The Scheme Programming Language, Third Edition [Electronic resources]

4.1. Variable References

Any identifier appearing as an expression in a program is a keyword or variable reference. It is a keyword reference if a lexical or top-level keyword binding for the identifier is visible; otherwise, it is a variable reference. After syntactic extensions have been expanded (see Chapter 8, no keyword references remain, so all remaining identifier expressions are variable references.

list ⇒ #<procedure>
(define x 'a)
(list x x) ⇒ (a a)
(let ((x 'b))
(list x x)) ⇒ (b b)
(let ((let 'let)) let) ⇒ let

It is an error to evaluate a top-level variable reference before the variable is defined at top-level, but it is not an error for such a reference to appear within a part of a that has not yet been evaluated. This permits mutually recursive procedures to be defined using top-level bindings.

i-am-not-defined ⇒ error
(define f
(lambda (x)
(g x)))
(define g
(lambda (x)
(+ x x)))
(f 3) ⇒ 6