scheme - While Loop Macro in DrRacket

Question

I am trying to create a macro for while loop in DrRacket. Here is what I wrote:

(require mzlib/defmacro)

(define-macro my-while
  (lambda (condition  body)
    (list 'local (list (list 'define (list 'while-loop)
                             (list 'if condition
                                   (list body (list 'while-loop))
                                   '(void))))
          '(while-loop))))


(define x 0)

(my-while (< x 10)
          (begin              
            (display x)
            (newline)
            (set! x (+ x 1))))

The output of this program is:

0
1
2
3
4
5
6
7
8
9
error:  procedure application: expected procedure, given: #<void>; arguments were: #<void>

Can someone help me with this? Why wouldn't this macro just terminate and return void. It seems that when the condition is not true, the system tries to apply the void as an argument to some procedure.

Answer 1

Ouch:

1. Using this style of while loop encourages excessive use of imperative programming.
2. Using define-macro creates unhygienic macros, which is a nightmare in Scheme.

While I don't encourage writing an imperative-style loop macro, for your reference, here's a non-define-macro version of the same macro:

(define-syntax-rule (my-while condition body ...)
  (let loop ()
    (when condition
      body ...
      (loop))))

It uses syntax-rules, which creates hygienic macros, and is much, much easier to read than what you have.

---

Now, for the actual answer for your question, first, let's write your original macro out in a more readable way:

(define-macro my-while
  (lambda (condition body)
    `(local ((define (while-loop)
               (if ,condition
                   (,body (while-loop))
                   (void))))
       (while-loop))))

Once you write it out this way, you can see where the real problem is: in the (,body (while-loop)) line, which should instead have been (begin ,body (while-loop)).