Preface

Types are the central organizing principle of the theory of programming languages. Language features are manifestations of type structure. The syntax of a language is governed by the constructs that define its types, and its semantics is determined by the interactions among those constructs. The soundness of a language design—the absence of ill-defined programs— follows naturally. The purpose of this book is to explain this remark. A variety of pro-gramming language features are analyzed in the unifying framework of type theory. A language feature is defined by its statics, the rules govern-ing the use of the feature in a program, and its dynamics, the rules defining how programs using this feature are to be executed. The concept of safety emerges as the coherence of the statics and the dynamics of a language. In this way we establish a foundation for the study of programming languages. But why these particular methods? The main justification is provided by the book itself. The methods we use are both precise and in