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From: adam@irvine.com
Subject: Re: Software landmines (was: Why C++ is successful)
Date: 1998/08/19
Message-ID: <6rf1f2$tqn$1@nnrp1.dejanews.com>#1/1
X-Deja-AN: 382686633
References: <6rd27s$ffi$1@nnrp1.dejanews.com>
 <ExwxsA.EEL.0.-s@inmet.camb.inmet.com>
Organization: Deja News - The Leader in Internet Discussion
X-Article-Creation-Date: Wed Aug 19 17:19:30 1998 GMT
Newsgroups: comp.lang.ada
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Date: 1998-08-19T00:00:00+00:00
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In article <ExwxsA.EEL.0.-s@inmet.camb.inmet.com>,
  stt@houdini.camb.inmet.com (Tucker Taft) wrote:
> adam@irvine.com wrote:
> : ...
> : Anyway, here's my main question:  Is it desirable to expand the syntax of
> : "exit" to cover cases like this?  I think it might be nice.  I'm thinking
that
> : code like this could look something like:
>
> :     block_to_do_stuff:
> :     begin
> :         blah-blah-blah;
> :         exit block_to_do_stuff when error-condition-1;
> :         keep-going;
> :         exit block_to_do_stuff when error-condition-2;
> :         still-keep-going;
> :         exit block_to_do_stuff when error-condition-3;
> :         everythings-still-fine-keep-going;
> :         etc.
> :     end block_to_do_stuff;
>
> : I think one of the Ada 9X proposals actually had a feature like this, but
> : unfortunately it also changed the semantics of "exit" in a way that would
> : change the meanings of a lot of Ada 83 code.  Probably, that's why it was
> : dropped from the final Ada 95 definition.
>
> The Ada 9x proposal was "upward consistent" in that it didn't change the
> meaning of any existing programs to a new legal meaning, but it did make
> some of them illegal (those that involved exits from blocks nested in loops,
> I believe).

Not just blocks--I think compound statements were involved.  If my
recollection is correct, it would have made the following code illegal:

    for I in Arr'Range loop
        if Arr (I) = Search_Value then
            Found_Index := I;
            exit;      <==== illegal according to 9X draft
        end if;
    end loop;

But I do remember now that this would have been illegal and not legal with a
different meaning.  As I recall, I did a grep through the code of my main
project for "exit;" (which would have always been illegal under this
proposal---the proposed syntax did not allow for "exit" all by itself) and
found it occurred about a hundred times.  I thought this was pretty obnoxious.


> If there were a solution that treated loops and blocks symmetrically,
> that would be nice.  But a proposal that only allows exiting from
> unnamed loops, while disallowing exiting from unnamed blocks, is
> a kludge.  I would prefer something which allowed an unnamed "exit" to be
> legal only if there were exactly one block or loop enclosing the point.

Actually, this would make a lot of sense to me.  I think I tend to do this
anyway when two loops are nested, but I'd have to look through my code to
make sure.  I'd support a proposal like this; the amount of previously legal
code that would be made illegal would be IMHO *much* less than the dropped
9X proposal did.

I also agree that this is a minor problem.  I'm willing to add this case to
the short list of situations where "goto" is appropriate.  Now all I have to
do is go against all my training and allow that 4-letter word into my code.
Easier said than done.  :)

				-- Adam

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