#lang racket

(define (calculate-RPN expr)
  (for/fold ([stack '()]) ([token expr])
    (printf "~a\t -> ~a~N" token stack)
    (match* (token stack)
     [((? number? n) s) (cons n s)]
     [('+ (list x y s ___)) (cons (+ x y) s)]
     [('- (list x y s ___)) (cons (- y x) s)]
     [('* (list x y s ___)) (cons (* x y) s)]
     [('/ (list x y s ___)) (cons (/ y x) s)]
     [('^ (list x y s ___)) (cons (expt y x) s)]
     [(x s) (error "calculate-RPN: Cannot calculate the expression:"
                   (reverse (cons x s)))])))