CLHS: Issue DEFINE-CONDITION-SYNTAX Writeup

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## Issue DEFINE-CONDITION-SYNTAX Writeup

    Issue:		DEFINE-CONDITION-SYNTAX
    Forum:		Cleanup
    References:	Condition System, Version 18
    		CLOS Specification, 88-002R
    		Issue CLOS-CONDITIONS
    		Issue CONDITION-ACCESSORS-SETFABLE
    Category:	CHANGE/CLARIFICATION
    Edit History:	Version 1, 1/3/90 by Kim A. Barrett

    Problem Description:
      Issue CLOS-CONDITIONS incompatibly changed the slot specification syntax
      for DEFINE-CONDITION to match the slot specification syntax for DEFCLASS.
      This leaves us with two defining macros whose general forms are very
      similar but which have small idiosyncratic differences.

    Notes on voting:
     Vote for just 1 of MORE-LIKE-DEFCLASS, INCOMPATIBLY-MORE-LIKE-DEFCLASS, or
       MORE-LIKE-DEFINE-CONDITION.
     In addition, vote yes or no to EMPHASIZE-READ-ONLY.

    Proposal (DEFINE-CONDITION-SYNTAX:MORE-LIKE-DEFCLASS):
      Define the syntax for DEFINE-CONDITION to be

        DEFINE-CONDITION name ({supertype}*) [({slot}*) {option}*]

        option ::= (:DEFAULT-INITARGS initarg-list) |
    	       (:DOCUMENTATION string) |
    	       (:REPORT exp)

      The :default-initarg option is defined in the same way as for DEFCLASS.  The
      :documentation and :report options are unchanged.

      If no supertypes are specified, then the condition being defined inherits
      directly from CONDITION.

    Proposal (DEFINE-CONDITION-SYNTAX:INCOMPATIBLY-MORE-LIKE-DEFCLASS):
      As DEFINE-CONDITION-SYNTAX:MORE-LIKE-DEFCLASS, but make the slot
      specification list required.

    Proposal (DEFINE-CONDITION-SYNTAX:MORE-LIKE-DEFINE-CONDITION):
      As DEFINE-CONDITION-SYNTAX:MORE-LIKE-DEFCLASS, but change the syntax of
      DEFCLASS so that the slot specification list is optional.

    Proposal (DEFINE-CONDITION-SYNTAX:EMPHASIZE-READ-ONLY)
      As either of the above proposals, but remove the :WRITER and :ACCESSOR
      slot options.

    Rational:
      This provides the added functionality provided by :DEFAULT-INITARGS, and
      the convenience of not having to explicitly write CONDITION as the only
      super for some conditions.

      INCOMPATIBLY-MORE-LIKE-DEFCLASS makes the basic syntax identical, but at
      the cost of an incompatible change, since the list of slot specifications
      will no longer be optional.

      EMPHASIZE-READ-ONLY emphasizes the point that it is an error to attempt
      to assign a condition's slots using SETF.  Condition objects are supposed
      to be at least conceptually read-only.

    Current Practice:
      IIM supports the :DEFAULT-INITARGS option.  They may support the behavior
      specified for an empty supertype list (if they don't, then they still
      require at least one supertype).

    Cost to Implementors:
      Supporting an empty supertype list in the specified way is probably fairly
      trivial.  For an implementation of conditions which is not based on an
      implementation of CLOS, adding the support for :default-initargs might not
      be trivial.

    Cost to Users:
      MORE-LIKE-DEFCLASS is upwardly compatible for the status quo.

      The other two proposals will probably require some programs to be modified.
      As an interim solution implementations could support MORE-LIKE-DEFCLASS and
      issue warnings for cases which were incompatible with the other two
      proposals.

    Benefits:
      Programmers would need to remember fewer idiosyncratic differences in the
      syntax of two very similar and important macros.

    Discussion:
      Barrett supports MORE-LIKE-DEFINE-CONDITION.  He thinks that
      EMPHASIZE-READ-ONLY would be a good idea if there isn't too much concern
      about the incompatibility issues.

      Pitman supports MORE-LIKE-DEFCLASS but mostly because the CLOS crew are
      a stubborn bunch and it seems more likely that we can bend the rest of the
      spec to be like their polished piece of prose than vice versa.
      (i.e., MORE-LIKE-DEFINE-CONDITION is technically just as good to him.)
      Pitman also supports EMPHASIZE-READ-ONLY.

    -----
    Additional comments in response to write-up:

    Moon (11 Jan 90):

      This issue raises good points that I think were overlooked in past
      discussions of unifying Conditions and CLOS.

      I am neutral among MORE-LIKE-DEFCLASS, INCOMPATIBLY-MORE-LIKE-DEFCLASS,
      or MORE-LIKE-DEFINE-CONDITION.  The incompatibilities here (either making
      the slot-specifier list optional where it's mandatory or mandatory where
      it is optional) are of little concern since the adjustment is tiny.  Any
      one of these would be a significant improvement over the status quo.

      I am definitely in favor of EMPHASIZE-READ-ONLY and not impressed by the
      compatibility arguments.  An existing program that is incompatible with
      this either actually modifies condition objects, in which case it can't
      work, or it just accidentally says :ACCESSOR but only uses the reader,
      in which case it is trivial to fix.

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