;; Run code in an environment that only creates a connection if the
;; object is accessed, and closes after execution in the case a
;; connection was created.

(module lazy-connect mzscheme

  (provide
   with-lazy-connect
   lazy-connection   
   )

  ;; Abstract type to distinguish parameter from delayed connect.
  
  (define-struct lazy-connect (thunk))

  (define connection
    (make-parameter
     (make-lazy-connect
      (lambda ()
        (error 'no-lazy-connection)))))

  (define (with-lazy-connect connect-thunk thunk close)
    (parameterize
        ((connection #f))
      (dynamic-wind
          (lambda ()
            (connection
             (make-lazy-connect connect-thunk)))
          thunk
          (lambda ()
            (unless (lazy-connect? (connection))
              (close (connection)))))))

  
  ;; a thunk evaluating to the object.
  (define (lazy-connection)
    (let ((c (connection)))
      (when (lazy-connect? c)
        (connection ((lazy-connect-thunk c)))))
    (connection))
    

  )