Re: Q: access to subprogram

[ncohen@watson.ibm.com (Norman H. Cohen)]

In article <ROGOFF.96Jul8155205@sccm.Stanford.EDU>, rogoff@sccm.Stanford.EDU
(Brian Rogoff) writes: 

|> ncohen@watson.ibm.com (Norman H. Cohen) writes: 
...
|>                            Bill Taylor had made what I considered an
|>    irrefutable case for downward closures, showing how much easier it would
|>    be to write iterators if downward closures were allowed.  It came down to
|>    a conflict between the interests of Ada programmers and the interests of
|>    a minority of Ada implementors, and in this case the interests of the few
|>    implementors using displays prevailed.)
|>
|> Could you summarize or provide a reference to Bill Taylor's work? Does it
|> answer the objection that downward closures impose an inefficiency on programs
|> that don't use them?

Bill Taylor's contribution was part of an ongoing discussion in the form
of comments sent to the Mapping/Revision Team during the design of Ada 9X.
All of these comments from June 1992 on are publicly available in the
following directory: 

ftp://sw-eng.falls-church.va.us/public/AdaIC/standards/95com/mrtcomments/

All the comments through April 1994 are in files with names of the form
YYMMmrt.zip, where YY and MM are the year and month the comment was
received.  (Thus all comments from May 1993 are in 9305mrt.zip.) Starting
with May 1994, there are individual daily files, such as 94.0501 for all
comments received on May 1, 1994.

The discussion on downward closures and their impact on display
implementations included the following comments: 

Date              Comment ID  Author          My summary

April 29, 1993    93.2622.b   Bill Taylor     The initial message about
                                              iterators mentioned above.

May 1, 1993       93-2628.b   Robert Duff     Says he likes the feature,
                                              but Robert Dewar has
                                              frightened WG9 members
                                              about the cost, and the MRT
                                              won't add downward closures
                                              until someone convinces WG9
                                              otherwise.  Claims the size
                                              of a display is not known
                                              at compile time.

May 3, 1993       93-2634.a   Norman Cohen    Reply to Duff, asserting
                                              that the size of a display
                                              IS known at compile time.
                                              Foreshadows an identical
                                              exchange that will take
                                              place on comp.lang.ada in
                                              July 1996.  :-)

May 3, 1993       93-2635.a   Robert Duff     Reply to Cohen.  Size of a
                                              display is not known at the
                                              site of a call through a
                                              pointer.  Says he hates
                                              arguing against a feature
                                              he likes.

May 3, 1993       93-2637.a   Randy Brukardt  Reports that Janus/Ada uses
                              (RR Software)   displays.  Proposes a
                                              workaround involving
                                              unchecked conversion.

May 3, 1993       93-2638.a   Robert Duff     Reply to Brukardt, asking
                                              him to explain workaround.

May 4, 1993       93-2644.a   Antoine Bertier Reports that Alsys uses
                              (Alsys)         displays for most of its
                                              compilers.

May 4, 1993       93-2645.a   Ted Baker       Calls displays an
                                              "anachronism", except on
                                              register-poor machines.

May 4, 1993       93-2646.a   Randy Brukardt  Reply to Duff, explaining
                                              his workaround and
                                              conceding that it is not
                                              portable to compilers that
                                              do NOT use displays.

May 5, 1993       93-2647.a   Brian Dobbing   Also reports that Alsys
                              (Alsys)         uses displays, states that
                                              Alsys cannot afford any
                                              more language changes with
                                              such great impact on their
                                              compilers, and states that
                                              neither the display
                                              approach nor the static
                                              link approach is always
                                              the right choice.

May 5, 1993       93-2651.a   Bill Taylor     Points out that Brukhardt's
                                              proposed workaround does
                                              not work when the pointed-
                                              to procedures reference
                                              data in surrounding scopes,
                                              as they do in his iterators
                                              example.

May 5, 1993       93-2655.a   Robert Dewar    Reply to Baker, defending
                                              displays and asserting
                                              that they can be quite
                                              efficient.

May 5, 1993       93-2656.a   Robert Eachus   Experiments (with a
                                              compiler that was altered
                                              to send Eachus e-mail every
                                              time code was generated for
                                              a call on a nested
                                              subprogram!) show that
                                              calls on nested procedure
                                              are rare, so the expense of
                                              supporting this feature is
                                              not justified.

May 6, 1993       93-2661.a   Ted Baker       Reply to Dewar, claiming
                                              that downward closures were
                                              important for the kind of
                                              applications that justified
                                              ANY inclusion of 'Access
                                              for nested subprograms in
                                              Ada 9X, and worth the cost;
                                              but admitting that Brian
                                              Dobbing's economic argument
                                              might be the deciding
                                              consideration.

May 11, 1993      93-2676.a   Randy Brukardt  Reply to Baker.

May 11, 1993      93-2676.b   Randy Brukardt  States that downward
                                              closures impose a
                                              distributed overhead on all
                                              uses of generics when
                                              generics are implemented by
                                              shared code for all
                                              instances, as in Janus/Ada.

January 14, 1994  94-3669.a   Bjorn Kallberg  An eloquent case for the
                                              importance of downward
                                              closures, pointing out that
                                              their omission would create
                                              the only case in which Ada
                                              is not a functional
                                              superset of Pascal.

February 15, 1994 LSN 1083    Robert Duff     A dispassionate analysis
                                              of all sides of the issue,
                                              explaining why the MRT
                                              decided not to include
                                              downward closures in Ada 9X.

March 4, 1994     94-3985.a   David Tombs     An empassioned plea to
                                              reconsider this decision.

--
Norman H. Cohen    ncohen@watson.ibm.com