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From: collberg@dna.lth.se (Christian S. Collberg)
Newsgroups: comp.lang.ada
Subject: Readonly variables in Ada?
Message-ID: <1990Jun5.085654.920@lth.se>
Date: 5 Jun 90 08:56:54 GMT
Sender: newsuser@lth.se (LTH network news server)
Reply-To: collberg@DNA.LTH.Se (Christian S. Collberg)
Organization: Department of Computer Science, Lund University
List-Id: <comp.lang.ada>

Is it possible to declare (the equivalent of) an exported readonly
variable in Ada? I'm referring to something similar to the READ_ONLY
attribute present in Mesa. I'm having a hard time reading the 
appropriate sections in the LRM, and I don't have a compiler at hand
to try it out, so maybe someone out there could enlighten me...

Maybe something along these lines might work (excuse the syntax, I'm not an
Ada programmer):

 ---------------------------
   package P;
       type T is limited private;
       v : T;            -- We want a client of P to be able to
                         -- perform any operation on v, except
                         -- assignment.
   private
      type T is BOOLEAN;
   end P;
 ---------------------------

 ---------------------------
   with P;
   declare
      x : BOOLEAN;
   begin 
      P.v := false;       -- Obviously illegal, as it should be 
                          -- for a read only variable.

      if P.v = true then  -- Doesn't seem like this would be 
          ...             -- legal since equality is undefined
                          -- for limited private types.
          

      if P.v then         -- legal ?
         ...

      if P.v OR x then    -- legal ?
         ...

   end;
---------------------------


Would one, in order to achieve the desired effect, have to provide,
in the interface of P, overloaded operators for the type T (=, OR, AND,
etc for this example)?

Thanks in advance for any help you may offer in clearing this up for me.

Chris Collberg
collberg@dna.lth.se