#lang plait
(define-type BinOp [Plus] [Or]) 

(define-type Exp
  [binE (operator : BinOp)
        (left : Exp)
        (right : Exp)]
  [numE (value : Number)]
  [boolE (value : Boolean)]
  [ifE (check : Exp) (then-e : Exp) (else-e : Exp)])

(define-type Value [boolV (value : Boolean)] [numV (value : Number)])

(define (interp expr)
  (type-case Exp expr
    [(numE n) (numV n)]
    [(boolE b) (boolV b)]
    [(ifE check then-e else-e) ....]
    [(binE o l r)
     (let ([l-val (interp l)]
           [r-val (interp r)])
       (type-case BinOp o
         [(Or) (boolV (or (boolV-value l-val) (boolV-value r-val)))]
         [(Plus) (numV (+ (numV-value l-val) (numV-value r-val)))]))]))

(test (interp (binE (Plus) (numE 3) (numE 4)))
        (numV 7))
(test (interp (binE (Or) (boolE #t) (boolE #f)))
      (boolV #t))
(test (interp (ifE (boolE #f) (numE 1) (numE 2)))
      (numV 2))