Lambda the Ultimate

In gbeta, object metamorphosis coexists with strict, static type-checking: It is possible to take an existing object and modify its structure until it is an instance of a given class, which is possibly only known or even constructed at run-time. Still, the static analysis ensures that message-not-understood errors can never occur at run-time.

It is possible to define relations between classes, e.g., to specify that the class MyPoint must be a subclass of YourPoint (without committing to exactly what classes they are). This makes it possible to define a kind of constraint graph of classes.