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From: adam@irvine.com
Subject: Mutually dependent private types
Date: 1998/05/21
Message-ID: <6k25ra$6j7$1@nnrp1.dejanews.com>#1/1
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Organization: Deja News - The Leader in Internet Discussion
X-Article-Creation-Date: Thu May 21 21:20:42 1998 GMT
Newsgroups: comp.lang.ada
Date: 1998-05-21T00:00:00+00:00
List-Id: <comp.lang.ada>


I've run into another problem trying to get something implemented in
Ada 95.  This problem seems superficially similar to John Volan's
with-ing problem, but I don't think it's quite the same.

(1) If I want to have two private types in two separate packages whose
    *implementations* are mutually dependent on each other, this is
    easy to do, if we're willing to make the private types accesses:

        package P1 is
            type T1 is private;
        private
            type T1_Info;
            type T1 is access T1_Info;
        end P1;

        package P2 is
            type T2 is private;
        private
            type T2_Info;
            type T2 is access T2_Info;
        end P2;

    and this will work since it's legal for P1's package body to
    depend on P2 and P2's package body to depend on P1.  The
    definitions of P1.T1_Info and P2.T2_Info would contain references
    to the other package's type.

(2) Another mutual-dependency situation is this: Suppose I want to
    have two types T1 and T2 in two different packages, and I want to
    provide operations on T1 that take T2 as a parameter, and
    operations on T2 that take T1 as a parameter.  You can't do this
    by declaring T1 and T1's operations in the same package and do the
    same for T2, since then the two package specs would have to be
    mutually dependent.  However, you can solve this problem by
    separating the type definition from the operations in two separate
    packages (preferably using child units):

        package P1 is
            type T1 is ...
        end P1;

        package P2 is
            type T2 is ...
        end P2;

        with P2;
        package P1.Operations is
            procedure Some_Operation (X : T1;  Y : P2.T2);
        end P1.Operations;

        with P1;
        package P2.Operations is
            procedure Some_Operation (X : T2;  Y : P1.T1);
        end P2.Operations;

    Actually, you don't have to do this for both packages; you can
    solve the problem by splitting the operations in just one of the
    packages.

(3) But what happens if you want to do both (1) and (2)?  Now the
    whole thing seems to break down.  If you try to do both of the
    above, something like:

        package P1 is
            type T1 is private;
        private
            type T1_Info;
            type T1 is access T1_Info;
        end P1;

        with P2;
        package P1.Operations is
            procedure Some_Operation (X : T1;  Y : P2.T2);
        end P1.Operations;

    you can't, because the body of Some_Operation cannot do anything
    useful with X, since the definition of T1_Info is not visible to
    it.

I've been trying to come up with some way around this problem; for
example, I thought about putting some routines in the private part of
P1 that P1.Operations could access to give it information about
T1_Info, or by setting up another private child package to declare
actual type info, but I couldn't quite get anything to work.  Does
anyone know of a good way to accomplish this, without using
Unchecked_Conversion?  (It's pretty easy to come up with a solution
using Unchecked_Conversion.)  I might be missing something obvious
here due to my incomplete knowledge of Ada 95.

P.S. I believe this would also be solved by Tucker Taft's "with type"
proposed language extension.

                                -- thanks, Adam


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