patterns in a highly generic way that is otherwise not supported by conventional techniques. For instance, morphing lets us write generic proxies (i.e., classes that can be parameterized with another class and export the same public methods as that class); default implementations (e.g., a generic do-nothing type, configurable for any interface); semantic extensions (e.g., specialized behavior for methods that declare a certain annotation); and more. MJ’s hallmark feature is that, despite its emphasis on generality, it allows modular type checking: an MJ class can be checked independently of its uses. Thus, the possibility of supplying a type parameter that will lead to invalid code is detected early—an invaluable feature for highly general components that will be statically instantiated by other programmers.

This is related to the paper by Turon and Reppy I linked to yesterday; it's another take on compile-time metaprogramming. This time they have a static iteration over the structure of a class, which makes their safety result rather impressive. I talked Huang, and she told me that their big-picture goal is to eventually build a full language for manipulating programs at compile time, while still preserving all the safety guarantees we expect.