Controlling Evaluation

The first category of special operators contains the three operators that provide basic control over the evaluation of forms. They’re **QUOTE**, **IF**, and **PROGN**, and I’ve discussed them all already. However, it’s worth noting how each of these special operators provides one fundamental kind of control over the evaluation of one or more forms. **QUOTE** prevents evaluation altogether and allows you to get at s-expressions as data. **IF** provides the fundamental boolean choice operation from which all other conditional execution constructs can be built.1 And **PROGN** provides the ability to sequence a number of forms.