The Expression Problem, Gracefully

The "Expression Problem" was brought to prominence by Wadler in 1998. It is widely regarded as illustrating that the two mainstream approaches to data abstraction---procedural abstraction and type abstraction---are complementary, with the strengths of one being the weaknesses of the other. Despite an extensive literature, the origin of the problem remains ill-understood. I show that the core problem is in fact the use of global constants, and demonstrate that an important aspect of the problem goes away when Java is replaced by a language like Grace, which eliminates them.