section-2.5.1, Ex-2.77-2.80

Ex-2.77
(magnitude z)
(apply-generic 'magnitude z)
(apply (get 'magnitude '(complex)) (list (contents z)))
Now let say content of z is z1 wihch is a complex number in polar notation and
(magnitude polar-z)
(apply-generic 'magnitude polar-z)
(apply (get 'magnitude '(polar)) (list (contents polar-z)))
This will find the concrete magnitude procedure to get magnitude of a complex number in polar representation. apply-generic is called two times as we have two level of abstraction. Number->Complex->Polar(or Rectangular).

Ex-2.78

(define (attach-tag type-tag contents)
  (if (number? contents) contents
      (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)))) 

Ex-2.79

(define (equ? x y)
  (apply-generic 'equ? x y))

(define (equ-scheme-number? x y)
  (= x y))
(put 'equ? '(scheme-number scheme-number) equ-scheme-number?)

(define (equ-rational-number? x y)
  (and (= (numer x) (numer y))
       (= (denom x) (denom y))))
(put 'equ? '(rational rational) equ-rational-number?)

(define (equ-complex-number? x y) 
  (and (= (real-part x) (real-part y))
       (= (imag-part x) (imag-part y))))
(put 'equ? '(complex complex) equ-complex-number?) 

Ex-2.80

(define (=zero? x)
  (apply-generic '=zero? x))

(put '=zero? '(scheme-numer)
     (lambda (x) (= x 0)))

(put '=zero? '(rational)
     (lambda (x) (= (numer x) 0)))

(put '=zero? '(complex)
     (lambda (x) (and
                  (= (real-part x) 0)
                  (= (imag-part x) 0))))