sicp/2_78.rkt

#lang sicp


;;;from section 1.2.5, for Section 2.1.1
(define (gcd a b)
  (if (= b 0)
      a
      (gcd b (remainder a b))))

;;;-----------
;;;from section 3.3.3 for section 2.4.3
;;; to support operation/type table for data-directed dispatch

(define (assoc key records)
  (cond ((null? records) false)
        ((equal? key (caar records)) (car records))
        (else (assoc key (cdr records)))))

(define (make-table)
  (let ((local-table (list '*table*)))
    (define (lookup key-1 key-2)
      (let ((subtable (assoc key-1 (cdr local-table))))
        (if subtable
            (let ((record (assoc key-2 (cdr subtable))))
              (if record
                  (cdr record)
                  false))
            false)))
    (define (insert! key-1 key-2 value)
      (let ((subtable (assoc key-1 (cdr local-table))))
        (if subtable
            (let ((record (assoc key-2 (cdr subtable))))
              (if record
                  (set-cdr! record value)
                  (set-cdr! subtable
                            (cons (cons key-2 value)
                                  (cdr subtable)))))
            (set-cdr! local-table
                      (cons (list key-1
                                  (cons key-2 value))
                            (cdr local-table)))))
      'ok)    
    (define (dispatch m)
      (cond ((eq? m 'lookup-proc) lookup)
            ((eq? m 'insert-proc!) insert!)
            (else (error "Unknown operation -- TABLE" m))))
    dispatch))

(define operation-table (make-table))
(define get (operation-table 'lookup-proc))
(define put (operation-table 'insert-proc!))

(define coercion-table (make-table))
(define get-coercion (coercion-table 'lookup-proc))
(define put-coercion (coercion-table 'insert-proc!))

;; Apply generic

;;(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))))))

;; Number tower.  Ignores possibility of one of the arguments
;; not being in the tower.  Maybe fix later.
(define (lower-in-tower? t1 t2)
  (define tower '(integer rational scheme-number complex))
  (define (tower-iter tower)
    (cond ((null? tower)
           (error "Not in tower" (list t1 t2)))
          ((eq? (car tower) t1) #t)
          ((eq? (car tower) t2) #f)
          (else (tower-iter (cdr tower)))))
  (if (eq? t1 t2)
      #f
      (tower-iter tower)))

(define tower '(integer rational scheme-number complex))

(define (next-type type tower)
  (cond ((or (null? tower)
             (null? (cdr tower))) '())
        ((eq? type (car tower)) (cadr tower))
        (else (next-type type (cdr tower)))))

(define (transform-type x tower)
  (and (type-tagged? x)
       (let* ((type (type-tag x))
              (proj-type (next-type type tower)))
         (if proj-type
             ((get-coercion type proj-type) (contents x))
             '()))))

(define (raise x)
  (transform-type x tower))

(define (project x)
  (transform-type x (reverse tower)))


(define (apply-generic op . args)
  (define (coerce-to t)
    (letrec ((coerce (lambda (arg)
                       (cond ((eq? (type-tag arg) t) arg)
                             ((lower-in-tower?
                               (type-tag arg)
                               t)
                              (coerce (raise arg)))
                             (else '())))))
      coerce))
  (define (any-empty-list l)
    (cond ((null? l) #f)
          ((eq? (car l) '()) #t)
          (else (any-empty-list (cdr l)))))
  (define (coerce-args args type-tags)
    (if (null? type-tags)
        '()
        (let* ((type (car type-tags))
               (coerced-args (map (coerce-to type) args)))
          (if (not (any-empty-list coerced-args))
              coerced-args
              (coerce-args args (cdr type-tags))))))
  (let* ((type-tags (map type-tag args))
         (proc (get op type-tags))
         (result
          (if proc
              (begin
                (display "Applying ")
                (display (list op args))
                (newline)
                (apply proc (map contents args)))
              (if (> (length args) 1)
                  (let ((coerced-args (coerce-args args type-tags)))
                    (if (not (null? coerced-args))
                        (begin
                          (display "Applying with coerced args ")
                          (display (list op coerced-args))
                          (newline)
                          (apply apply-generic (cons op coerced-args)))
                        (error "Can't coerce arguments to a common type"
                               (list op type-tags))))
                  (error "No matching operator for args" op args)))))
    (if (and (pair? result)
             (not (or (eq? op 'raise)
                      (eq? op 'project)
                      (eq? op 'real-part)
                      (eq? op 'imag-part)
                      (eq? op 'angle)
                      (eq? op 'magnitude))))
        (begin
          (display "Drop ")
          (display result)
          (newline)
          (drop result))
        (begin
          (display "Result ")
          (display result)
          (newline)
          result))))

;; scheme-number is for reals only
(define (scheme-number? x)
  (and (real? x)
       (not (integer? x))))

(define (attach-tag type-tag contents)
  (cond ((or (scheme-number? contents)
             (integer? contents))
         contents)
        (else (cons type-tag contents))))

(define (type-tag datum)
  (cond ((scheme-number? datum) 'scheme-number)
        ((integer? datum) 'integer)
        ((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 type-tagged?
  (lambda (x)
    (or (number? x) (pair? x))))

;; Scheme numbers

(define (install-scheme-number-package)
  (define (tag x)
    (attach-tag 'scheme-number x))
  (define (exp x y) (apply-generic 'exp x y))
  (put 'add '(scheme-number scheme-number)
       (lambda (x y) (tag (+ x y))))
  (put 'add3 '(scheme-number scheme-number scheme-number)
       (lambda (x y z) (tag (+ x y z))))
  (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 'negate '(scheme-number)
       (lambda (x) (tag (* -1 x))))
  (put 'sine '(scheme-number)
       (lambda (x) (tag (sin x))))
  (put 'cosine '(scheme-number)
       (lambda (x) (tag (cos x))))
  (put 'square-root '(scheme-number)
       (lambda (x) (tag (sqrt x))))
  (put 'equ? '(scheme-number scheme-number) =)
  (put '=zero? '(scheme-number)
       (lambda (x) (= x 0)))
  (put-coercion 'scheme-number 'complex
                (lambda (x)
                  (make-complex-from-real-imag
                   (tag x)
                   (tag 0))))
  (put-coercion 'scheme-number 'rational
                (lambda (x)
                  (make-rational
                   (round (* x 1000))
                   1000)))
  (put 'exp '(scheme-number scheme-number)
       (lambda (x y) (tag (expt x y))))
  (put 'make 'scheme-number
       (lambda (x) (tag x)))
  'done)

;; Integers

(define (install-integer-package)
  (define (tag x)
    (attach-tag 'integer x))
  (define (raise-int x)
    (make-rational x 1))
 
  (put 'add '(integer integer)
       (lambda (x y)
         (make-integer (+ x y))))
  (put 'sub '(integer integer)
       (lambda (x y)
         (make-integer (- x y))))
  (put 'div '(integer integer)
       (lambda (x y)
         (make-integer (/ x y))))
  (put 'mul '(integer integer)
       (lambda (x y)
         (make-integer (* x y))))
  (put 'negate '(integer)
       (lambda (x) (* -1 x)))
  (put 'sine '(integer)
       (lambda (x) (make-scheme-number (sin x))))
  (put 'cosine '(integer)
       (lambda (x) (cos x)))
  (put 'square-root '(integer)
       (lambda (x) (make-scheme-number (sqrt x))))
  (put 'arctan '(integer integer)
       (lambda (x y) (make-scheme-number (atan x y))))
  (put 'equ? '(integer integer)
       (lambda (x y)
         (= x y)))
  (put-coercion 'integer 'rational raise-int)
  (put 'make 'integer
       (lambda (x) (tag (round x))))
  'done)

;; Rationals

(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 (eq-rat x y)
    (and (= (numer x) (numer y))
         (= (denom x) (denom y))))
  (define (raise-rat 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 (add-rat x (add-rat 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 'negate '(rational)
       (lambda (x)
         (make-rational (* -1 (numer x)) (denom x))))
  (put 'sine '(rational)
       (lambda (x) (sine (raise-rat x))))
  (put 'cosine '(rational)
       (lambda (x) (cosine (raise-rat x))))
  (put 'square-root '(rational)
       (lambda (x) (square-root (raise-rat x))))
  (put 'arctan '(rational rational)
       (lambda (x y) (arctan (raise-rat x)
                             (raise-rat y))))
  (put 'equ? '(rational rational) eq-rat)
  (put '=zero? '(rational)
       (lambda (x) (equ? (numer x) (make-integer 0))))
  (put-coercion 'rational 'scheme-number raise-rat)
  (put-coercion 'rational 'integer
                (lambda (x)
                  (make-integer
                  (/ (numer x) (denom x)))))

 (put 'make 'rational
       (lambda (n d) (tag (make-rat n d))))
  'done)

;;;-----------
;;;SECTION 2.4.3

;; uses get/put (from 3.3.3) -- see ch2support.scm

(define (install-rectangular-package)
  ;; internal procedures
  (define (real-part z) (car z))
  (define (imag-part z) (cdr z))
  (define (make-from-real-imag x y) (cons x y))
  (define (magnitude z)
    (square-root (add (square (real-part z))
                      (square (imag-part z)))))
  (define (angle z)
    (arctan (imag-part z) (real-part z)))
  (define (make-from-mag-ang r a) 
    (cons (mul r (cosine a))
          (mul r (sine a))))
  (define (negate z)
    (make-from-real-imag
     (mul (make-scheme-number -1) (real-part z))
     (mul (make-scheme-number -1) (imag-part z))))

  ;; interface to the rest of the system
  (define (tag x) (attach-tag 'rectangular x))
  (put 'real-part '(rectangular) real-part)
  (put 'imag-part '(rectangular) imag-part)
  (put 'magnitude '(rectangular) magnitude)
  (put 'angle '(rectangular) angle)
  (put 'negate '(rectangular)
       (lambda (z) (tag (negate z))))
  ;;(put 'project '(rectangular)
   ;;    (lambda (z) (real-part z)))
  (put 'make-from-real-imag 'rectangular
       (lambda (x y) (tag (make-from-real-imag x y))))
  (put 'make-from-mag-ang 'rectangular
       (lambda (r a) (tag (make-from-mag-ang r a))))
  'done)

(define (install-polar-package)
  ;; internal procedures
  (define (magnitude z) (car z))
  (define (angle z) (cdr z))
  (define (make-from-mag-ang r a) (cons r a))
  (define (real-part z)
    (mul (magnitude z) (cosine (angle z))))
  (define (imag-part z)
    (mul (magnitude z) (sine (angle z))))
  (define (make-from-real-imag x y) 
    (cons (square-root (add (square x) (square y)))
          (arctan y x)))
  (define pi (make-scheme-number (* 2 (asin 1)))) 
  (define (negate z)
    (make-from-mag-ang
     (magnitude z)
     (add pi (angle z))))

  ;; interface to the rest of the system
  (define (tag x) (attach-tag 'polar x))
  (put 'real-part '(polar) real-part)
  (put 'imag-part '(polar) imag-part)
  (put 'magnitude '(polar) magnitude)
  (put 'angle '(polar) angle)
  (put 'negate '(polar)
       (lambda (z) (tag (negate z))))
  ;;(put 'project '(polar)
       ;;(lambda (z) real-part z))
  (put 'make-from-real-imag 'polar
       (lambda (x y) (tag (make-from-real-imag x y))))
  (put 'make-from-mag-ang 'polar
       (lambda (r a) (tag (make-from-mag-ang r a))))
  '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))
  ;; internal procedures
  (define (add-complex z1 z2)
    (make-from-real-imag (add (real-part z1) (real-part z2))
                         (add (imag-part z1) (imag-part z2))))
  (define (sub-complex z1 z2)
    (make-from-real-imag (sub (real-part z1) (real-part z2))
                         (sub (imag-part z1) (imag-part z2))))
  (define (mul-complex z1 z2)
    (make-from-mag-ang (mul (magnitude z1) (magnitude z2))
                       (add (angle z1) (angle z2))))
  (define (div-complex z1 z2)
    (make-from-mag-ang (div (magnitude z1) (magnitude z2))
                       (sub (angle z1) (angle z2))))

  ;; interface to rest of the system
  (define (tag z) (attach-tag 'complex z))
  (put 'add '(complex complex)
       (lambda (z1 z2) (tag (add-complex z1 z2))))
  (put 'add3 '(complex complex complex)
       (lambda (z1 z2 z3) (tag (add-complex z1 (add-complex z2 z3)))))
  (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 'equ? '(complex complex)
       (lambda (z1 z2)
         (and (equ? (real-part z1) (real-part z2))
              (equ? (imag-part z1) (imag-part z2)))))
  (put '=zero? '(complex)
       (lambda (z) (equ? (magnitude z) (make-scheme-number 0))))
  ;;(put 'negate '(complex)
       ;;(lambda (z) (tag (negate z))))
  (put 'negate '(complex)
       (lambda (z)
         (tag
          (make-from-real-imag
           (mul (real-part z) (make-scheme-number -1))
           (mul (imag-part z) (make-scheme-number -1))))))
         
  (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-coercion 'complex 'scheme-number
                (lambda (z)
                  (real-part z)))
  'done)

;; *incomplete* skeleton of package
(define (install-polynomial-package)
  ;; internal procedures
  ;; representation of poly
  (define (make-poly variable term-list)
    (cons variable term-list))
  (define (variable p) (car p))
  (define (term-list p) (cdr p))
  ;;[procedures same-variable? and variable? from section 2.3.2]
  (define (variable? x) (symbol? x))

  (define (same-variable? v1 v2)
    (and (variable? v1) (variable? v2) (eq? v1 v2)))

  ;; representation of terms and term lists
  ;;[procedures adjoin-term ... coeff from text below]
  (define (adjoin-term term term-list)
    (if (=zero? (coeff term))
        term-list
        (cons term term-list)))

  (define (the-empty-termlist) '())
  (define (first-term term-list) (car term-list))
  (define (rest-terms term-list) (cdr term-list))
  (define (empty-termlist? term-list) (null? term-list))

  (define (make-term order coeff) (list order coeff))
  (define (order term) (car term))
  (define (coeff term) (cadr term))

  (define (add-terms L1 L2)
    (cond ((empty-termlist? L1) L2)
          ((empty-termlist? L2) L1)
          (else
           (let ((t1 (first-term L1)) (t2 (first-term L2)))
             (cond ((> (order t1) (order t2))
                    (adjoin-term
                     t1 (add-terms (rest-terms L1) L2)))
                   ((< (order t1) (order t2))
                    (adjoin-term
                     t2 (add-terms L1 (rest-terms L2))))
                   (else
                    (adjoin-term
                     (make-term (order t1)
                                (add (coeff t1) (coeff t2)))
                     (add-terms (rest-terms L1)
                                (rest-terms L2)))))))))

  (define (mul-terms L1 L2)
    (if (empty-termlist? L1)
        (the-empty-termlist)
        (add-terms (mul-term-by-all-terms (first-term L1) L2)
                   (mul-terms (rest-terms L1) L2))))

  (define (mul-term-by-all-terms t1 L)
    (if (empty-termlist? L)
        (the-empty-termlist)
        (let ((t2 (first-term L)))
          (adjoin-term
           (make-term (+ (order t1) (order t2))
                      (mul (coeff t1) (coeff t2)))
           (mul-term-by-all-terms t1 (rest-terms L))))))

  ;;(define (add-poly p1 p2) ... )
  ;;[procedures used by add-poly]
  (define (add-poly p1 p2)
    (if (same-variable? (variable p1) (variable p2))
        (make-poly (variable p1)
                   (add-terms (term-list p1)
                              (term-list p2)))
        (error "Polys not in same var -- ADD-POLY"
               (list p1 p2))))

  ;;(define (mul-poly p1 p2) ... )
  ;;[procedures used by mul-poly]
  (define (mul-poly p1 p2)
    (if (same-variable? (variable p1) (variable p2))
        (make-poly (variable p1)
                   (mul-terms (term-list p1)
                              (term-list p2)))
        (error "Polys not in same var -- MUL-POLY"
               (list p1 p2))))

  (define (=zero-poly? p)
    (let ((terms (term-list p)))
      (if (null? terms)
          #t
          (and (= (coeff (first-term terms)) 0)
               (=zero-poly? (rest-terms terms))))))

  ;; interface to rest of the system
  (define (tag p) (attach-tag 'polynomial p))
  (put 'add '(polynomial polynomial) 
       (lambda (p1 p2) (tag (add-poly p1 p2))))
  (put 'mul '(polynomial polynomial) 
       (lambda (p1 p2) (tag (mul-poly p1 p2))))
  (put '=zero? '(polynomial)
       (lambda (p) (empty-termlist? (term-list p))))

  (put 'make 'polynomial
       (lambda (var terms) (tag (make-poly var terms))))
  'done)

;; Generic constructors

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

(define (make-integer x)
  ((get 'make 'integer) x))

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

(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 (make-polynomial var terms)
  ((get 'make 'polynomial) var terms))

;; Generic selectors

(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))

(put 'real-part '(complex) real-part)
(put 'imag-part '(complex) imag-part)
(put 'magnitude '(complex) magnitude)
(put 'angle '(complex) angle)

;; Generic operators

(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 (sine x) (apply-generic 'sine x))
(define (cosine x) (apply-generic 'cosine x))
(define (square-root x) (apply-generic 'square-root x))
(define (arctan x y) (apply-generic 'arctan x y))
(define (negate x) (apply-generic 'negate x))
(define (square x) (mul x x))

(define (equ? x y) (apply-generic 'equ? x y))
;;(put 'equ? '(complex complex) equ?)

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


(define (drop x)
  (and (type-tagged? x)
       (let ((p (project x)))
         (begin
           (display "drop: ")
           (display x)
           (newline)
           (cond ((null? p)
                  (display "drop: p Null ")
                  (newline)
                  x)
                 ((equ? x (raise p))
                  (display "drop: x equal to raise p, dropping ")
                  (display p)
                  (newline)
                  (drop p))
                 (else
                  (display "drop: returning ")
                  (display x)
                  (newline)
                  x))))))

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


;; Coercion


(define (scheme-number->complex n)
  (make-complex-from-real-imag (contents n) 0))
(define (scheme-number->rational n)
  (make-rational n 1))

(define (scheme-number->scheme-number n) n)
(define (complex->complex z) z)

;; 2.81a: If we call exp with two complex arguments, apply-generic fails to find an
;; operation with the correct argument list, so tries to coerce t1->t2.
;; b: If the arguments are of the same type, and if there isn't an operation defined for
;; that combination of arguments, then apply-generic will try to coerce a type to itself,
;; so there is a need change it.

;; 2.82: The approach of coercing to a common type would not work where there is a function
;; that takes mixed types.  For example, if we have an operator to add a complex to a rational.
;; It isn't possible to coerce from one to the other, so a mixed-type operator is needed, and this
;; approach would fail.