Essentials of Programming Languages (EOPL) is a textbook on programming languages by Daniel P. Friedman, Mitchell Wand, and . 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 interpreter is formulate