Type Systems: Introduction and State of the Art

The most significant difference between persistent and non-persistent programming languages is that, in a persistent language, the long-term data is typed. This results in a major shift in the emphasis of type system protection, from one of a safety mechanism over programs to that of a safety mechanism over the entire software system, including both programs and long-term data. Type systems are normally viewed as providing two aids to the programmer: a modelling framework to aid the task of data abstraction, and a protection mechanism which prevents this modelling framework from being improperly used by a program. The difference in persistent systems is that the integrity of the data modelling is enforced over data which outlive the program invocation in which they are created, and are shareable by other program invocations.