Introduction

Work a long time on your code, and you will have countless opportunities to experiment with software design. Today, I'd like to describe how I moved some classes to a new model for separating queries and commands.

The need for this came to me because of complex assertions.

When I first learned Eiffel, I learned the design pattern for getting results back from commands - storing the result in an attribute of the class being changed. And I dutifully followed it religously.

More recently, I have encountered situations where this pattern seems to be undesirable. If the command can be re-invoked recursively, then things can get tricky.