infix-to-prefix.rkt

#lang racket

(define operators (hash '+ 5 '- 5 '* 10 '/ 10))

(define (priority operator)
  (hash-ref operators operator 0))

(define (operator? operator) 
  (hash-has-key? operators operator))

(define (max-priority lst)
  (argmax priority lst))

;упорядочить оператор
;target-op искомый оператор
;left - левый операнд; op - оператор; args - остальная часть выражения
(define (reverse-operator target-op left op . args)
  (let ((right (car args)) (righter (cdr args)))
    (if (eq? target-op op) 
        (append (list (list op left right)) righter)
        (append (list left op) (apply reverse-operator target-op args))
    )))

(define (reverse-operators expr) 
  (if (and (list? (car expr)) (eq? (cdr expr) '())) 
      (car expr) 
      (reverse-operators (apply reverse-operator (max-priority expr) expr))
      )) 

(define (infix-to-prefix expression)
  (let ((sub-parsed-list (map (λ (x) (if (list? x) (infix-to-prefix x) x)) expression)))
    (reverse-operators sub-parsed-list)
    ))

(define-syntax (eval-infix stx)
  (syntax-case stx ()
    ((eval-infix expr ...)
  #'(infix-to-prefix (expr ...)))))

;simple example
(define (infix2prefix left operator right)
  (operator left right)
  )

(define a (eval-infix 2 + 2 * 2))