Exp. Exact types. Both dependent classes and prefix types of depen- dent classes are exact types [6]: all instances of these types must have the same run-time class. Simple types may be exact too. If A is a class, the exact type A! represents values of the run-time class A. Even if class B in- herits from A, neither B nor B! is a subtype of A!. Exactness ap- plies to the entire type preceding “!”, so supertypes of a sim- ple exact type can be obtained by shifting the exactness outward. For example, ASTDisplay.Exp! is not a subtype of AST.Exp!,
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