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Subject: Re: Is the Writing on the Wall for Ada?
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From: Matthew Heaney <matthewjheaney@earthlink.net>
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Xref: archiver1.google.com comp.lang.ada:42842
Date: 2003-09-24T03:52:43+00:00
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Hyman Rosen <hyrosen@mail.com> writes:

> Are the functions allowed to modify the state of the object?  I assume
> no, since functions only take IN parameters.

Never assume.  The answer is, "it depends."

Functions can of course have access parameters:

   type T is record ... end record;

   function Modify (O : access T) return Whatever;

This is a function that can modify its parameter O, because it's passed
an access parameter which is basically the same as inout.

If the type is limited, then you can use the trick discovered by that
other Rosen:

   type T;
   type Rosen_Trick (O : access T) is limited null record;

   type T is limited record
      Jean_Pierre : Rosen_Trick (T'Access);
      ...
   end record;

   function Modify (O : in T) return Whatever is
      Modifiable : T renames O.Jean_Pierre.O.all;
   begin
      ... -- modify Modifiable as desired
   end;

In fact the bounded linked-list containers in my AI-302 proposal are
implemented using the Rosen Trick, precisely because I have in-params
that need to change container state.

If you have a non-limited type that is pass-by-reference, then you can
use address-to-access conversions to get a variable view of the in-param
and then modify it that way.  You have to leave the type system very
briefly so be careful.

Functions in Ada are basically value-returning subprograms that can
modify state just like procedures, but aren't allowed to say that they
modify state.  This is one thing Ada got wrong and C++ got right.
Ideology should never trump reality.