SICP/ex-2_77-97.scm

(load "util.scm")

(display "\nexample - generic arithmetic operations\n")

; (define (display x) ())
; (define (newline) ())
; can be used to import stuff silently

; Put and get functions. We could have implemented this via a list of
; three-tuples, but I don't know how to create global variables yet so we just
; use this code from SO. Doesn't look too complicated.
; https://stackoverflow.com/questions/5499005/how-do-i-get-the-functions-put-and-get-in-sicp-scheme-exercise-2-78-and-on
(define *op-table* (make-hash-table))
(define (put op type proc)
  (hash-table/put! *op-table* (list op type) proc))
(define (get op type)
  (hash-table/get *op-table* (list op type) #f))

(define *coercion-table* (make-hash-table))
(define (put-coercion type1 type2 proc)
  (hash-table/put! *coercion-table* (list type1 type2) proc))
(define (get-coercion type1 type2)
  (hash-table/get *coercion-table* (list type1 type2) #f))

;; Helpers for generic arithmetic operations
(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))))

(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))))))

(define (install-integer-package)
  (define (make-integer x)
    (if (integer? x)
      (tag x)
      (error "Not an integer -- MAKE-INTEGER " x)))
  (define (integer->rational x)
    (make-rational x 1))
  (define (tag x)
    (attach-tag 'integer x))
  (put 'add '(integer integer)
       (lambda (x y) (tag (+ x y))))
  (put 'sub '(integer integer)
       (lambda (x y) (tag (- x y))))
  (put 'mul '(integer integer)
       (lambda (x y) (tag (* x y))))
  (put 'div '(integer integer)
       (lambda (x y) (tag (/ x y))))
  (put 'equ? '(integer integer)
       (lambda (x y) (= x y)))
  (put 'exp '(integer integer)
       (lambda (x y) (tag (expt x y)))) ; using primitive expt
  (put '=zero? '(integer)
       (lambda (x) (= x 0)))
  (put 'make 'integer
       (lambda (x) (make-integer x)))
  (put 'raise '(integer) integer->rational)
  (display "[install-integer-package]\n")
  'done)

(define (install-rational-package)
  ;; internal procedures
  (define (numer x) (car x))
  (define (denom x) (cdr x))
  (define (make-rat n d)
    (let ((g (gcd n d)))
      (cons (/ n g) (/ d g))))
  (define (add-rat x y)
    (make-rat (+ (* (numer x) (denom y))
                 (* (numer y) (denom x)))
              (* (denom x) (denom y))))
  (define (sub-rat x y)
    (make-rat (- (* (numer x) (denom y))
                 (* (numer y) (denom x)))
              (* (denom x) (denom y))))
  (define (mul-rat x y)
    (make-rat (* (numer x) (numer y))
              (* (denom x) (denom y))))
  (define (div-rat x y)
    (make-rat (* (numer x) (denom y))
              (* (denom x) (numer y))))
  (define (add3-rat x y z)
    (add-rat (add-rat x y) z))
  (define (equ? x y)
    (= (* (numer x) (denom y))
       (* (numer y) (denom x))))
  (define (rational->real x)
    (make-scheme-number (/ (numer x) (denom x))))
  ;; interface to rest of the system
  (define (tag x) (attach-tag 'rational x))
  (put 'add '(rational rational)
       (lambda (x y) (tag (add-rat x y))))
  (put 'add3 '(rational rational rational)
       (lambda (x y z) (tag (add3-rat x y z))))
  (put 'sub '(rational rational)
       (lambda (x y) (tag (sub-rat x y))))
  (put 'mul '(rational rational)
       (lambda (x y) (tag (mul-rat x y))))
  (put 'div '(rational rational)
       (lambda (x y) (tag (div-rat x y))))
  (put 'equ? '(rational rational) equ?)
  (put '=zero? '(rational)
       (lambda (x) (= (numer x) 0)))
  (put 'make 'rational
       (lambda (n d) (tag (make-rat n d))))
  (put 'raise '(rational) rational->real)
  (display "[install-rational-package]\n")
  'done)

(define (install-scheme-number-package)
  (define (tag x)
    (attach-tag 'scheme-number x))
  (define (real->complex x)
    (make-complex-from-real-imag x 0))
  (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 'equ? '(scheme-number scheme-number)
       (lambda (x y) (= x y)))
  (put 'exp '(scheme-number scheme-number)
       (lambda (x y) (tag (expt x y)))) ; using primitive expt
  (put '=zero? '(scheme-number)
       (lambda (x) (= x 0)))
  (put 'make 'scheme-number
       (lambda (x) (tag x)))
  (put 'raise '(scheme-number) real->complex)
  (display "[install-scheme-number-package]\n")
  'done)

(define (install-rectangular-package)
  (define (real-part z) (car z))
  (define (imag-part z) (cdr z))
  (define (magnitude z)
    (sqrt (+ (square (real-part z))
             (square (imag-part z)))))
  (define (angle z)
    (atan (imag-part z)
          (real-part z)))
  (define (tag z) (attach-tag 'rectangular z))
  (define (make-from-real-imag x y)
    (tag (cons x y)))
  (define (make-from-mag-ang r a)
    (tag (cons (* r (cos a)) (* r (sin a)))))
  ; interface to the rest of the system
  (put 'real-part '(rectangular) real-part)
  (put 'imag-part '(rectangular) imag-part)
  (put 'magnitude '(rectangular) magnitude)
  (put 'angle '(rectangular) angle)
  (put '=zero? '(rectangular) (lambda (z) (= (real-part z) (imag-part z) 0)))
  (put 'make-from-mag-ang 'rectangular make-from-mag-ang)
  (put 'make-from-real-imag 'rectangular make-from-real-imag)
  (display "[install-rectangular-package]\n")
  'done)

(define (install-polar-package)
  (define (real-part z)
    (* (magnitude z) (cos (angle z))))
  (define (imag-part z)
    (* (magnitude z) (sin (angle z))))
  (define (magnitude z) (car z))
  (define (angle z) (cdr z))
  (define (tag z) (attach-tag 'polar z))
  (define (make-from-real-imag x y)
    (tag (cons (sqrt (+ (square x) (square y)))
               (atan y x))))
  (define (make-from-mag-ang r a) (tag (cons r a)))
  ; interface to rest of the system
  (put 'real-part '(polar) real-part)
  (put 'imag-part '(polar) imag-part)
  (put 'magnitude '(polar) magnitude)
  (put 'angle '(polar) angle)
  (put '=zero? '(polar) (lambda (z) (= (magnitude z) 0)))
  (put 'make-from-mag-ang 'polar make-from-mag-ang)
  (put 'make-from-real-imag 'polar make-from-real-imag)
  (display "[install-polar-package]\n")
  'done)

(define (install-complex-package)
  ;; imported procedures from rectangular and polar packages
  (define (make-from-real-imag x y)
    ((get 'make-from-real-imag 'rectangular) x y))
  (define (make-from-mag-ang r a)
    ((get 'make-from-mag-ang 'polar) r a))
  ;; getters
  (define (real-part z) (apply-generic 'real-part z))
  (define (imag-part z) (apply-generic 'imag-part z))
  (define (magnitude z) (apply-generic 'magnitude z))
  (define (angle z) (apply-generic 'angle z))
  ;; internal procedures
  (define (add-complex z1 z2)
    (make-from-real-imag (+ (real-part z1) (real-part z2))
                         (+ (imag-part z1) (imag-part z2))))
  (define (sub-complex z1 z2)
    (make-from-real-imag (- (real-part z1) (real-part z2))
                         (- (imag-part z1) (imag-part z2))))
  (define (mul-complex z1 z2)
    (make-from-mag-ang (* (magnitude z1) (magnitude z2))
                       (+ (angle z1) (angle z2))))
  (define (div-complex z1 z2)
    (make-from-mag-ang (/ (magnitude z1) (magnitude z2))
                       (- (angle z1) (angle z2))))
  (define (equ? z1 z2)
    (and (= (magnitude z1) (magnitude z2))
         (= (angle z1) (angle z2))))
  ;; interface to rest of the system
  (put 'real-part '(complex) real-part)
  (put 'imag-part '(complex) imag-part)
  (put 'magnitude '(complex) magnitude)
  (put 'angle '(complex) angle)
  (define (tag z) (attach-tag 'complex z))
  (put 'add '(complex complex)
       (lambda (z1 z2) (tag (add-complex z1 z2))))
  (put 'sub '(complex complex)
       (lambda (z1 z2) (tag (sub-complex z1 z2))))
  (put 'mul '(complex complex)
       (lambda (z1 z2) (tag (mul-complex z1 z2))))
  (put 'div '(complex complex)
       (lambda (z1 z2) (tag (div-complex z1 z2))))
  (put 'make-from-real-imag 'complex
       (lambda (x y) (tag (make-from-real-imag x y))))
  (put 'make-from-mag-ang 'complex
       (lambda (r a) (tag (make-from-mag-ang r a))))
  (put 'equ? '(complex complex) equ?)
  (put '=zero? '(complex) =zero?)
  (display "[install-complex-package]\n")
  'done)

;; constructors
(define (make-integer n)
  ((get 'make 'integer) n))

(define (make-rational n d)
  ((get 'make 'rational) n d))

(define (make-scheme-number n)
  ((get 'make 'scheme-number) n))

(define (make-complex-from-real-imag x y)
  ((get 'make-from-real-imag 'complex) x y))

(define (make-complex-from-mag-ang r a)
  ((get 'make-from-mag-ang 'complex) r a))

(define (real-part z) ((get 'real-part '(complex)) z))
(define (imag-part z) ((get 'imag-part '(complex)) z))
(define (magnitude z) ((get 'magnitude '(complex)) z))
(define (angle z) ((get 'angle '(complex)) z))

;; generic operations
(define (add x y) (apply-generic 'add x y))
(define (add3 x y z) (apply-generic 'add3 x y z))
(define (sub x y) (apply-generic 'sub x y))
(define (mul x y) (apply-generic 'mul x y))
(define (div x y) (apply-generic 'div x y))
(define (equ? x y) (apply-generic 'equ? x y))
(define (=zero? x) (apply-generic '=zero? x))
(define (exp x y) (apply-generic 'exp x y))
(define (raise x) (apply-generic 'raise x))

(install-integer-package)
(install-rational-package)
(install-scheme-number-package)
(install-rectangular-package)
(install-polar-package)
(install-complex-package)

(assert (add (make-scheme-number 10) (make-scheme-number 20)) (make-scheme-number 30))
(define p1 (make-complex-from-mag-ang 14.142135623730951 0.7853981633974483))
(define e1 (make-complex-from-real-imag 10 10))
(assert (add e1 e1) (make-complex-from-real-imag 20 20))

(newline) (display "ex-2.77 - see comments") (newline)

; real-part (and all other selectors are implemented via calls to apply
; generic. The first call to apply generic has the type 'magnitude '(complex).
; By adding the code from Alyssa that call gets dispatched a second time which
; results in a call to apply generic with 'magnitude '(rectangular). This calls
; the actual magnitude function from the rectangular package.

(newline) (display "ex-2.78 - simplify scheme number") (newline)

; Solution at the beginning of this file.
(assert (add 5 3) 8)

(newline) (display "ex-2.79 - equ?") (newline)

; Extended each of the packages and defined generic procedure
(assert (equ? (make-scheme-number 10) (make-scheme-number 10)) #t)
(assert (equ? (make-rational 3 4) (make-rational 6 8)) #t)
(assert (equ? (make-complex-from-mag-ang 3 4) (make-complex-from-real-imag 6 8)) #f)
(assert (equ? p1 e1) #t) ; define above

(newline) (display "ex-2.80 - =zero?") (newline)

; Extended each of the packages and defined generic procedure
(assert (=zero? 0) #t)
(assert (=zero? 1) #f)
(assert (=zero? (make-rational 0 1)) #t)
(assert (=zero? (make-rational 1 1)) #f)
(assert (=zero? e1) #f)
(assert (=zero? p1) #f)

(newline) (display "ex-2.81 - Louis trying things") (newline)

(define (scheme-number->complex n)
  (make-complex-from-real-imag (contents n) 0))

(put-coercion 'scheme-number 'complex scheme-number->complex)

(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))
          (if (= (length args) 2)
              (let ((type1 (car type-tags))
                    (type2 (cadr type-tags))
                    (a1 (car args))
                    (a2 (cadr args)))
                (let ((t1->t2 (get-coercion type1 type2))
                      (t2->t1 (get-coercion type2 type1)))
                  (cond
                    ((eq? type1 type2) (error "No need to coerce identical types"
                                              (list op type-tags)))
                    (t1->t2 (apply-generic op (t1->t2 a1) a2))
                    (t2->t1 (apply-generic op a1 (t2->t1 a2)))
                    (else (error "No method for these types" (list op type-tags))))))
              (error "No method for these types"
                     (list op type-tags)))))))

(display "[see comments]\n")
(assert (exp 3 3) 27)
(assert (add (make-scheme-number 3) (make-complex-from-real-imag 3 4))
        (make-complex-from-real-imag 6 4))

; a. This is an endless loop. Louis change is not necessary, because if we
; coerce the arguments into the same type we would have found the respective
; procedure already.

; (define (scheme-number->scheme-number n) n)
; (define (complex->complex z) z)
; (put-coercion 'scheme-number 'scheme-number scheme-number->scheme-number)
; (put-coercion 'complex 'complex complex->complex)

; (exp (make-complex-from-real-imag 3 4) (make-complex-from-mag-ang 2 3))

; b. apply-generic already handles arguments of the same type correctly. It
; will simply not find a coercion procedure and return.

; c. added check for identical types to apply-generic. The following now just
; causes an error and no endless loop.
; (exp (make-complex-from-real-imag 3 4) (make-complex-from-mag-ang 2 3))


(newline) (display "ex-2.82 - multi argument coercion") (newline)


(define (scheme-number->rational n)
  (make-rational (contents n) 1))
(put-coercion 'scheme-number 'rational scheme-number->rational)

(define (coerce-args target-type args)
  (define (coerce-arg arg)
    (let ((t1->t2 (get-coercion (type-tag arg) target-type)))
      (if (procedure? t1->t2) (t1->t2 arg) arg)))
  (map coerce-arg args))

(define (apply-generic op . args)
  (define (try-args args-list)
    (if (null? args-list)
      (error "No method for these types" (list op (map type-tag args)))
      (let ((proc (get op (map type-tag (car args-list))))
            (args-contents (map contents (car args-list))))
        (if (procedure? proc)
          (apply proc args-contents)
          (try-args (cdr args-list))))))
  (define (coerce-to-arg arg)
    (coerce-args (type-tag arg) args))
  (try-args (cons args (map coerce-to-arg args))))

(assert (add3 (make-rational 1 3) 2 (make-rational 3 9)) (make-rational 8 3))

; This approach does not work if there exist procedures for mixed types or if
; the coerced type that would work is different from any of the existing
; arguments' types.

(display (coerce-args 'rational (list (make-rational 1 3) 2 3))) (newline)

(newline) (display "ex-2.83") (newline)

; Our scheme-number package supports real numbers so we use that as our
; real-number package without further changes. Additionally, we create an
; integer package that only accepts integers in the constructor.

(assert (sub (make-integer 3) (make-integer 1)) (make-integer 2))

(define i (make-integer 3))
(display i) (newline)
(display (raise i)) (newline)
(display (raise (raise i))) (newline)
(display (raise (raise (raise i)))) (newline)

(newline) (display "ex-2.84") (newline)

; All we have to do is update coerce-args to do consecutive raises
; to reach the target type.
(define (coerce-args target-type args)
  (define (coerce-arg arg)
    (if (eq? (type-tag arg) target-type)
      arg
      (let ((proc (get 'raise (list (type-tag arg)))))
         (if (procedure? proc)
           (proc (contents arg))
           arg))))
  (let ((coerced-args (map coerce-arg args)))
    (if (equal? args coerced-args)
      coerced-args ; no more raising possible
      (coerce-args target-type coerced-args))))

(assert (equ? (make-integer 3) (make-complex-from-real-imag 3 0)) #t)
(assert (equ? (make-integer 3) (make-complex-from-real-imag 3 1)) #f)
(assert (equ? (make-integer 3) (make-rational 3 1)) #t)
(assert (add3 (make-rational 1 3) (make-integer 2) (make-rational 3 9)) (make-rational 8 3))
;(display (coerce-args 'scheme-number (list (make-rational 1 3) 2 (make-rational 3 9))))

(newline) (display "ex-2.85 - drop it") (newline)


(newline) (display "ex-2.86") (newline)