Re: friend classes in ada95

[Robert Dewar <robert_dewar@my-deja.com>]

In article <38F887AE.8CDA24E0@acm.org>,
  jrcarter@acm.org wrote:
> Stefan Folkesson wrote:
> >
> > In ADA95, is there a way to make the private part of one
class
> > accessable from another (see friend classes in C++)?
>
> From ARM N.6:
>
> Class.  A class is a set of types that is closed under
> derivation, which means that if a given type is in the class,
> then all types derived from that type are also in the class.
> The set of types of a class share common properties, such as
> their primitive operations.

Some interesting Ada history, probably not known to many
readers of this list, since it's quite a while ago now :-)

It's always a hard sell when you choose terminology that is
arbitrarily different from the cultural norm. It is interesting
to recall the big debate at the Salem meeting between Tucker
Taft and Jean Ichibiah.

Jean argued that Ada 95 would be far more accessible to the
object oriented cognoscenti if it was willing to embrace the
term class more enthusiastically, and he suggested that

  type x is tagged record ....

be replaced by

  class type x is record ....

The distinguished reviewers had previously voted overwhelmingly
approving the idea that the term class should appear in the
declaration, although there was somewhat less enthusiasm for
the prefix notation (although of course it is quite consistent
with the notation used for task types and protected types).

Tucker argued strongly (one almost might say frantically :-)
that this was a bad idea, since class meant something different
in Ada 95.

The national delegations at Salem were all a bit appalled by
the ferocity of the argument on both sides, and in particular
at what was clearly a huge amount of effort the design team
had spent in preparing the counter argument.

The vote was even, with several countries abstaining, including
the US. The US abstention reflected substantial disagreement in
the US delegation (I can't rememeber everyone's individual
positions, and indeed it would not be appropriate for me to
divulge them in any case, but I can say I strongly supported
the use of the term CLASS in this context, and not suprisingly
Tucker strongly opposed it). The US delegation decided in
advance that it would support the winning side. When the vote
was tied, I got up and said:

"Since it would take a majority decision to undo a previous
decision, this means that the motion (to change to using class)
fails, and the US delegation therefore changes its vote to
opposed."

I knew perfectly well at the time that if I had instead said

"I really think we cannot have an important issue like this
left open with an even vote, I propose that we discuss
this further and revote, that then most likely CLASS would
have been approved."

But I worried about the dynamics. For whatever reason it
was clear that the Ada 95 design team was truly ferociously
opposed (in fact it reminded me of Jean's opposition to the
unanimous recommendation of the Ada 83 DR's for removing
derived types from Ada 83).

Jean's motion was rejected partly because he insisted on
the prefix notation. Indeed the Japanese delegation voted
against it solely on this basis, and I think if they had
realized that tactically their negative vote would sink
the whole proposal, they would have reconsidered. Even
if they had abstained, then the motion would have carried
(by a majority of 2 countries, since the US would then have
voted FOR the motion.)

I tried before the discussion to get Jean to separate the
two points, but he refused. I knew that was a
tactical error, based on previous discussions in the DR
group, but he was adamant. Personally I prefer the prefix
notation, but I knew others were quite opposed to it.

The history at that point is that Jean was so fed up with what
he felt was a significantly bad decision that would affect
acceptance of Ada 95 that he gave up on participation in the
design effort.

All water under the bridge of course, but everytime I see the
kind of confusion that this decision causes among OO folks
seeing Ada for the first time, I wonder whether the DR's who
voted in favor of putting CLASS into the declaration and doing
away with TAGGED might have been right.

I think Jean was wrong in his fear that this "mistake" would
cause major disaffectation with Ada 95, and I think that history
has shown that this was not the case, but still the odd word
TAGGED still does cause a lot of raised eyebrows for people
coming to the language for the first time.

There are many other intersting stories in the Ada 95 design
process, but the Salem vote on class was probably the high
point in drama :-)

If any of the other participants wish to add details, or correct
me if they think I got any details wrong, feel free (it's quite
a while ago and perhaps I have forgotten some details).

Robert Dewar


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