3.1.6 Definitions

The variables bound by a let expression are local, because their bindings are visible only in let’s body. Scheme also allows creating top–level bindings for identifiers as follows:

(define x 23)
(define y 42)
(+ x y) ⇒ 65

these are actually “top–level” in the body of a top–level program or library. Libraries.

The first two parenthesized structures are definitions; they create top–level bindings, binding ‘x’ to ‘23’ and ‘y’ to ‘42’. Definitions are not expressions, and cannot appear in all places where an expression can occur. Moreover, a definition has no value.

Bindings follow the lexical structure of the program: When several bindings with the same name exist, a variable refers to the binding that is “closest” to it, starting with its occurrence in the program and going from inside to outside, and referring to a top–level binding if no local binding can be found along the way:

(define x 23)
(define y 42)
(let ((y 43))
  (+ x y)) ⇒ 66

(let ((y 43))
  (let ((y 44))
    (+ x y))) ⇒ 67