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Subject: Re: why only in-parameters in functions
References: <cjejo4$62e$00$1@news.t-online.com>
 <mailman.131.1096477841.390.comp.lang.ada@ada-france.org>
From: Matthew Heaney <matthewjheaney@earthlink.net>
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Date: Thu, 30 Sep 2004 01:55:08 GMT
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"Alexander E. Kopilovich" <aek@VB1162.spb.edu> writes:

> In practice, where you really need in-parameters for a function you
> may either use a procedure that returns value (if you use GNAT
> compiler) or you may use the construct, presented here in c.l.a by
> Robert I. Eachus (see his message in comp.lang.ada from 2003-07-15
> with Subject: Re: What evil would happen?)
> 
> ----------------------------------------------------------------------------
> 
> function Random(Gen: in Generator) return ...
>    Local: Generator;
>    for Local'Address use Gen'Address;
> begin
>    ...
> end Random;
> 
> ----------------------------------------------------------------------------


Actually, if the type is limited, then you can use the Rosen Trick,
which is somewhat cleaner:

private

   type Handle (G : access Generator) is 
      limited null record;

   type Generator is limited record
      H : Handle (Generator'Access);
      ... -- rest of state
   end record;

end;

   function Random (G : Generator) return ...
      GG : Generator renames G.H.G.all;
   begin
      ... -- modify GG as desired
   end Radom;

The Rosen Trick was discovered in the fall of 1999.  See CLA archives
around that time for more info.