Essentials of Programming Languages (EOPL) is a textbook on programming languages by Daniel P. Friedman, Mitchell Wand, and Christopher T. Haynes. EOPL surveys the principles of programming languages from an operational perspective. It starts with an interpreter in Scheme for a simple functional core language similar to the lambda calculus and then systematically adds constructs. For each addition, for example, variable assignment or thread-like control, the book illustrates an increase in expressive power of the programming language and a demand for new constructs for the formulation of a direct interpreter. The book also demonstrates that systematic transformations, say, store-passing style or continuation-passing style, can eliminate certain constructs from the language in which the int