(define-datatype primitive primitive?
  (add-prim)
  (subtract-prim)
  (mult-prim)
  (incr-prim)
  (decr-prim))

(define-datatype expression expression?
  (lit-exp
   (datum number?))
  (var-exp
   (id symbol?))
  (primapp-exp
   (rator primitive?)
   (rands (list-of expression?)))
  (if-exp
   (test-exp expression?)
   (then-exp expression?)
   (else-exp expression?))
  (let-exp
   (ids (list-of symbol?))
   (rands (list-of expression?))
   (body expression?)))