;; Self referencial expressions

;; Expr: can be
;; -- Number
;; --(make-add (Expr Expr)
;; --(make-mul (Expr Expr)
;; --(make-sub (Expr Expr)
(define-struct add (left right))
(define-struct mul (left right))
(define-struct sub (left right))


;;TODO: write the template for Expr
;; Ask the 4 questions when writing a template!


;; Lets define a function, compute
;;  -that when given a number, returns that number
;;  -when given add, adds the two expressions, sub, subtracts the two expressions
;;  -and mul multiplies the two expressions

;;examples:
;; (compute 3) => 3
;; (compute (make-add 1 2))=> 3
;; (compute (make-sub 2 1))=> 1


  

  
;; Expr2 can be
;; --number
;; --(make-expression Symbol Expr2 Expr2)

  (define-struct expression ( op left right))
  
  ;;TODO: Create the template for Expr2
  ;; Ask the 4 questions again!
  
  
  
  
;;Create a function, expr2 that performs the same function as compute, except we only use one struct
;; Only allow Symbol to be one of the following, +,-,*. Any other inputed symbol should return the string "error"

;; (expr2 expression) -> number

  
  
  
  
  
;; examples:
;; (expr2 2) => 2
;; (expr2 (make-expression '+ 1 2)) => 3
;; (expr2 (make-expression '- 4 3)) => 1
;; (expr2 (make-expression '* (make-expression '- 3 1) 1)) => 2
;; (expr2 (make-expression '/ 1 2)) => "error"