35 lines
1.2 KiB
Racket
35 lines
1.2 KiB
Racket
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(define (point x y)
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(define (self m)
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(cond ((eq? m 'getx) (lambda () x))
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((eq? m 'gety) (lambda () y))
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((eq? m 'type) (lambda () 'point))
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((eq? m 'setx!) (lambda (nx) (set! x nx)))
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((eq? m 'sety!) (lambda (ny) (set! y ny)))
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((eq? m 'info) (lambda () (list 'point x y)))
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((eq? m 'add) (lambda (p)
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(let ((px ((p 'getx)))
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(py ((p 'gety))))
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(point (+ x px) (+ y py)))))
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((eq? m 'sub) (lambda (p)
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(let ((px ((p 'getx)))
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(py ((p 'gety))))
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(point (- x px) (- y py)))))
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(else (lambda () (display (string-append "Message not understood: " (symbol->string m) "\n"))))))
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self)
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(define p1 (point 1 2))
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(define p2 (point 3 4))
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((p1 'getx)) ; returns 1
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((p1 'gety)) ; returns 2
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((p2 'getx)) ; returns 3
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((p2 'gety)) ; returns 4
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(define pp ((p1 'add) p2)) ; returns a new point pp
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((pp 'info)) ; returns (point 4 6)
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(define pn ((p1 'sub) p2)) ; returns a new point pn
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((pn 'info)) ; returns (point -2 -2)
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((pp 'foo)) ; should display "Message not understood" error
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((p1 'setx!) 5)
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((p1 'sety!) 5)
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((p1 'info)) ; returns (point 5 5)
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