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Date: Tue, 30 Jun 2009 00:45:38 +0200
From: Georg Bauhaus <rm.tsoh.plus-bug.bauhaus@maps.futureapps.de>
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Newsgroups: comp.lang.ada
Subject: Re: coroutines (Maping of Ada tasks on the plateforme OS threads
 model)
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Date: 2009-06-30T00:45:37+02:00
List-Id: <comp.lang.ada>

Hibou57 (Yannick Duch�ne) wrote:

> By the ways, I'm looking for the best way to do coroutines with Ada,
> without tasking (none-recursive coroutines, so no separate stack is
> needed, just global storage for local variables). Yes, this can be
> achieved with a select/when statement and a state enumeration, but
> this way is not very clean nor nice.

Not necessarily the best, or even a good way, the
following is an outline. (Yes, there is recursion BTW,
but it isn't crucial.)

procedure News32 is

   Max_Statements: constant := 10;
   type Statement_Index is range 1 .. Max_Statements;

   A_Index,
   B_Index : Statement_Index;

   procedure A;
   procedure B;

   Waiting: Boolean;
   Result: Integer;

   procedure A is
   begin
      case A_Index is
         when 1 => goto STMT1;
         when 2 => goto STMT2;
         when others => raise Program_Error;
      end case;

      <<STMT1>>
      if Waiting then
         B;
         return;
      end if;
      Result := 1;

      <<STMT2>>
      Result := 2;

   end A;

   procedure B is
   begin
      case B_Index is
         when 1 => goto STMT1;
         when others => raise Program_Error;
      end case;

      <<STMT1>>
      A_Index := A_Index + 1;
      A;  -- resumes A at A.STMT2
   end B;

begin
   Result := 0;
   Waiting := True;
   A_Index := 1;
   B_Index := 1;
   A;
   pragma Assert(Result = 2);
end News32;