Package Instances???

Package Instances???

[Yates]

I am new to Ada. I'd like to know if it is possible to create multiple
instances of a package. For example, if I have a package 'My_Package',  can
I do something like:

p1 : My_Package := new My_Package
p2 : My_Package := new My_Package
.....

I tried this and didn't work. Is there other ways to do it?

Thanks

Yates

Re: Package Instances???

[Larry Kilgallen]

In article <721b6d80.0202131302.1ccce2ed@posting.google.com>, yates_1900@yahoo.com (Yates) writes:
> I am new to Ada. I'd like to know if it is possible to create multiple
> instances of a package. For example, if I have a package 'My_Package',  can
> I do something like:
> 
> p1 : My_Package := new My_Package
> p2 : My_Package := new My_Package
> .....
> 
> I tried this and didn't work. Is there other ways to do it?

You just said that.

Re: Package Instances???

[chris.danx]

"Yates" <yates_1900@yahoo.com> wrote in message
news:721b6d80.0202131302.1ccce2ed@posting.google.com...
> I am new to Ada. I'd like to know if it is possible to create multiple
> instances of a package. For example, if I have a package 'My_Package',
can
> I do something like:
>
> p1 : My_Package := new My_Package
> p2 : My_Package := new My_Package
> .....
>
> I tried this and didn't work. Is there other ways to do it?

Why would you need to for ordinary work?  All the public types and
subroutines will be available to you when you with it the first time, so you
only need to 'with' it in a package once to use it in that package.  In the
case of generics you can instantiate multiple packages for different types
(*maybe* even the same ones over again), but if your just starting you
probably don't need generics.

Re: Package Instances???

[Marin David Condic]

Sounds like you're trying to treat a package as if it was a C++ class and
you're trying to create a new instance of it because it presumably holds a
set of variables you want to duplicate? First, as you discovered, you can't
do that. Second, it isn't really the right idiom for Ada.

The way to do it would be to have a record type declared in the package that
holds all your variables. Then you create objects of the record type & use
the operations contained in the package on the individual objects of that
type. Something like this:

package My_Package is
    type My_Rec is record
        A    : Integer ;
        B    : Integer ;
        C    : Integer ;
    end record ;
    function Some_Operation (Rec : My_Rec) return Integer ;
end My_Package ;

--  Then in the unit that uses it...

X : My_Package.My_Rec ;  --  One instance of the A, B, C variables
Y : My_Package.My_Rec ;  --  A whole different instance of them.
.......
--  Operates on X.A, X.B, X.C
Some_Int := My_Package.Some_Operation (X) ;
.......
--  Operates on Y.A, Y.B, Y.C
Another_Int := My_Package.Some_Operation (Y) ;
.......


If that's what you were trying to accomplish, I hope this clarifies. If it
was something else, see if you can explain what you are trying to do and
maybe we can help you find The Ada Way of doing it.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Yates" <yates_1900@yahoo.com> wrote in message
news:721b6d80.0202131302.1ccce2ed@posting.google.com...
> I am new to Ada. I'd like to know if it is possible to create multiple
> instances of a package. For example, if I have a package 'My_Package',
can
> I do something like:
>
> p1 : My_Package := new My_Package
> p2 : My_Package := new My_Package
> .....
>
> I tried this and didn't work. Is there other ways to do it?
>
> Thanks
>
> Yates

Re: Package Instances???

[Eric Merritt]

Yates,
 
  You may actually be getting packages mixed up with
classes as used in other languages. Packages are not
so much a coherent whole as a collection of types and
variables that you may use in your client program. So
although you would not do

p1 : My_Package := new My_Package;

you would do something like

p1: My_Package.My_Type : Create_New_My_Type;

(If there was a function named Create_New_My_Type in
the package). 

I would suggest you hit some of the excellent
tutorials available to get you over this hump.

--- Yates <yates_1900@yahoo.com> wrote:
> I am new to Ada. I'd like to know if it is possible
> to create multiple
> instances of a package. For example, if I have a
> package 'My_Package',  can
> I do something like:
> 
> p1 : My_Package := new My_Package
> p2 : My_Package := new My_Package
> .....
> 
> I tried this and didn't work. Is there other ways to
> do it?
> 
> Thanks
> 
> Yates
> _______________________________________________
> comp.lang.ada mailing list
> comp.lang.ada@ada.eu.org
> http://ada.eu.org/mailman/listinfo/comp.lang.ada


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Re: Package Instances???

[Yates]

Thanks to all of you!!!

Yates

"Yates" <yates_1900@yahoo.com> wrote in message
news:721b6d80.0202131302.1ccce2ed@posting.google.com...
> I am new to Ada. I'd like to know if it is possible to create multiple
> instances of a package. For example, if I have a package 'My_Package',
can
> I do something like:
>
> p1 : My_Package := new My_Package
> p2 : My_Package := new My_Package
> .....
>
> I tried this and didn't work. Is there other ways to do it?
>
> Thanks
>
> Yates

Re: Package Instances???

[Hyman Rosen]

Marin David Condic wrote:
> Sounds like you're trying to treat a package as if it was a C++ class and
> you're trying to create a new instance of it because it presumably holds a
> set of variables you want to duplicate? First, as you discovered, you can't
> do that. Second, it isn't really the right idiom for Ada.

Wouldn't it work by just making the package generic?

Re: Package Instances???

[Marc A. Criley]

Hyman Rosen wrote:
> 
> Marin David Condic wrote:
> > Sounds like you're trying to treat a package as if it was a C++ class and
> > you're trying to create a new instance of it because it presumably holds a
> > set of variables you want to duplicate? First, as you discovered, you can't
> > do that. Second, it isn't really the right idiom for Ada.
> 
> Wouldn't it work by just making the package generic?

I got a clear impression from the original posting that he was thinking
that packages were very much like classes, which is what his example
strongly suggested.  So I think the postings about generics were
answering a question that wasn't being asked :-)

Marc A. Criley
Consultant
Quadrus Corporation
www.quadruscorp.com

Re: Package Instances???

[Simon Wright]

Hyman Rosen <hyrosen@mail.com> writes:

> Marin David Condic wrote:
> > Sounds like you're trying to treat a package as if it was a C++ class and
> > you're trying to create a new instance of it because it presumably holds a
> > set of variables you want to duplicate? First, as you discovered, you can't
> > do that. Second, it isn't really the right idiom for Ada.
> 
> Wouldn't it work by just making the package generic?

But different instantiations would be different "classes". I guess
that depends on what the generic actually says.

Re: Package Instances???

[Marin David Condic]

In the simplest case where the generic package had no generic parameters and
only a collection of data objects & subprograms, it would look very similar
to a C++ "class" in many respects. The important part being that it would
only *look* like a C++ class. Semantically the whole analogy starts to fall
apart once you start picking at the loose threads.

Obviously, as you observe, once you start changing what the generic actually
says, the distinction between a generic package and a "class" starts
becoming more apparent. In general, I'd think it would be wise not to try to
use a generic package to represent a "class" in the way it appears to have
been intended. Its just the wrong idiom in Ada to do what a C++ class does.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Simon Wright" <simon@pushface.org> wrote in message
news:x7v8z9u7dsk.fsf@smaug.pushface.org...
>
> But different instantiations would be different "classes". I guess
> that depends on what the generic actually says.

Re: Package Instances???

[Matthew Heaney]

"Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org> wrote in
message news:a4tm89$ajg$1@nh.pace.co.uk...
> Obviously, as you observe, once you start changing what the generic
actually
> says, the distinction between a generic package and a "class" starts
> becoming more apparent. In general, I'd think it would be wise not to try
to
> use a generic package to represent a "class" in the way it appears to have
> been intended. Its just the wrong idiom in Ada to do what a C++ class
does.

Yes.  An Ada package is analogous to a C++ namespace, not a class.

Actually, there's a straight-forward conversion from C++ to Ada:

namespace N
{
   class C
   {
   public:
      //...
   private:
      //...
   };
} //end namespace N

package P is
   type T is private;
   --...
private
   type T is record ...;
   --...
end P;

Re: Package Instances???

[Marin David Condic]

Excellent illustration. Lat time I checked though, there were several C++
compilers not supporting namespaces though - notably MSVC++, IIRC. Not
involved with it at the moment, so maybe its changed. Are more C++ compilers
supporting it? (It sure does overcome some of the big limitations I found
with C++...)

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Matthew Heaney" <mheaney@on2.com> wrote in message
news:u75v5l119fc64c@corp.supernews.com...
>
> Yes.  An Ada package is analogous to a C++ namespace, not a class.
>
> Actually, there's a straight-forward conversion from C++ to Ada:
>
> namespace N
> {
>    class C
>    {
>    public:
>       //...
>    private:
>       //...
>    };
> } //end namespace N
>
> package P is
>    type T is private;
>    --...
> private
>    type T is record ...;
>    --...
> end P;
>

Re: Package Instances???

[Hyman Rosen]

Matthew Heaney wrote:
> Yes.  An Ada package is analogous to a C++ namespace, not a class.

Except that in Ada, a package can be used as a parameter for a
generic instantiation, but in C++ a namespace cannot be used as
a template argument. Given that C++ classes can have arbitrary
things defined in them (types, static functions, constants, etc.)
considering them as an analogue of Ada packages is not wrong.

Re: Package Instances???

[Pat Rogers]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C73F5F6.7050704@mail.com...
> Matthew Heaney wrote:
> > Yes.  An Ada package is analogous to a C++ namespace, not a class.
>
> Except that in Ada, a package can be used as a parameter for a
> generic instantiation, but in C++ a namespace cannot be used as
> a template argument. Given that C++ classes can have arbitrary
> things defined in them (types, static functions, constants, etc.)
> considering them as an analogue of Ada packages is not wrong.

Yes, I agree with that sentiment, but I don't agree that one can make the
analogies in isolation from other language elements.  I would express it thusly:
a C++ class declared is analogous to a tagged type declared within a package
visible part.  The package, the type, and the corresponding primitive operations
work together to form an abstraction.  This atomicity is illustrated, for
example, by the fact that subprograms are not primitive operations of the type
unless they are declared in the same package as the tagged type declaration
itself (in addition to the other requirement, namely, that they mention the type
in their parameter and (for functions) result type profile).  Subprograms that
meet the second requirement but not the first are not primitive operations of
the type and thus are not inherited and are not dynamically dispatched.

A package not exporting a tagged type with primitive operations could correspond
to a C++ namespace.  What's missing is that, like Java's packages, C++
namespaces can be extended dynamically -- they can be "re-opened".  An Ada
package is a module, with a definite end.  But then there are subsystems...

You might be interested in the Ada generated from UML by a product called "UML
Studio" (www.pragsoft.com).  It uses the specified namespace of the UML class,
if present, as the package name, and the UML class name for the tagged type name
within that package.  If the namespace is not specified, it uses the class name
as the package name and "Object" for the tagged type name.

---
Patrick Rogers                       Consulting and Training in:
http://www.classwide.com          Real-Time/OO Languages
progers@classwide.com               Hard Deadline Schedulability Analysis
(281)648-3165                                 Software Fault Tolerance

Re: Package Instances???

[Matthew Heaney]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C73F5F6.7050704@mail.com...
> Given that C++ classes can have arbitrary
> things defined in them (types, static functions, constants, etc.)
> considering them as an analogue of Ada packages is not wrong.

But it is wrong if your purpose is to implement a type, so that you can
declare objects, which you can pass to subprograms, etc.  The fact that in
C++ you can declare things inside the class is irrelevent.

Re: Package Instances???

[Hyman Rosen]

Matthew Heaney wrote:
> "Hyman Rosen" <hyrosen@mail.com> wrote
> 
>>Given that C++ classes can have arbitrary
>>things defined in them (types, static functions, constants, etc.)
>>considering them as an analogue of Ada packages is not wrong.
> 
> But it is wrong if your purpose is to implement a type, so that you can
> declare objects, which you can pass to subprograms, etc.  The fact that in
> C++ you can declare things inside the class is irrelevent.

But my purpose isn't to implement a type, it's to find a C++ analogue
for Ada packages. C++ namespaces are useless for anything except what
the name implies, namely establishing a namespace.

Re: Package Instances???

[Marin David Condic]

But why fight it? When programming in C++, the fundamental idiom is the
class - maybe with a namespace around it to isolate the identifiers &
produce better encapsulation. When programming in Ada, the fundamental idiom
is the package - which gives you encapsulation and lots of other cool
stuff - but isn't an "object" or a "type" so it can't really do what a C++
class does. It needs to contain a tagged type to complete the picture.

I don't think that C++ really has an analog for what Ada does with a
package - at least nothing that provides anything resembling a mirror image.
It has things that will give you some of the features of a package - but
because it carries so much legacy from C the whole concept of what Ada does
with a package is rather foreign and it would be difficult to add something
to C++ that would create a "package" and still maintaining upward
compatibility.

I think Ada's packages are a big advantage and a good reason to do things in
Ada instead of C++ (or C, for that matter). The encapsulation, separation of
interface and implementation and information hiding alone is worth it. You
can get something similar out of C++, sort of, if you work at it, but it
always seemed to me to be something you had to cobble together out of spare
parts in a junk drawer rather than something that was inherently engineered
into the language.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C7487D6.5060603@mail.com...
>
> But my purpose isn't to implement a type, it's to find a C++ analogue
> for Ada packages. C++ namespaces are useless for anything except what
> the name implies, namely establishing a namespace.
>

Re: Package Instances???

[Matthew Heaney]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C7487D6.5060603@mail.com...
> But my purpose isn't to implement a type, it's to find a C++ analogue
> for Ada packages. C++ namespaces are useless for anything except what
> the name implies, namely establishing a namespace.

Well, if you don't need a type, then you really do want an Ada package.
Clearly the C++ analogue is a namespace.

package P is
  procedure Op;
  I : Integer;
end P;

namespace N
{
   void f();
   extern int i;
}

In Ada, you have P.Op and P.I, and in C++ you have N::f() and N::i.  What's
the problem?

(Note that in C++ you could use a class, and declare f() and i as static
members, but you said you didn't want a type.)

Re: Package Instances???

[Matthew Heaney]

"Marin David Condic" <dont.bother.mcondic.auntie.spam@[acm.org> wrote in
message news:a50kfe$n39$1@nh.pace.co.uk...
> Excellent illustration. Lat time I checked though, there were several C++
> compilers not supporting namespaces though - notably MSVC++, IIRC. Not
> involved with it at the moment, so maybe its changed. Are more C++
compilers
> supporting it? (It sure does overcome some of the big limitations I found
> with C++...)

Visual C++ v6 works fine, as does g++ v2.96.

Re: Package Instances???

[Hyman Rosen]

Matthew Heaney wrote:
> In Ada, you have P.Op and P.I, and in C++ you have N::f() and N::i.  What's
> the problem?

In Ada, P can be passed as a package parameter to an instantiation
of a generic. P can be made generic and then there could be multiple
instantiations of it. P can have a body which does some initialization.
No equivalent is possible in C++ if N is a namespace, only if N is a
class or a class template.

Re: Package Instances???

[Hyman Rosen]

Depending on how one builds it, a C++ class or class template
can fulfill the role of an Ada package, record, or tagged type.
That's just the way it is.

To examine equivalent capabilities in the two languages, you
*must* use a C++ class to emulate an Ada package, otherwise
there will be all sorts of things that you can do in Ada that
won't be possible in C++.

It is simplistic and wrong to argue that a C++ class is just
like an Ada tagged type. That just is not how classes are used
in modern C++. It may appear to be cobbled together because of
the C heritage, and because a lot of the idioms were discovered
late in the game, but the non-tagged-type usages are at least
as important and useful as the OO stuff.

Re: Package Instances???

[Marin David Condic]

O.K. But please take a moment to recall that I never said that a C++ class
was "just like" an Ada tagged type. I've been stating consistently that you
can't presume that a feature of Language X is going to have an exact
parallel in Language Y. I wouldn't make the claim that a C++ class and an
Ada tagged type are plug compatible. They both have their own semantics that
aren't going to mirror-image each other and attempting to force them to be
the same is a mistake.

What I *am* claiming is that if you want to do something similar to a C++
class in Ada, that attempting to do it with a package is going to lead to
disappointing results. Using a package in conjunction with a tagged type is
the correct idiom in Ada for representing a "class" (general concept - not
C++ class).

When comparing C++ and Ada, it really is important to remember that you can
get the same end results from either language, but you can't necessarily get
those results in an identical manner.

MDC
--
Marin David Condic
Senior Software Engineer
Pace Micro Technology Americas    www.pacemicro.com
Enabling the digital revolution
e-Mail:    marin.condic@pacemicro.com
Web:      http://www.mcondic.com/


"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C7676AF.9020404@mail.com...
>
> It is simplistic and wrong to argue that a C++ class is just
> like an Ada tagged type. That just is not how classes are used
> in modern C++. It may appear to be cobbled together because of
> the C heritage, and because a lot of the idioms were discovered
> late in the game, but the non-tagged-type usages are at least
> as important and useful as the OO stuff.
>

Re: Package Instances???

[Matthew Heaney]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C7676AF.9020404@mail.com...
> Depending on how one builds it, a C++ class or class template
> can fulfill the role of an Ada package, record, or tagged type.

We all agree it's not a perfect comparison, since a class in C++ has type
semantics and module semantics, and in Ada they are orthogonal.

> That's just the way it is.

This is not a description of ontic reality, it is a description of your
subjective experience.  I suggest you study instrumentalism (Dewey) or
pragmatic philosophy (James, Peirce).  See also Ernst von Glasersfeld and
Heinz von Foerster.  Indeed, any book on general systems theory.


> To examine equivalent capabilities in the two languages, you
> *must* use a C++ class to emulate an Ada package, otherwise
> there will be all sorts of things that you can do in Ada that
> won't be possible in C++.

Wrong.  An Ada package is a C++ namespace.  An Ada private type is a C++
class.  The fact that genericity is attached to modules in Ada rather than
to the type itself is irrelevant.

> It is simplistic and wrong to argue that a C++ class is just
> like an Ada tagged type.

Well, if a C++ class has virtual methods, then it has a vtable, which
exactly corresponds to an Ada tagged type.  A C++ class sans virtual methods
has no vtable, and hence is equivalent to a nontagged Ada type.

A C++ class is an Ada type.  The fact that a C++ class has module semantics
too is irrelevant.

> That just is not how classes are used
> in modern C++. It may appear to be cobbled together because of
> the C heritage, and because a lot of the idioms were discovered
> late in the game, but the non-tagged-type usages are at least
> as important and useful as the OO stuff.

On that we most certainly agree.  The Ada community has been saying this
from the very beginning, but certain pedantic computer scientists (and lots
of obsequious programmers who followed the party line) weren't convinced.
Thankfully Stroustrup has tempered this unbridled enthusiasm for all things
"pure object-oriented" by demonstrating the benefits of non-extensible types
and static method binding.

Re: Package Instances???

[Hyman Rosen]

Matthew Heaney wrote:
> "Hyman Rosen" <hyrosen@mail.com> wrote
>>Depending on how one builds it, a C++ class or class template
>>can fulfill the role of an Ada package, record, or tagged type.
> 
> We all agree it's not a perfect comparison, since a class in C++ has type
> semantics and module semantics, and in Ada they are orthogonal.

Of course it's not a perfect comparison! We are talking about
two different and largely independent programming languages.


> Wrong.  An Ada package is a C++ namespace.  An Ada private type is a C++
> class.  The fact that genericity is attached to modules in Ada rather than
> to the type itself is irrelevant.

An Ada package is not a C++ namespace, since it's written
in Ada, not in C++. The issue we're dealing with is how to
best represent the construct of one programming language in
another. When one construct has many aspects to it, it is
likely that such cross-language representation will depend
on which aspect is pre-eminent.

In Ada, a package may be passed as a parameter to a generic
instantiation. A package body may have a begin-end section
which runs when the package is elaborated. You mat have a
generic package. The proper way to represent these abilities
in C++ is as a class or class template, not a namespace. You
may have some Ada packages which don't make use of any of
these features, and those can be represented as namespaces,
but that is not the case in general.


> Well, if a C++ class has virtual methods, then it has a vtable, which
> exactly corresponds to an Ada tagged type.  A C++ class sans virtual methods
> has no vtable, and hence is equivalent to a nontagged Ada type.

An Ada tagged type is best represented as a C++ class with virtual
methods. An Ada untagged record type is best represented as a C++
class without virtual methods. Notice that the direction here in
both cases is how to represent an Ada construct in C++. Your second
statement is completely false, however, because a C++ class may
contain type definitions, constant definitions, and non-member
functions, none of which are present in an untagged Ada type, but
which *are* present in an Ada package.

> A C++ class is an Ada type.  The fact that a C++ class has module
 > semantics too is irrelevant.

You may assert that all you like, but that will not make it true.
Here's a C++ class: class c { typedef int t; enum { v = 3 }; };
What is the corresponding Ada type?

> Thankfully Stroustrup has tempered this unbridled enthusiasm for all things
> "pure object-oriented" by demonstrating the benefits of non-extensible types
> and static method binding.

Actually, all classes in C++ are extensible, but whatever.
Anyway, the exciting C++ stuff these days is in template
metaprogramming, which is pretty much inexpressible in Ada.

Re: Package Instances???

[Matthew Heaney]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C76B04C.7050108@mail.com...
> Here's a C++ class: class c { typedef int t; enum { v = 3 }; };
> What is the corresponding Ada type?

package P is
   type C is private;
private
   type C is null record;
   subtype T is Integer;
   V : constant := 3;
end P;

Re: Package Instances???

[Hyman Rosen]

Matthew Heaney wrote:
> "Hyman Rosen" <hyrosen@mail.com> wrote in message
> news:3C76B04C.7050108@mail.com...
> 
>>Here's a C++ class: class c { typedef int t; enum { v = 3 }; };
>>What is the corresponding Ada type?
>>
> 
> package P is
>    type C is private;
> private
>    type C is null record;
>    subtype T is Integer;
>    V : constant := 3;
> end P;

Thus illustrating my point about the correspondence between
Ada packages and C++ classes!

Re: Package Instances???

[Hyman Rosen]

Marin David Condic wrote:
> What I *am* claiming is that if you want to do something similar to a C++
> class in Ada, that attempting to do it with a package is going to lead to
> disappointing results. Using a package in conjunction with a tagged type is
> the correct idiom in Ada for representing a "class" (general concept - not
> C++ class).

Yes, that is exactly what I believe! I think it's Matthew Heaney
who is arguing otherwise.

Re: Package Instances???

[Matthew Heaney]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C7898A4.2080102@mail.com...
> Matthew Heaney wrote:
> Thus illustrating my point about the correspondence between
> Ada packages and C++ classes!

Well, if that's how you'd translate it too, then it sounds like we're in
violent agreement...

Re: Package Instances???

[Matthew Heaney]

"Hyman Rosen" <hyrosen@mail.com> wrote in message
news:3C78998A.10006@mail.com...
> Yes, that is exactly what I believe! I think it's Matthew Heaney
> who is arguing otherwise.

Well, if you say "implement it as a type in Ada," that also implies the use
of a package, because you can't declare library-level types, only packages.
Therefore I didn't think it was necessary to call out the use of a package
explicitly.

But if only say "implement it as a package in Ada," then that is ambiguous,
because someone (like me) will think you mean to use only a package, and not
an abstract data type too.

Perhaps the locution "implement as an abstract data type declared in a
package" is necessary to militate against any misunderstanding.