On the unification of data and program abstraction in Ada

Ada is rich in the variety of its abstraction mechanisms. It has both a data abstraction mechanism (packages with private data types) that supports a functional programming style and a program abstraction mechanism (generic program units) that supports an object-oriented program style. Tradeoffs between data and program abstraction are examined and it is pointed out that Ada discourages program abstraction because program units are not first-class objects. It is shown how program units could be made into first-class objects by introducing closures as values for functions and records with function components as values for packages. Further unification by allowing types to be first-class objects conflicts with the requirement of compile-time type invariance. The relaxation of this requirement in a manner that preserves type consistency is examined and leads to a notion of value for types as tuples of operations. It is suggested in the conclusion that our understanding of abstraction for object-oriented languages and of other language design, implementation, and environment issues will have progressed sufficiently by 1985 to warrant the design of a successor to Ada by the late 1980s.