#lang plait
(define-type Exp
  [num (n : Number)]
  [plus (left : Exp) (right : Exp)]
  [times (left : Exp) (right : Exp)])

(define (calc e)
  (type-case Exp e
    [(num n) n]
    [(plus l r) (+ (calc l) (calc r))]
    [(times l r) (* (calc l) (calc r))]))

(test (calc (num 1)) 1)
(test (calc (num 2.3)) 2.3)
(test (calc (plus (num 1) (num 2))) 3)
(test (calc (plus (plus (num 1) (num 2))
                  (num 3)))
      6)
(test (calc (plus (num 1)
                  (plus (num 2) (num 3))))
      6)
(test (calc (plus (num 1)
                  (plus (plus (num 2)
                              (num 3))
                        (num 4))))
      10)