#lang r5rs

;You found the second flag as well >:(
;Fine, I'm no longer loading the flag...

;Assoc containing defined variables
(define env '())

;Add a variable to the environment
(define (add-variable name value)
  (set! env (cons (cons name value) env))
  "Variable added!")

;Read a variable from the environment
(define (read-variable name)
  (define pair (assq name env))
  (if pair
    (cdr pair)
    (begin
      (display "Undefined variable")
      (newline))))

;Load the very secret flag
(define (load-flag)
  'NO!)

;Allowed operators
(define operators '(+ - * /))

;Evaluate an expression of the form (<op> <exp> ... <exp>)
(define (evaluate-pair expression)
  (define operator (car expression))
  (define arguments (cdr expression))
  (cond 
    ((eq? operator 'def)
     (add-variable (cadr expression) (evaluate (caddr expression))))
    (else
     (apply (eval operator (scheme-report-environment 5)) (map evaluate arguments)))))

;Evaluate any expression
(define (evaluate expression)
  (cond 
    ((pair? expression)
     (evaluate-pair expression))
    ((symbol? expression)
     (read-variable expression))
    (else
      expression)))

;Start the read-eval-print loop
(define (read-eval-print)
  (display ">> ")
  (let*
    ((expression (read))
     (result (if (eq? expression 'flag)
               "Wait, thats illegal!"
               (evaluate expression))))
    (display result)
    (newline)
    (read-eval-print)))

(load-flag)
(read-eval-print)