Implicit in the study of let, let*, and letrec expressions is the concept of scope. Scope is a concept that programmers encounter in every language. Since scope is often so tightly woven into the semantics of a language, we unconsciously understand it and rarely ever give it a second thought. In this chapter, we examine the details more closely.
In a program, variables appear as either references or declarations—even in typeless languages like Lisp that use manifest typing. The value named by a variable is called its denotation. Consider the following Scheme expression:
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