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From: mheaney@on2.com (Matthew Heaney)
Newsgroups: comp.lang.ada
Subject: Re: "access constant" discriminant
Date: 24 Feb 2003 08:03:32 -0800
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Lutz Donnerhacke <lutz@iks-jena.de> wrote in message news:<slrnb5c0ih.nq.lutz@taranis.iks-jena.de>...
> * Matthew Heaney wrote:
> > Access discriminants are very powerful.  No serious Ada95 program can
> > be written without them.
> 
> If there is a serious program without access discriminants, let's call it a
> Spark program, or trivial, ok?

What I meant was that access discriminants are the sine qua non of
Ada95 programming.  If you're not using them, you're losing something
essential, not accidental (per se vs. per accidens), about an Ada95
program.

Of course, this idiom is just a reification of a more general design
pattern called the "Adapter Pattern," or more specifically an "Object
Adapter."

It shows up in myriad languages.  You often see C++ written like this:

   X x;
   ...
   Y y(&x);

where the object y is bound to object x.  Class Y would be implemented
as

class Y
{
   X* const p
public:
   Y(X* pp) : p(pp) {}
   //...
private:
   Y& operator=(const Y&);
   Y(const Y&);
};

This shows up even in some Ada95 iterator implementations:

   Iter : Iterator_Type (Container'Access);

Here, you're viewing a container object through an iterator object.  

Of course, the effect of an object adapter can probably be implemented
sans access discriminants (persumably that is the case in SPARK), but
I don't consider these alternate locutions as elegant as an access
discriminant.

In your case you lose access discriminants in order to gain provable
correctness -- which is of course the correct tradeoff.