🚀 go-pugleaf

  In article <1993Jan3.023306.25733@cc.umontreal.ca> kardank@ERE.UMontreal.CA (Kardan Kaveh) writes:

  I would like to be able to place expressions in class slots, and have them
  transparently evaluated when accessed.  Is there a standard way of doing
  this?  Do I need the Meta Object Protocol for this?


  And then, in article <1i7g64INN2tt@early-bird.think.com> barmar@think.com (Barry Margolin) writes:

  To get precisely what you described in your example, you need the MOP.

  If you're willing to allow SLOT-VALUE to return the expression, but have
  SLOT-1 perform the computation, you can do this without the MOP.  You could
  write...

  I think this should be sufficient, since functions outside the class should
  not use SLOT-VALUE instead of the accessor function.


I want to make a brief comment about this, having to do with how I think
one should go about making this decision.  That is, how does one decide,
for a given application, whether the approach proposed by Barmar is good
enough or whether one wants to use the MOP to change the behavior of
slot-value.

I believe the way to think about this is to enumerate to yourself all
the different `calls' to slot-value there are, and decide whether you
want all of those calls to perform the special action (i.e. evaluate the
form) or whether it is ok for some of them just return the value in the
slot.  The key point is that the calls to slot-value that come from
calling an accessor are not the only calls to slot-value -- various
environment tools like inspectors (e.g. describe) also call slot-value.

So, if all you want is for calls to certain accessors (i.e. slot-1) to
perform the special action, Barmar's proposed solution works fine.  But,
it means that any other tool that just calls slot-value -- notably
describe and other kinds of inspectors -- won't perform the special
action, they will just return the raw value in the slot.  That may be
unnacceptable to you, and it is for cases where that is unnacceptable
that this part of the MOP is provided.  It really depends on what you
want. 

Gregor