🚀 go-pugleaf

jbk@world.std.com (Jeffrey B Kane) writes:

  (defclass test-class ()
    ((slot-1	:initarg :slot-1
		:accessor slot-1)
     (slot-2	:initarg :slot-2
		:accessor slot-2)))

  (defun make-test-class (&key (slot-1 3) slot-2)
    (unless (boundp slot2)
      (setf slot-2 slot-1))
    (make-instance 'test-class :slot-1 slot-1 :slot-2 slot-2))

  The problem with this approach is that the function "boundp" does
  not test if the user typed in a particular key word.

  Calling:

  (make-test-class :slot-2 4)

  results in the error:

  > Error: Unbound variable: SLOT2
  > While executing: SYMBOL-VALUE

Uh, that's 'cause you mistyped SLOT-2 as SLOT2.  But the variable SLOT-2 would
be bound in any case.  You can't use BOUNDP to tell if a keyword or optional
parameter was supplied, because they have default values.

That aside:

  The reason I think that the constructor approach is bettter is than
  using initialize-instance :after is that it allows other key word
  and a higher level of abstraction.

But it doesn't allow you to specialize the class.  And it assumes that
SLOT-1 is only initialized in one way, through the initarg :SLOT-1.
Hardly more abstract.

  The the bottom line question is:

  Other than defining a keyword with some kind of a value to flag it
  hasn't been used, how can you test if it was used similar to
  slot-boundp for slots?

I'm not sure what this means, but are you asking how to tell if a
keyword was used?  Use what CLtL calls "svars" (page 76, CLtL2),
presumably "s" for "supplied".  Minimally modifying your constructor:

  (defun make-test-class (&key (slot-1 3) (slot-2 NIL SLOT-2-SUPPLIED?)
    (unless SLOT-2-SUPPLIED?
      (setf slot-2 slot-1))
    (make-instance 'test-class :slot-1 slot-1 :slot-2 slot-2))

John Burger
john@mitre.org