The topic of this and the next column is how to use instance and temporary variables. I came upon the information (some of it is patterns, some isn't yet) in the course of writing my forthcoming book of Smalltalk coding patterns. I looked at temporary variables first, and found patterns for four ways temps are used:

1. Reuse the value of a side-effecting expression (like ∧SStream≫next∧T).

2. Cache the value of an expensive expression.

3. Explain the meaning of a complex expression.

4. Collect the results of several expressions.

Success in hand, I went to look for a similar set of canonical uses of instance variables. No dice. I came up with a taxonomy of the uses of instance variables, but no patterns. Many of the uses are bound up in other patterns (like the instance variable that holds the current State object). Others are too vague to make good patterns. I'll present what I have so far, because I have found it useful even in its unpolished state.

Ward Cunningham and I had a good long talk about this. We decided that the reason I was having so much trouble was scope. In the first book I am looking for coding patterns, the tactics of successful Smalltalk. Choosing to use a temporary variable is a tactical decision.