CS4400-Spr24-Calc

calc.rkt

#lang plait

(define-type Exp
    [num (n : Number)]
    [plus (left : Exp) (right : Exp)])

(parse : (S-Exp -> Exp))
(define (parse s)
  (cond
    [(s-exp-number? s)
     (num (s-exp->number s))]
    [(s-exp-list? s)
     (let ([l (s-exp->list s)])
       (cond
         [(empty? l) (error 'parse "empty list")]
         [(symbol=? '+
                     (s-exp->symbol (first l)))
           (plus (parse (second l))
                 (parse (third l)))]
         [else (error 'parse "unrecognized symbol")]))]))

;; evaluates a Calc expression to a number
(calc : (Exp -> Number))
(define (calc e)
  (type-case Exp e
    [(num n) n]
    [(plus l r) (+ (calc l) (calc r))]))