expr.scm

(define (expr x)
  (define (calc op oprnd ans)
    (if (null? op)
    ans
    (calc (cdr op) (cdr oprnd) ((car op) ans (car oprnd)))))
  (define (expr-sub x op-stack oprnd-stack)
    (cond [(number? x) 
           (lambda (y) (expr-sub y op-stack (cons x oprnd-stack)))]
	  [(member x '("+" "*" "-" "/")) 
	   (lambda (y) (expr-sub y (cons (eval (string->symbol x)) op-stack) oprnd-stack))]
	  [else
            (let ((r-op-stack (reverse op-stack))
                  (r-oprnd-stack (reverse oprnd-stack)))
             (calc (cdr r-op-stack)
		  (cddr r-oprnd-stack )
		  ((car r-op-stack) (car r-oprnd-stack) (cadr r-oprnd-stack))))]))
    (expr-sub x '() '()))
 
((((((((expr 4) "+") 3) "*") 2) "-") 1) "=")