Issue DESCRIBE-INTERACTIVE Writeup

Status:		Proposal NO passed Jan 89 X3J13

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 they're 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.