Re: detecting uninitialized components of structures.

[Norman Adams <adams@YALE.ARPA>]

    I need a copy-structure that also copies components (instead of making
    them EQ)...    Unfortunately, there is no way
    to tell if a component is uninitialized without causing an error,
    ...

The easiest thing to do is to initialize the components somehow:
         
  (define-predicate empty?)

  (define (empty id)
      (object nil
          ((print self stream) (format stream "#{Empty~_~s}" id))
          ((empty? self) t)))

  (define-structure-type employee
    name
    age)

  (let ((emp (stype-master employee-stype)))
    (set (employee-name emp) (empty employee-name))
    (set (employee-age emp)  (empty employee-age))
    )

If you want the EMPTY? checks to be fast, then you could go
a little lower tech:
                          
  (define *empty*
    (object nil ((print self stream) (writes stream "#{Empty}"))))

  (define-integrable (empty? obj) (eq? obj *empty*))

  (let ((emp (stype-master employee-stype)))
    (set (employee-name emp) *empty*)
    (set (employee-age emp)  *empty*)
    )

-Norman
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