Reflections on metaclass programming in SOM

Abstract

This paper reports on the evolution of metaclass programming in SOM (the IBM System Object Model). Initially, SOM's use of explicit metaclasses introduced metaclass incompatibilities. This was cured by having SOM dynamically derive an appropriate metaclass by interpreting the “metaclass declaration” as a constraint. In effect, inheritance is given a new dimension, because the constraint is also inherited. The derived metaclass is the least solution to all these constraints. Subsequently, this cure led to the possibility of metaclasses conflicting over the need to assign meaning to a method. The cure for this problem is a framework that facilitates the programming of metaclasses that cooperate on the assignment of meaning to methods.