CS4400-Spr24-Ite

ite.rkt

#lang plait

(define-type Value
  [vbool (b : Boolean)]
  [vnum (v : Number)])

(define-type Exp
  [num (n : Number)]
  [bool (b : Boolean)]
  [plus (left : Exp) (right : Exp)]
  [cnd (test : Exp) (thn : Exp) (els : Exp)])

(define (get-num-or-err value)
  (type-case Value value
    [(vnum n) n]
    [else (error 'value "value conversion error")]))

(define (get-bool-or-err value)
  (type-case Value value
    [(vbool b) b]
    [else (error 'value "value conversion error")]))

(define (add v1 v2)
  (+ (get-num-or-err v1) (get-num-or-err v2)))

(define (calc e)
  (type-case Exp e
    ([num n] (vnum n))
    ([bool b] (vbool b))
    ([plus l r] (vnum (add (calc l) (calc r))))
    ([cnd test thn els] (if (get-bool-or-err (calc test))
                            (calc thn)
                            (calc els)))))