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