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From: stt@inmet.inmet.com
Newsgroups: comp.lang.ada
Subject: Re: layering with discriminants
Message-ID: <20600087@inmet>
Date: 6 Mar 91 20:12:00 GMT
References: <90348@tut.cis.ohio-state.edu>
Lines: 46
Nf-ID: #R:tut.cis.ohio-state.edu:90348:inmet:20600087:000:1501
Nf-From: inmet.inmet.com!stt    Mar  6 15:12:00 1991


In article <44963@ut-emx.uucp> hasan@emx.utexas.edu (David A. Hasan) writes:
 . . .
>My problem revolves around the fact that the DEC compiler rejects
>the presence of the discriminant in the full declaration of the adt in
>the private part of g_highLevel, since the declaration is not a record.
>But the declaration is *derived* from a record.

>My conclusion is that it is not legal to derive
>types with discriminants from parents with discriminants.  
>If this is true, is there anyway to achieve an equivalent effect?

This is true, and it is admittedly annoying.  One way is
to make the private type into a record containing the parent
as a component.  Some compilers are clever enough to share
the space occupied by the component's discriminants for
the enclosing record's discriminants (since they must always
have the same value).

For example:

    type Blah(X : Integer) is private;

private

    type Blah(X : Integer) is record
        Parent : Parent_Type(X);
    end record;

-S. Tucker Taft
Ada 9X Mapping/Revision Team
Intermetrics, Inc.
Cambridge, MA  02138

P.S. This annoyance has been identified as something which
might be fixable in Ada 9X without too much change, perhaps
by allowing:

    type Blah(X : Integer) is new Parent_Type(X);

in the private part which would make Blah be a derivative of 
Parent_Type, but also allow the declaration to 
fulfill "type Blah(X : Integer) is private;"

It is too early to tell whether such a proposal will survive the
9X review process.