The abstraction boundary extends from the adt keyword the terminating

Figure 24-1: Existential Types

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A conventional abstract data type (or ADT) consists of (1) a type name A, (2) a concrete representation type T, (3) implementations of some operations for creating, querying, and manipulating values of type T, and (4) an abstraction boundary enclosing the representation and operations. Inside this bound ary, elements of the type are viewed concretely (with type T). Outside, they are viewed abstractly, with type A. Values of type A may be passed around, stored in data structures, etc., but not directly examined or changed-the only operations allowed on A are those provided by the ADT.

For example, here is a declaration of an abstract data type of purely functional counters, in a pseudocode notation similar to Ada (U.S. Dept. of Defense, 1980) or Clu (Liskov et al., 1981).