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 14.4.9 Example 4: Object Orientation
 ------------------------------------
 
 Closure is the capture of an environment, containing persistent variable
 bindings, within the definition of a procedure or a set of related
 procedures.  This is rather similar to the idea in some object oriented
 languages of encapsulating a set of related data variables inside an
 "object", together with a set of "methods" that operate on the
 encapsulated data.  The following example shows how closure can be used
 to emulate the ideas of objects, methods and encapsulation in Scheme.
 
      (define (make-account)
        (let ((balance 0))
          (define (get-balance)
            balance)
          (define (deposit amount)
            (set! balance (+ balance amount))
            balance)
          (define (withdraw amount)
            (deposit (- amount)))
 
          (lambda args
            (apply
              (case (car args)
                ((get-balance) get-balance)
                ((deposit) deposit)
                ((withdraw) withdraw)
                (else (error "Invalid method!")))
              (cdr args)))))
 
    Each call to `make-account' creates and returns a new procedure,
 created by the expression in the example code that begins "(lambda
 args".
 
      (define my-account (make-account))
 
      my-account
      =>
      #<procedure args>
 
    This procedure acts as an account object with methods `get-balance',
 `deposit' and `withdraw'.  To apply one of the methods to the account,
 you call the procedure with a symbol indicating the required method as
 the first parameter, followed by any other parameters that are required
 by that method.
 
      (my-account 'get-balance)
      =>
      0
 
      (my-account 'withdraw 5)
      =>
      -5
 
      (my-account 'deposit 396)
      =>
      391
 
      (my-account 'get-balance)
      =>
      391
 
    Note how, in this example, both the current balance and the helper
 procedures `get-balance', `deposit' and `withdraw', used to implement
 the guts of the account object's methods, are all stored in variable
 bindings within the private local environment captured by the `lambda'
 expression that creates the account object procedure.
 

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