[pro] Initialization of shared slots

[Scott L. Burson]

On Wed, Jan 5, 2011 at 7:17 AM, Didier Verna <didier at lrde.epita.fr> wrote:
> Until now, I've been uncounsiously relying on something that the
> standard does not seem to specify: the fact that shared slots equipped
> with an initform are initialized before the first instance is created:
>
> (defclass test () ((slot :allocation :class :initform t)))
>
> (defmethod initialize-instance :before ((test test) &key)
>    (format t "Slot boundp: ~A~%" (slot-boundp test 'slot)))
>
>
> * (make-instance 'test)
> Slot boundp: T
>
> * (make-instance 'test)
> Slot boundp: T
>
>
> I just realized that because with ABCL (and contrary to the other
> implementations I've tried), the result is NIL, and then T.
>
> But now, I'm a little confused. Section 7.1.3 of CLHS says:
>
>        An :initform form is used to initialize a slot only if no
>        initialization argument associated with that slot is given as an
>        argument to make-instance or is defaulted by :default-initargs.
>
> So after re-reading this (the "only" part notably), it now appears to me
> somewhat abusive to initialize shared slots earlier than at first
> instance creation time (like most lisps seem to do), because there's no
> way to know, at that time, whether the fist call to make-instance will
> provide an initarg.
>
> WDYT?

It's possible that the CLHS is not perfectly consistent on this point,
but there are a couple of places where it endorses the familiar
interpretation.  In section 7.1 it says: "The :initform for a shared
slot may be used when defining or redefining the class."  On the
DEFCLASS page it says "... for shared slots, the dynamic environment
[to be used when evaluating the :initform form] is the dynamic
environment in which the defclass form was evaluated."

I think the sentence from 7.1.3 that you've quoted is intended to
apply only to local slots; I think these other two passages make that
clear by establishing that shared slots are indeed initialized at
defclass time.

Oh, and this appears to be a bug in ABCL, since the second passage I
quoted above _requires_ the :initform to be evaluated at defclass
time.

-- Scott