Re: Idea for Ada 200x: Arguments that are procedures

[swhalen@netcom.com (Steve Whalen)]

Brian Rogoff (bpr@shell5.ba.best.com) wrote:

: I think you would agree with Robert Duff, who floated the limited access
: type for procedure parameters a while ago, in 

: http://sw-eng.falls-church.va.us/AdaIC/standards/95lsn/LSN1083.GeneralAccess

: as I pointed out in my reply to Van Snyder. If you really want to go
: digging, I've appended a post from Norman Cohen on this topic; you can use
: the thread title to trace the entire thread under DejaNews. 

Thanks for the additional background.  I hadn't read the LSN1083 
note you referenced at the time I posted. Interesting history of
this "feature". 

Also interesting is the thread you attached, especially the note that
this is the only thing that keeps Ada95 from being a proper superset
of Pascal! I guess that's one reason that Ada95's lack of downward
closures feels so "wrong" to me.

: I think more than one implementation uses displays, and there is also the
: issue of problems caused in implementations with shared generics. 

: FWIW, I started out with your view (omission of downward closures was an 
: egregious mistake) and ended up agreeing that it was the only right thing
: to do for Ada *95*. However, I have wanted this in code I write and I
: think the workarounds are really clumsy and impair readability. I'd rather 
: that generics were never shared, if the interaction of the two features
: is problematic! 

I agree completely.  Either dump shared generics (because *I* don't
often/ever? need them <g>) or add another piece of programmer control
syntax to "limited" (maybe "limited controlled?") That way the programmer
can "promise" the compiler that the procedure pointer REALLY won't be
abused/confused and permit downward closure of procedure pointers.

The code that is needed to work around this "feature" is just TOO
UGLY!  It seriously detracts from Ada95's "elegance" and a main reason
for Ada's existance (if I wanted unreadable, difficult to maintain
code, I use C++ <g>).

Steve
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                Steve Whalen     swhalen@netcom.com
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