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From: stt@houdini.camb.inmet.com (Tucker Taft)
Subject: Re: Extending A Generic Signature Package
Date: 1997/03/18
Message-ID: <E78yEu.15.0.-s@inmet.camb.inmet.com>#1/1
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References: <5gkv23INN3rn@thalamus.cis.ohio-state.edu>
Organization: Intermetrics, Inc.
Newsgroups: comp.lang.ada
Date: 1997-03-18T00:00:00+00:00
List-Id: <comp.lang.ada>


david scott gibson (dgibson@thalamus.cis.ohio-state.edu) wrote:

: Hi.  Is any way in Ada to extend a generic signature package?  For
: example, if I had the signature package Base_Interface:

: generic

:     type T is limited private;

:     with procedure Op1(x: in out T);

: package Base_Interface is end;

: I would like to extend it to get a new interface with Op1 inherited.
: The following code is illegal, but gives an idea of what I'd like to
: do.

: with Base_Interface;

: generic

:     type T is new Base_Interface.T;  -- ILLEGAL!!

:     with procedure Op2(x: in out T);

: package Extended_Interface is end;

: I'd be particularly interested if there were some way of doing this
: without using tagged types.  For the language lawyers, what would be
: the technical barriers to allowing an extension of this sort (not
: including there are other ways in Ada to achieve a similar result :-)?

Well, I'm not exactly sure what is your goal, but the following
might satisfy you:

    with Base_Interface;
    generic
        with package Base is new Base_Interface(<>);
        with procedure Op2(x : in out Base.T);
    package Extended_Interface is end;

Given any instantiation of Base_Interface and an appropriate
"Op2", you can create an instantiation of Extended_Interface as follows:

    with Extended_Interface;
    with My_Base;
    with ...appropriate other stuff...
    package Extended is new Extended_Interface(My_Base, My_Op2);

: Dave
: dgibson@cis.ohio-state.edu

-Tucker Taft   stt@inmet.com   http://www.inmet.com/~stt/
Intermetrics, Inc.  Burlington, MA  USA