Issue: DESCRIBE-INTERACTIVE (Version 4)

[Dan L. Pierson <pierson@mist>]

Issue:        DESCRIBE-INTERACTIVE
References:   DESCRIBE (p441)
Category:     CLARIFICATION (EXPLICITLY-VAGUE) / CHANGE (NO)
Edit history: 12-Sep-88, Version 1 by Pitman
              23-Sep-88, Version 2 by Masinter
	      19-Oct-88, Version 3 by Pierson, invert
              15-Nov-88, Version 4 by Pierson, two-proposal version

Problem Description:

  CLtL is not clear about whether DESCRIBE may be interactive.
  While CLtL describes INSPECT as an interactive as an
  interactive version of DESCRIBE, it doesn't make explicit
  that DESCRIBE is non-interactive. In some implementations it is, 
  and in other implementations it is not.

  Users of systems in which DESCRIBE is not interactive may presume
  that it is safe to call DESCRIBE in a batch applications without
  hanging the application, which can lead to problems.

Proposal (DESCRIBE-INTERACTIVE:EXPLICITLY-VAGUE):

    Specify that DESCRIBE is permitted (though not required) to
    require user input, and that such input should be negotiated
    through *QUERY-IO*.

    Descriptive information would continue to go to *STANDARD-OUTPUT*.

  Test Case:

    The following kind of interaction would be permissible in
    implementations which chose to do it:

     (DEFVAR *MY-TABLE* (MAKE-HASH-TABLE))
     (SETF (GETHASH 'FOO *MY-TABLE*) 1)
     (SETF (GETHASH 'BAR *MY-TABLE*) 2)
     (SETF (GETHASH 'FOOBAR *MY-TABLE*) 3)
     (DESCRIBE *MY-TABLE*)
     #<EQ-HASH-TABLE 259> has 3 entries.
     Do you want to see its contents? (Yes or No) Yes

  Rationale:

    This validates current implementations.

  Current Practice:

    Symbolics Genera asks some questions interactively when describing
    some kinds of structured data structures, such as hash tables.
    Since users can define their own DESCRIBE methods and took their cue
    from the system, describing some user structures also require such
    interactions.

  Cost to Implementors:

    None.

  Cost to Users:

    User code which depended on DESCRIBE running without user interaction
    would have to be modified. Such code is not currently fully portable,
    however.

  Cost of Non-Adoption:

    Users would not know the straight story about whether they should
    expect interaction from DESCRIBE.

  Benefits:

    Implementations which don't do interactive querying in DESCRIBE only
    because their not 100% sure it's kosher would be free to do it.

  Aesthetics:

    Some people might think it's not aesthetic for DESCRIBE to require user
    intervention. Not saying whether it's permissible is probably less
    aesthetic, though.

Proposal (DESCRIBE-INTERACTIVE:NO):

    Specify that DESCRIBE is forbidden to prompt for or require
    user input.  Permit implementations to extend describe via keyword
    arguments to prompt for or require user input as long as the default
    action if no keyword arguments are supplied does not prompt for or
    require user input.

    This is an incompatible change for some implementations.

  Test Case:

    The following kind of interaction would be permissible in
    implementations which chose to do it:

     (DEFVAR *MY-TABLE* (MAKE-HASH-TABLE))
     (SETF (GETHASH 'FOO *MY-TABLE*) 1)
     (SETF (GETHASH 'BAR *MY-TABLE*) 2)
     (SETF (GETHASH 'FOOBAR *MY-TABLE*) 3)
     (DESCRIBE *MY-TABLE* :INTERACTIVE T)
     #<EQ-HASH-TABLE 259> has 3 entries.
     Do you want to see its contents? (Yes or No) Yes

  Rationale:

    DESCRIBE is the only hook a portable program has for providing
    information about objects to the user.  The potential interactive
    functions of DESCRIBE are also likely to be available via INSPECT.

  Current Practice:

    Symbolics Genera asks some questions interactively when describing
    some kinds of structured data structures, such as hash tables.
    Since users can define their own DESCRIBE methods and took their cue
    from the system, describing some user structures also require such
    interactions.

  Cost to Implementors:

    Symbolics Genera and other non-conforming implementations will have
    to change.

  Cost to Users:

    User code which depends on DESCRIBE running with user interaction
    will have to be modified. Such code is not currently portable,
    however.

  Cost of Non-Adoption:

    Users would not have any portable way to have progams inform the
    user about object they are dealing with.  This might be important in
    traces and logs of portable progams or in portable debugging tools.

  Benefits:

    It will be easier to provide some types of portable functionality.

  Aesthetics:

    This simplifies the language slightly.

Discussion:

  Pitman thinks it's important to clarify this issue, but he isn't fussy
  about the particulars.

  EXPLICITY-VAGUE proposal is the minimal proposal for compatibility
  with current behavior.

  Some members of the cleanup committee think that EXPLICITLY-VAGUE is
  really a change from the intent of CLtL. However, the current
  sentiment is to be less rather than more specific about the behavior
  of debugging tools (25.3 of CLtL).

  It might be possible to extend DESCRIBE to have additional 
  keywords (:VERBOSE, :INTERACTIVE-ALLOWED) to cover 
  additional behavior. 

  Pierson supports NO because he thinks it's important for there to be
  some way for portable programs to present this sort of information
  to the user.  While the exact data and format presented is
  implementation dependent and thus outside of the bounds of
  standardization, the existance of such data is neither.

  Moon opposes NO because "it creates extra work for implementors and
  users of at least one implementation, for no compelling reason."

  Moon suggested as a compromise only allowing DESCRIBE to require
  user input from "interactive streams".  Several other members of the
  committee like this in principle but question whether it's feasible
  since we have neither defined "interactive streams" nor provided any
  portable way to tell if a stream is interactive.