2.4.1 Top-Level Definitions

A top-level definition,

(define variable expression)

has essentially the same effect as this assignment expression, if variable is bound:

(set! variable expression)

If variable is not bound, however, define binds variable to a new location in the current environment before performing the assignment (it is an error to perform a set! on an unbound variable). If you omit expression, the variable becomes unassigned; an attempt to reference such a variable is an error.

(define add3
   (lambda (x) (+ x 3)))                ⇒  unspecified
(add3 3)                                ⇒  6

(define first car)                      ⇒  unspecified
(first '(1 2))                          ⇒  1

(define bar)                            ⇒  unspecified
bar                                     error→ Unassigned variable