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.

My research interests are in the area of programming languages, operating systems, object-oriented systems and distributed computing, and more specifically in the region where they overlap (such as language design for distributed Object-oriented computing.) My favorite system for trying out ideas is Squeak Smalltalk. I have written a few Squeak goodies that I believe are generally useful. I have been actively working on Traits with the folks from SCG in Bern; see Nathanael Schärli’s Traits page for details. I’m a co-author of two books on Smalltalk programming, Squeak by Example, and Pharo By Example.