Decimal to Binary conversion options in Scheme

dec->bin.scm

;; Very clean, but is not tail recursive!
(define (dec->bin n)
  (cond ((zero? n) '())
        (else (cons (remainder n 2)
                    (dec->bin (quotient n 2))))))

;; Not so clean, but is tail recursive.
(define (dec->bin n)
  (letrec ((recc (lambda (acc n)
                   (cond ((zero? n) acc)
                         (else (recc (cons (remainder n 2) acc)
                                     (quotient n 2)))))))
    (recc '() n)))

;; Not much worse than the cleanest one, but uses optional args.
(define* (dec->bin n #:optional (acc '()))
  (cond ((zero? n) acc)
        (else (dec->bin (quotient n 2)
                        (cons (remainder n 2) acc)))))

(dec->bin 9) ;; => '(1 0 0 1)