ABSTRACT
The fundamental purpose of a type system is to prevent the occurrence of execution errors during the running of a program. This informal statement motivates the study of type systems, but requires clarification. Its accuracy depends, first of all, on the rather subtle issue of what constitutes an execution error, which we will discuss in detail. Even when that is settled, the absence of execution errors is a nontrivial property. When such a property holds for all the program runs that can be expressed within a programming language, we say that the language is type sound. It turns out that a fair amount of careful analysis is required to avoid false and embarrassing claims of type soundness for programming languages. As a consequence, the classification, description, and study of type systems has emerged as a formal discipline.
