Chapter 6: Compound Data and Data Abstraction

Data Abstraction Revisited Again

A: This is not the right question, because we care only about how a game state behaves.

The Data Abstraction Principle:

If it acts like an X (i.e., is suitable for operations that operate on X's), then it is an X.
Conversely, if it is an X, only expect it to act in X-like ways.
Don't use constructors and selectors in ways that assume anything about the data representation. For example, don't do any of these:
```
      (size-of-pile (* 6 6) 1)        Assumes game state is represented as a number
      (size-of-pile sqrt 1)           Assumes game state is represented as a __________
```