14 - INSTANCE-SPECIFIC BEHAVIOR: HOW AND WHY

Summary

After the brief pause in the previous column to discuss philosophy, this column descends again into the depths of techno-minutiae. At least I talked about why you'd want to use instance-specific behavior, and didn't just present it as:“Here's a cool thing you can do with Smalltalk.”

The pattern Scriptable Objects still looks pretty good to me. In fact, I like it better as a pattern than many of the patterns that follow.

This and the next column will discuss technical and philosophical matters. The technical material covers implementing and using instance-specific behavior, the idea that you can attach methods to individual instances rather than to a class. You might use it in animation or in building a Hypercard-like system. It is not a new idea. Lisp-based object systems have had it for years, and languages like Self rely on it exclusively. It is not well known in the Smalltalk community, though, and deserves a place in the mature Smalltalker's bag of tricks.

The philosophical material illuminates the differences between Digitalk's and ParcPlace's view of good Smalltalk style. ParcPlace grew out of a research atmosphere where truth and beauty were admired. Although established in business now, ParcPlace continues to favor elegant solutions. Digitalk has always been driven by the desire to build commercial software and has often been staffed with engineers whose experience comes from other languages. Digitalk's solutions tend to be more pragmatic and the workings easier to follow operationally, even if they don't have the most elegant high-level models.