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From: stefan-lucks@see-the.signature
Newsgroups: comp.lang.ada
Subject: Re: Operation can be dispatching in only one type
Date: Tue, 1 Dec 2009 06:45:16 +0100
Organization: Bauhaus-Universitaet Weimar
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Date: 2009-12-01T06:45:16+01:00
List-Id: <comp.lang.ada>

On Tue, 1 Dec 2009, Dmitry A. Kazakov wrote:

> On Mon, 30 Nov 2009 15:43:21 -0500, Robert A Duff wrote:

> > Right.  I think they should be equivalent.  My solution is to use
> > two different symbols for (initial) assignment and (subsequent)
> > reassignment.
> 
> But they cannot be, otherwise the semantics of ":=" would depend on the
> things done before:
> 
>     X : T;
> begin
>     X := F (Y); -- Initialization

No, at this point of time, X has been initialised to *some* value, even if 
the value itself is undefined. So this is just a proper assignment. 

>     X : T;
> begin
>     X := Z;
>     X := F (Y); -- [Re]assignment

That is a proper assignment, as well. The only difference is that we 
can be sure the before-assignment value of X is defined (assuming the 
value of Z is a defined one).  

An initialisation would be

     X : T ::= F(Y);
  begin
     ...

But you are right, Dmitry, nobody would not want to distinguish an
assignment to overwrite a potentially undefined value from an assignment 
ovwerwriting a previously defined value.

-- 
------ Stefan Lucks   --  Bauhaus-University Weimar  --   Germany  ------
               Stefan dot Lucks at uni minus weimar dot de
------  I  love  the  taste  of  Cryptanalysis  in  the  morning!  ------