Scheme - range function

range.scm

;;; validate-step validates whether a given step is a valid one.
(define validate-step
  (lambda (step)
    (let ((s
	   (cond
	    ((and (list? step) (not (null? step))) (car step))
	    ((and (list? step) (null? step)) 1)
	    ((string? step) (string->number step))
	    ((integer? step) step))))
      (if (integer? s)
	  (if (= s 0) (error "step cannot be zero") s)
	  (error 'validate-step "invalid step given" s)))))

;;; range creates a sequence of integer values from min to max with the given step.
;;;
;;; Note: the code below does not validate range input, e.g. (range 1 10 -1) is invalid
(define range
  (lambda (min max . step)
    (let ((step (validate-step step)))
      (if (< min max)
	  (cons min (range (+ min step) max step))
	  '()))))