Data types and program correctness

One of the most important current software issues is reliability, and accordingly, a major criterion of programming language design must be that the language contribute to the production of reliable programs. Although there are other important aspects of software reliability (e.g., fault tolerance), the most fundamental is program correctness: does the program do what it is supposed to do? A language can contribute to this goal by enhancing the provability of its programs. This paper discusses the impact of user-defined data types on program provability.