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  1. (define (country-population/area this)  (/ (this 'population) (this    'area      )))
  2. (define (country-gdp/population  this)  (/ (this 'gdp       ) (this    'population)))
  3. (define (country-area> this another  )  (> (this 'area      ) (another 'area      )))
  4.  
  5. (define (new-country gdp area population)
  6.   (letrec
  7.     (
  8.     (this.props
  9.       (list
  10.         (cons 'gdp            gdp                    )
  11.         (cons 'area           area                   )
  12.         (cons 'population     population             )
  13.         (cons 'density        country-population/area)
  14.         (cons 'gdp-per-capita country-gdp/population )
  15.         (cons 'is-bigger      country-area>          )
  16.       )
  17.     )
  18.     (this
  19.       (lambda (operation . args)
  20.         (case operation
  21.           ((set!)
  22.             (let
  23.               ((pair (assq (car args) this.props)))
  24.               (if pair (set-cdr! pair (cadr args)))
  25.               (car args)
  26.             )
  27.           )
  28.           ((get)
  29.             (let
  30.               ((pair (assq (car args) this.props)))
  31.               (if pair (cdr pair) (if (null? (cdr args)) #f (cadr args)))
  32.             )
  33.           )
  34.           (else
  35.             (set! operation (assq operation this.props))
  36.             (if operation
  37.               (begin
  38.                 (set! operation (cdr operation))
  39.                 (if (procedure? operation) (apply operation (cons this args)) operation)
  40.               )
  41.             "Invalid Option"
  42.             )
  43.           )
  44.         )
  45.       )
  46.     )
  47.     )
  48.     this
  49.   )
  50. )
  51.  
  52. (define make-country-mp new-country)
  53.  
  54. ; ------------------------------------------------------------------------------
  55.  
  56. (define Country-A (make-country-mp 1 10 20))
  57. (define Country-B (make-country-mp 2500 5000 50))
  58.  
  59. (display "gdp           : ") (display (Country-B 'gdp                )) (newline)
  60. (display "area          : ") (display (Country-B 'area               )) (newline)
  61. (display "population    : ") (display (Country-B 'population         )) (newline)
  62. (display "density       : ") (display (Country-A 'density            )) (newline)
  63. (display "gdp-per-capita: ") (display (Country-B 'gdp-per-capita     )) (newline)
  64. (display "is-bigger     : ") (display (Country-B 'is-bigger Country-A)) (newline)
  65. (newline)
  66. (display "get 'gdp     : ") (display (Country-B 'get 'gdp    )) (newline)
  67. (display "set! 'gdp 12 : ") (display (Country-B 'set! 'gdp 12)) (newline)
  68. (display "get 'gdp     : ") (display (Country-B 'get 'gdp    )) (newline)
  69. (display "get 'area    : ") (display (Country-B 'get 'area   )) (newline)
  70. (display "get 'density : ") (display (Country-B 'get 'density)) (newline)
  71. (display "get 'Joey    : ") (display (Country-B 'get 'Joey   )) (newline)
  72. (display "get 'Joey 'u : ") (display (Country-B 'get 'Joey 'u)) (newline)
Success #stdin #stdout 0.03s 14020KB
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gdp           : 2500
area          : 5000
population    : 50
density       : 2
gdp-per-capita: 50
is-bigger     : #t

get 'gdp     : 2500
set! 'gdp 12 : gdp
get 'gdp     : 12
get 'area    : 5000
get 'density : #<procedure country-population/area (a)>
get 'Joey    : #f
get 'Joey 'u : u
https://ideone.com/m26rzD
language:
Scheme (guile 2.2.4)
created:
8 days ago
visibility:
public

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