Lisp code to work with ranges of integers

;; Lisp code to work with ranges of integers
;; (range 2 7) --> (2 3 4 5 6 7)
;; [3 5]       --> (3 4 5)
;; (iota 4)    --> (1 2 3 4)
;; (ι 3)       --> (1 2 3)
;; (sum [1 4]) --> 10
;; Inspired by the comp.lang.lisp discussions:
;;   1) "My opinion re LISP"
;;   2) "APL-like operators defined in Lisp?"

; (range i f) produces the range of integers from i(nitial) to f(inal)
(defun range (i f &optional (a nil))
  (if (eq i f)
    (cons i a)
    (range i (1- f) (cons f a))))

; read the bracketed expression [i f] as (range i f)
(set-macro-character #\] (get-macro-character #\)))

(set-macro-character #\[
  #'(lambda (stream char)
    (let ((pair (read-delimited-list #\] stream t)))
      (list 'quote (range (car pair) (cadr pair))))))

; (sum list) produces the sum of the numbers in the given list
(defun sum (l) (apply #'+ l))

; emulate APL's iota operator ι and []IO index origin variable
(defvar *IO* 1) ; index origin

(defun iota (f) (range *IO* f))

; shortcut for the above
(defun ι (f) (iota f))

; examples of the above
(sum [2 7])     ; produces 27

(sum [1 100])   ; produces 5050

(sum (iota 10)) ; produces 55

(sum (ι 4))     ; produces 10