ANNOUNCE: data structures with limited items pattern

[Matthew Heaney <matthew_heaney@acm.org>]

I have submitted an article to the ACM patterns archive, on how to
implement a collection of items in which the item-type is limited.  A
description appears below.

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

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Data Structures Containing Limited Items

In this article I show how to implement a stack whose items are limited.

Most data structures you see are implemented this way:

generic
  type Item_Type is private;
package Stacks is

  type Stack_Type is limited private;

  procedure Push (Item : in     Item_Type;
                  On   : in out Stack_Type);
...
end Stacks;

The generic formal type Item_Type is non-limited, so it comes with
assignment.  This allows you to implement operation Push (for a bounded
stack) like this:

  procedure Push (Item : in     Item_Type;
                  On   : in out Stack_Type) is
  begin
     On.Top := On.Top + 1;
     On.Items (On.Top) := Item;
  end;

But suppose you need a stack of, say, files?  Type File_Type, in
Text_IO, is limited:

  type File_Type is limited private;

so you wouldn't be able to instantiate the stack package above.  But
even if you could, it's not clear how you'd implement Push, which
requires assignment.