Chapter 2, Building Abstractions with Data
Section - 2.5 - Systems with Generic Operations
Exercise 2.78
I have included the complete program so that it can be tested/executed easily.
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#lang sicp
(#%require (only racket/base error))
(#%require (only racket/base make-hash))
(#%require (only racket/base hash-set!))
(#%require (only racket/base hash-ref))
; helper procedures
(define *op-table* (make-hash))
(define (put op type proc)
(hash-set! *op-table* (list op type) proc)
)
(define (get op type)
(hash-ref *op-table* (list op type) '())
)
(define (attach-tag type-tag contents)
(if (number? contents)
contents
(cons type-tag contents)
)
)
; procedures implemented as per the requirement of exercise
(define (type-tag datum)
(cond ((number? datum) 'scheme-number)
((pair? datum) (car datum))
(error "Bad tagged datum -- TYPE-TAG" datum)
)
)
(define (contents datum)
(cond ((number? datum) datum)
((pair? datum) (cdr datum))
(error "Bad tagged datum -- CONTENTS" datum)
)
)
;remaining procedures
(define (install-scheme-number-package)
(define (tag x)
(attach-tag 'scheme-number x))
(put 'add '(scheme-number scheme-number)
(lambda (x y) (tag (+ x y))))
(put 'sub '(scheme-number scheme-number)
(lambda (x y) (tag (- x y))))
(put 'mul '(scheme-number scheme-number)
(lambda (x y) (tag (* x y))))
(put 'div '(scheme-number scheme-number)
(lambda (x y) (tag (/ x y))))
(put 'make 'scheme-number
(lambda (x) (tag x)))
'done
)
(install-scheme-number-package)
(define (make-number n)
((get 'make 'scheme-number) n)
)
(define (apply-generic op . args)
(let ((type-tags (map type-tag args)))
(let ((proc (get op type-tags)))
(if proc
(apply proc (map contents args))
(error
"No method for these types -- APPLY-GENERIC"
(list op type-tags))))))
I shall first add the output generated without the changes in apply-tag
, tag
or contents
:
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> (display (make-number 5))
(scheme-number 5)
> (apply-generic 'add 5 10)
(scheme-number 15)
> (apply-generic 'mul 5 10)
(scheme-number 50)
>
Now, lets see the output after the changes:
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> (make-number 5)
5
> (display (make-number 5))
5
> (apply-generic 'add 5 10)
15
> (apply-generic 'mul 5 10)
50
>