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From: Matthew Heaney <matthew_heaney@acm.org>
Subject: Re: Can't export object of private type
Date: 1999/03/02
Message-ID: <m3n21w7cnb.fsf@mheaney.ni.net>#1/1
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References: <7b8cvc$ii5$3@plug.news.pipex.net> <F7xuGK.K6n@syd.csa.com.au>
NNTP-Posting-Date: Mon, 01 Mar 1999 20:09:25 PDT
Newsgroups: comp.lang.ada
Date: 1999-03-02T00:00:00+00:00
List-Id: <comp.lang.ada>

nospam@thanks.com.au (Don Harrison) writes:

> There is always a semantic gap between design and implementation in a
> particular language. Better languages, IMO, are those where that gap
> is uniformly narrow. For Ada, I think the gap is quite wide in places
> (eg. polymorphic singletons) and narrow in others (eg. low-level
> mapping).

I disagree about the polymorphic singletons.  The spec looks like this:


package Singletons is

  type Root_Singleton_Type (<>) is abstract tagged limited null record;

  procedure Do_Something
   (Singleton : access Root_Singleton_Type) is abstract;

  <other primitive operations>


  type Singleton_Access is access all Root_Singleton_Type'Class;

  function Singleton return Singleton_Access;

end;


I wouldn't describe the semantic gap as "quite wide."  The problem is
that few Ada programmers have learned the limited-and-indefinite
idiom.  

So this idiom just needs to be publicized.  You can find examples of
this idiom and others in the ACM patterns archive.

<http://www.acm.org/archives/patterns.html>