4.1. Variable References

variable	syntax
returns: the value of variable

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