Chapter 10
Local Binding and Conditional Evaluation
The interpreter for a computer language is just another program.
— Hal Abelson in Foreword to Essentials of Programming Languages (Friedman, Wand, and Haynes 2001)
Les yeux sont les interprètes du coeur, mais il n’y a que celui qui y a intérêt qui entend leur langage.
(Translation: The eyes are the interpreters of the heart, but only those who have an interest can hear their language.)
— Blaise Pascal
THIS book is about programming language concepts. One approach to learning language concepts is to implement them by building interpreters for computer languages. Interpreter implementation also provides the operational semantics for the interpreted programs. In this and the following two chapters we put into practice the language concepts we have encountered in Chapters 1–9.