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From: jonas.nygren@telia.com (jn)
Newsgroups: comp.lang.ada
Subject: An improved Ada?
Date: 27 Sep 2004 12:38:51 -0700
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Date: 2004-09-27T12:38:51-07:00
List-Id: <comp.lang.ada>

Hi,

after spending numerous hours to do a simple double linked list
package - yes, I have seen charles - a 5 hours tedious car drive came
up with the following example of an improved Ada language:
  - prefix notation (as for tasks and protected types)
  - Ada OO syntax in line with tasks and protected objects
  - reference types 
  - controlled, allocates from Storage_Pools.Garbage_Collected :)
  - implicit up- and down-conversions

Here is my small example, based on the simset-library from Simula, not
complete but it should be easy for you to fill out the rest:)

package Simset is
   tagged type List_Type is private
   begin
      function First return Link_Type;
   end List_Type;

   tagged type Link_Type is private
   begin
      function Next return Link_Type;
   end Link_Type;
private
   tagged type Linkage_Type is
      Suc, Pred : Linkage;
   begin
      function Next return Linkage_Type;
      procedure Into (List : List_Type);
      procedure Initialize;
   end Linkage_Type;

   tagged type List_Type is new Linkage_Type with
   begin
      null;
   end List_Type;

   tagged type Link_Type is new Linkage_Type with
   begin
      null;
   end Link_Type;
end Simset;

package body Simset is
   tagged body Linkage_Type is
      function Next return Linkage_Type is
      begin
	if not Suc in List_Type then
           return Suc;
        else
           return null;
      end Suc;

      procedure Initialize is
      begin
         Suc  := This;
         Pred := This;
      end Initialize;
   end List_Type; 
 
   tagged body List_Type is
      function First return Link_Type is
      begin
	return Next;
      end First;
   end List_Type;
  
   tagged body Link_Type is
      function Next return Link_Type is
      begin
	return Linkage_Type(This).Next;
      end Suc;

      procedure Into (List : List_Type) is
      begin
          -- Unlink; -- remove from whichever list This is in
          -- manipulate pointers to put This last into List
      end Into;
   end List_Type;
end Simset;

-- Simset example usage

with Simset and use;
with Book_Intrinsics rename as BI;

procedure Example is
   tagged Book_Shelf is new List_Type;
   tagged Shopping_Cart is new List_Type;

   tagged Book_In_Shelf is new Link_Type with
      �uthor : BI.Author_Type;
      Title  : BI.Title_Type;
   end Book_In_Shelf;

   function Contains_Ada_In_Title (Title : BI.Title_Type) return
Boolean is
   begin ... end; 
   -- not part of example
   
   Shelf      : Book_Shelf := new Book_Shelf;
   Book, Buy  : Book_In_Shelf;
   Cart       : Shopping_Cart := new Shopping_Cart;
begin
   -- populate Shelf with books, not part of example

   Book := Shelf.First,
   while Book /= null loop
      if Ada_In_Title (Book.Title) then
         Buy := Book;
         Book := Book.Next
         Buy.Into (Cart);
      else
         Book := Book.Next;
      end if;
   end loop;
   -- pay for books in cart
end Example;