Hex0 assembler in scheme

hex0.scm

(import (scheme base)
	(scheme char)
	(scheme file)
	(scheme read)
	(scheme write))
  
(define hold
  (make-parameter 0))

(define toggle
  (make-parameter #f))
  
(define (hex c)
  (cond
   ((<= 48 c 57)  (- c 48))
   ((<= 65 c 70)  (- c 55))
   ((<= 97 c 102) (- c 87))
   ((or (= c 35)  (= c 59))
    (begin (read-line) #f))
   (else #f)))

(define (main)
  ;; foreach char
  (let ((c (read-u8)))
    (if (not (eof-object? c))
	(begin
	  ;; foreach hexdigit
	  (let ((v (hex c)))
	    (if v
		(begin
		  (if (toggle)
		      ;; foreach pair
		      (let ((u (+ (* (hold) 16) v)))
			(write-u8 u)
			(hold 0))
		      (hold v))
		  (toggle (not (toggle))))))
	  ;; loop
	  (main)))))

(main)