Racket pure-functional RPN calculator

rpn.rkt

#lang racket

(define (op? e)
  (member e '(+ *)))

(define (op->procedure op)
  (case op
    [(+) +]
    [(*) *]))

(define (eval-rpn expr stack)
  (cond
    [(empty? expr) stack] ;; we're done, return the stack
    [(number? (car expr)) ;; number
      (eval-rpn (cdr expr) (cons (car expr) stack))] ;; push number on the stack
    [(op? (car expr)) ;; operation
      (let* ([op (car expr)]
             [rhs (car stack)]
             [lhs (cadr stack)]
             [result ((op->procedure op) lhs rhs)]) ;; eval operation
       (eval-rpn (cdr expr) (cons result (cddr stack))))] ;; push result; pop lhs/rhs
    [else (error "unknown symbol")]))
  
(printf "result: ~a~%" (eval-rpn '(3 2 + 2 * 2 *) '()))