ex-2.78.scm

;;; Exercise 2.78.  The internal procedures in the scheme-number package are
;;; essentially nothing more than calls to the primitive procedures +, -, etc.
;;; It was not possible to use the primitives of the language directly because
;;; our type-tag system requires that each data object have a type attached to
;;; it. In fact, however, all Lisp implementations do have a type system, which
;;; they use internally. Primitive predicates such as symbol? and number?
;;; determine whether data objects have particular types. Modify the
;;; definitions of type-tag, contents, and attach-tag from section 2.4.2 so
;;; that our generic system takes advantage of Scheme's internal type system.
;;; That is to say, the system should work as before except that ordinary
;;; numbers should be represented simply as Scheme numbers rather than as pairs
;;; whose car is the symbol scheme-number.

(define (attach-tag type-tag contents)
  (cond
    [(eq? type-tag 'scheme-number)
     contents]
    [else
      (cons type-tag contents)])

(define (type-tag datum)
  (cond
    [(number? datum)
     'scheme-number]
    [(pair? datum)
     (car datum)]
    [else
      (error "Bad tagged datum -- TYPE-TAG" datum)]))

(define (contents datum)
  (cond
    [(number? datum)
     datum]
    [(pair? datum)
     (cdr datum)]
    [else
      (error "Bad tagged datum -- CONTENTS" datum)]))