encapsulation

compare these

• procedure that modifies global var
  (define down1 (lambda (i) (set! x (- x i)) x))
  
• procedure that modifies local var
  (define down2 (let ((x 100)) (lambda (i) (set! x (- x i)) x)))
  
• what does this interaction produce when down is down1 down2?
  (define x 100)
  
  (down 5) 95 95
  x 95 100
  (down 5) 90 90
  (set! x 100)
  (down 5) 95 85
  

note

• down2’s x cannot be accessed (read or write!) at the top level, except by executing down2
• that x is said to be encapsulated

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