PIT calculation

pit.scm

#!/usr/bin/scheme-r5rs

;; Calculate Vietnam Personal Income Tax
;; Input in million VND
(let* ((thresholds '((09 . 0.00)
                     (02 . 0.05)
                     (03 . 0.10)
                     (05 . 0.15)
                     (10 . 0.20)
                     (20 . 0.25)
                     (40 . 0.30)
                     ( 0 . 0.35)))
       (args (cond-expand
              (gauche *argv*)
              (gambit-c (cdr (command-line)))
              (chibi (cdr (command-line)))))
       (total (string->number (car args)))
       (tax
        (let loop ((value total)
                   (thresholds thresholds))
          (let* ((limit (caar thresholds))
                 (rate  (cdar thresholds))
                 (next  (cdr  thresholds))
                 (amount (if (and (pair? next)
                                  (>= value limit))
                             limit
                             value)))
;           (println amount " * " rate " = " (* amount rate))
            (+ (* amount rate)
               (if (pair? next)
                   (loop (- value amount) next)
                   0))))))
  (display tax)
  (newline))