factorial.sel

; vim:ft=lisp ts=2 sts=2 sw=2 expandtab

; (list args...) := (args...)
func list (args)

val nil ()

; (inc x) := x + 1
func inc (add (car-args) "1")


; (foreach f list) := nil (with side effects)
func foreach (if (cadr-args)
    (nil
        ((car-args) (caadr-args))
        (foreach (car-args) (cdadr-args)))
    nil)


; (upto n m) := (n n+1 n+2 ... m)
func upto (if (eq? (car-args) (cadr-args))
  (cdr-args)
  (cons (car-args) (upto (inc (car-args)) (cadr-args))))


; (mul-tail n m acc) := n * m + acc
func mul-tail (if (eq? (car-args) "0")
  (caddr-args)
  (mul-tail (dec (car-args)) (cadr-args) (add (cadr-args) (caddr-args))))

; (mul n m) := n * m
func mul (mul-tail (car-args) (cadr-args) "0")


; (fact-tail n acc) := n! * acc
func fact-tail (if (eq? (car-args) "0")
  (cadr-args)
  (fact-tail (dec (car-args)) (mul (car-args) (cadr-args))))

; (factorial n) := n!
func factorial (if (eq? (car-args) "0")
  "1"
  (fact-tail (dec (car-args)) (car-args)))


func print-factorial (print (str-concat
  (car-args)
  "! = "
  (factorial (car-args))))

val main (foreach print-factorial (upto "0" "10"))