Principles of Programming Languages

In this open book, our goal is to study the fundamental concepts in programming languages, as opposed to learning a range of specific languages. Languages are easy to learn, it is the concepts behind them that are difficult. The basic features we study in turn include higher-order functions, data structures in the form of records and variants, mutable state, exceptions, objects and classes, and types. We also study language implementations, both through language interpreters and language compilers. Throughout the book we write small interpreters for toy languages, and in Chapter 8 we write a principled compiler. We define type checkers to define which programs are well-typed and which are not. We also take a more precise, mathematical view of interpreters and type checkers, via the concepts of operational semantics and type systems. These last two concepts have historically evolved from the logician's view of programming.

The material has evolved from lecture notes used in a programming languages course for juniors, seniors, and graduate students at Johns Hopkins University. While the book uses formal mathematical techniques such as operational semantics and type systems, it does not emphasize proofs of properties of these systems. We will nonetheless sketch the intuitions of some proofs.