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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Question on type invariants
Date: Fri, 27 Jan 2017 10:47:49 +0100
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Date: 2017-01-27T10:47:49+01:00
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On 27/01/2017 10:20, reinkor wrote:
> On Thursday, January 26, 2017 at 9:38:21 AM UTC+1, Dmitry A. Kazakov wrote:
>
>> What you have described looks like a lazy evaluation scheme.
>
> Exactly.
>
> But no way to express this intention in a somehow explicite way in Ada?
> The point is to produce updates only when you need it (i.e. when you read
> from (use) S and A has changed since last update (of S).

private
    type Lazy_Object is ... record
       Self : not null access Lazy_Object :=
              Lazy_Object'Unchecked_Access; -- Rosen's trick
       Cached : Value_Type; -- Last evaluated value
       Queue  : FIFO_Type;  -- Queue of update requests
    end record;

procedure Update (Value : in out Lazy_Object; Change : Request_Type) is
begin
    if Value.Queue.Is_Full then
       Value.Flush; -- Make space
    end if;
    Value.Queue.Put (Change);
end Update;

function Get (Value : Lazy_Object) return Value is
begin
    if not Value.Queue.Is_Empty then
       Value.Self.Flush; -- Collapse the queue
    end if;
    return Value.Cached;
end Get;

procedure Flush (Value : in out Lazy_Object) is
begin
    while not Value.Queue.Is_Empty loop
       Value.Cached := Execute (Value.Queue.Top, Value.Cached);
       Value.Queue.Delete;
    end loop;
end Flush;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de