Error with define in Racket

Question

I just discovered Racket a few days ago, and I\'m trying to get more comfortable with it by writing a little script that generates images to represent source code using #l

Answer 1

Definitions are allowed in a 'body' context, like in lambda and let among others. The consequent and alternate clauses of if are not body contexts; they are expression contexts and therefore definitions are not allowed.

begin is special - begin in a body context allows definitions, but begin in an expression contexts forbids definitions. Your case falls in to the later.

For example:

(define (foo . args)     #| body context #|)
(define foo (lambda args #| body context |#))
(define (foo . args)
  (let (...)
    #| body context |#))

Syntactic keywords that requires expressions: if, cond, case, and, or, when, unless, do, begin. Check out the formal syntax in any Scheme report (r{4,5,6,7}rs); look for , , , and .

Also, if you need a body context in an expression, just wrap a let syntactic form, as such:

(if test
    (let ()
      (define foo 'foo)
      (list foo foo))
    alternate)