Issue SLOT-MISSING-VALUES Writeup

Issue:         SLOT-MISSING-VALUES


References:    88-002R p.2-78..79


Category:      CLARIFICATION


Edit history:  29-Apr-90, Version 1 by Moon


Problem description:


  The document says that all values returned by a SLOT-UNBOUND or

  SLOT-MISSING method are returned from SLOT-VALUE, SETF, SLOT-BOUNDP, or

  SLOT-MAKUNBOUND.  However, the language mandates particular values to be

  returned by SETF and by SLOT-MAKUNBOUND, and mandates that SLOT-VALUE and

  SLOT-BOUNDP return exactly one value.  If methods can change this,

  efficient compilation becomes very difficult.


  This is Symbolics issue #19.


Proposal (SLOT-MISSING-VALUES:SPECIFY):


  Specify exactly what is done with the values returned from a SLOT-UNBOUND

  or SLOT-MISSING method, as follows:


  If the operation was SETF or SLOT-MAKUNBOUND, the values are ignored.


  If the operation was SLOT-VALUE or SLOT-BOUNDP, exactly one value is

  returned; extra values returned by the method are ignored, and if the

  method returns no values, NIL is returned.


  If the operation was SLOT-BOUNDP and the method returns a value other

  than NIL, SLOT-BOUNDP might return any object other than NIL that it can

  validly return; only the truth or falsity of the method's value matters.


Current practice:


  It's likely that all CLOS implementations already conform to the proposal,

  since implementing what the document says would be extremely difficult.

  I have not actually tested any implementations.


Cost to Implementors:


  What's proposed is what's easiest to implement.


Cost to Users:


  None, since it's unlikely that any users would depend on being able to

  force SLOT-VALUE to return two values, or to force SETF of SLOT-VALUE to

  return something different from new-value.


Cost of non-adoption:


  CLOS will be seriously difficult to implement correctly.


Performance impact:


  Implementing CLOS correctly will cause a serious efficiency impact on

  SLOT-VALUE if this proposal is not adopted, since it will have to be

  capable of dynamically deciding at run time to return multiple values.


Benefits:


  Simpler and more consistent language.


Esthetics:


  Simpler and more consistent language.


Discussion:


  None.