Universal type in Ada

Universal type in Ada

[zw]

Hi, I am trying to create a Universal type in Ada such as "Object" in
Java so that I could define a function that returns a value in this
Universal type, then I could lower-cast value in this type to its
specific type such as String, Float or any user-defined types. Because
in Java if a method returns an Object, it could be casted to any other
types that are subtypes of Object. Is there a Universal type in Ada?
Could anyone tell me how to create such types in Ada, please?

Re: Universal type in Ada

[Larry Kilgallen]

In article <bf72da73.0506150507.6058c851@posting.google.com>, zw@cs.man.ac.uk (zw) writes:
> Hi, I am trying to create a Universal type in Ada such as "Object" in
> Java so that I could define a function that returns a value in this
> Universal type, then I could lower-cast value in this type to its
> specific type such as String, Float or any user-defined types. Because
> in Java if a method returns an Object, it could be casted to any other
> types that are subtypes of Object. Is there a Universal type in Ada?
> Could anyone tell me how to create such types in Ada, please?

That is not a very Ada-like way to program.

Are absolutely certain you are not trying to transfer programming habits
from other languages over to Ada ?

Re: Universal type in Ada

[Dmitry A. Kazakov]

On 15 Jun 2005 06:07:12 -0700, zw wrote:

> Hi, I am trying to create a Universal type in Ada such as "Object" in
> Java so that I could define a function that returns a value in this
> Universal type, then I could lower-cast value in this type to its
> specific type such as String, Float or any user-defined types. Because
> in Java if a method returns an Object, it could be casted to any other
> types that are subtypes of Object. Is there a Universal type in Ada?

Yes. What you describe is a contravariant result. In Ada it is achieved by
returning a class-wide result. A function of this sort is also called
"abstract factory".

> Could anyone tell me how to create such types in Ada, please?

type Object is abstract tagged null record;
function Factory (...) return Object'Class;

type My_Special_Object is new Object with ...;

declare
   X : My_Special_Object;
begin
   ...
   X := My_Special_Object Factory (...));

Here the result of Factory is explicitly converted to My_Special_Object. If
that is impossible (because the result is not a My_Special_Object)
Constraint_Error exception is propagated at run-time.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Universal type in Ada

[Preben Randhol]

zw <zw@cs.man.ac.uk> wrote on 15/06/2005 (13:11) :
> Hi, I am trying to create a Universal type in Ada such as "Object" in
> Java so that I could define a function that returns a value in this
> Universal type, then I could lower-cast value in this type to its
> specific type such as String, Float or any user-defined types. Because
> in Java if a method returns an Object, it could be casted to any other
> types that are subtypes of Object. Is there a Universal type in Ada?
> Could anyone tell me how to create such types in Ada, please?

Why would you want to do something like this? Generally you don't want
to use subtypes over types as you loose the advantage of the Ada type
checking...

Remember that in Ada one can have:

     function Method (Some_Input : Input_Type) return Integer;
     function Method (Some_Input : Input_Type) return Float;
     function Method (Some_Input : Input_Type) return String;

And do:

   Number : Integer := Method (Input);
   Height : Float   := Method (Input);
   Text   : String  := Method (Input);

then the correct function will be called depending on return type.

Please explain what you want to do and we can give better hints to do it
in more Ada-wise ways.

Preben

Re: Universal type in Ada

[Preben Randhol]

"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote on 15/06/2005 (14:00) :
> 
> type Object is abstract tagged null record;
> function Factory (...) return Object'Class;
> 
> type My_Special_Object is new Object with ...;
> 
> declare
>    X : My_Special_Object;
> begin
>    ...
>    X := My_Special_Object Factory (...));

    X := My_Special_Object (Factory (...));

right?

Preben

Re: Universal type in Ada

[Georg Bauhaus]

zw wrote:


> Because
> in Java if a method returns an Object, it could be casted to any other
> types that are subtypes of Object.

Depends on what you mean by "could". You will have to
know the type of your object unless a ClassCastException
is o.K.. You could start expressing a minimal set of
expectations that your Ada program is going to have of a
type. Then use polymorphism down that type's hierarchy.

Re: Universal type in Ada

[Dmitry A. Kazakov]

On Wed, 15 Jun 2005 16:37:46 +0200, Preben Randhol wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote on 15/06/2005 (14:00) :
>> 
>> type Object is abstract tagged null record;
>> function Factory (...) return Object'Class;
>> 
>> type My_Special_Object is new Object with ...;
>> 
>> declare
>>    X : My_Special_Object;
>> begin
>>    ...
>>    X := My_Special_Object Factory (...));
> 
>     X := My_Special_Object (Factory (...));
> 
> right?

Yes, of course. (:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Universal type in Ada

[Ole-Hjalmar Kristensen]

Please do not do this. As someone (may have been John Max Skaller) on
comp.lang.c++ said once, "casting is something you do when fishing".
Think about what you are trying to do, and try to figure out how this
can be accomplished by the tools available to you in Ada, like
generics or run-time polymorphism.

>>>>> "z" == zw  <zw@cs.man.ac.uk> writes:

    z> Hi, I am trying to create a Universal type in Ada such as "Object" in
    z> Java so that I could define a function that returns a value in this
    z> Universal type, then I could lower-cast value in this type to its
    z> specific type such as String, Float or any user-defined types. Because
    z> in Java if a method returns an Object, it could be casted to any other
    z> types that are subtypes of Object. Is there a Universal type in Ada?
    z> Could anyone tell me how to create such types in Ada, please?

-- 
The Sun also rises

Re: Universal type in Ada

[Dmitry A. Kazakov]

On 16 Jun 2005 10:18:44 +0200, Ole-Hjalmar Kristensen wrote:

> Please do not do this. As someone (may have been John Max Skaller) on
> comp.lang.c++ said once, "casting is something you do when fishing".
> Think about what you are trying to do, and try to figure out how this
> can be accomplished by the tools available to you in Ada, like
> generics or run-time polymorphism.

Yes, but unfortunately, there is one case in Ada 95 where apparently
unnecessary downcasting is necessary: pool-specific access types to
class-wide objects:

type Object is tagged ...;
type Object_Ptr is access Object'Class;

function Create_My_Object (...) return Object_Ptr is
   Result  : Object_Ptr;
begin
   Result := new My_Object (...);
   declare
      This : My_Object renames  M_Object (Result.all);
         -- Casting is required because a pool-specific access
         -- to My_Object is not convertible to a pool-specific
         -- access to Object'Class!
   begin
      -- Do whatever needed to complete initialization of
      -- This
      ...
   end;
   return Result;
end Create_My_Object;

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Universal type in Ada

[zw]

Preben Randhol wrote:
> zw <zw@cs.man.ac.uk> wrote on 15/06/2005 (13:11) :
> > Hi, I am trying to create a Universal type in Ada such as "Object" in
> > Java so that I could define a function that returns a value in this
> > Universal type, then I could lower-cast value in this type to its
> > specific type such as String, Float or any user-defined types. Because
> > in Java if a method returns an Object, it could be casted to any other
> > types that are subtypes of Object. Is there a Universal type in Ada?
> > Could anyone tell me how to create such types in Ada, please?
>
> Why would you want to do something like this? Generally you don't want
> to use subtypes over types as you loose the advantage of the Ada type
> checking...
>
> Remember that in Ada one can have:
>
>      function Method (Some_Input : Input_Type) return Integer;
>      function Method (Some_Input : Input_Type) return Float;
>      function Method (Some_Input : Input_Type) return String;
>
> And do:
>
>    Number : Integer := Method (Input);
>    Height : Float   := Method (Input);
>    Text   : String  := Method (Input);
>
> then the correct function will be called depending on return type.
>
> Please explain what you want to do and we can give better hints to do it
> in more Ada-wise ways.
>
> Preben

Hi, Thank you very much.

We could define a function "Method" that takes same inputs and return
different types of results. However, we must know the type of result
from a "Method" so that we could declare "Result" as an Integer, Float
or String, but the problem is when we don't know the "Result" type, we
could not use the function "Method". For example, in Java,

..........
Object Method(Input_Type Some_Input){....}
..........
Input_Type S1;
Object result = Method(S1);
..........

where result could an Integer, a Float or an instance of a user-defined
Object.

Re: Universal type in Ada

[zw]

Yes, I am trying to transfer a program in Java to Ada. Because as far
as I know there is no reflection mechanisms in Ada, so what I do is to
try to create a new API for reflection in Ada that would do someting
similar to java.lang.reflect

Re: Universal type in Ada

[zw]

Hi, thank you very much. This is a great idea to create pointers to an
Object. I also have the same idea, but my problem is a pointer must be
declared with a specific type such as a pointer to an Integer. However,
in Ada, when we declared Object as an abstract superclass, the existing
types like Integer, Float, String are not an Object, we could easily
define a superclass called "Object" and define all of our types
inherited from the "Object", this way we will get roughly same
functionalities like Object as the root type. However, there is no way
to cope with existing types like Integer. I would like to know that
whether this is possible to define a root type like "Object" in Java so
that whatever a "Method" is, it will always return an "Object"? because
all classes are inherited from "Object", so an Integer is an Object, a
Float is an Object, ...........

Re: Universal type in Ada

[jimmaureenrogers]

zw@cs.man.ac.uk wrote:
> Hi, thank you very much. This is a great idea to create pointers to an
> Object. I also have the same idea, but my problem is a pointer must be
> declared with a specific type such as a pointer to an Integer. However,
> in Ada, when we declared Object as an abstract superclass, the existing
> types like Integer, Float, String are not an Object, we could easily
> define a superclass called "Object" and define all of our types
> inherited from the "Object", this way we will get roughly same
> functionalities like Object as the root type. However, there is no way
> to cope with existing types like Integer. I would like to know that
> whether this is possible to define a root type like "Object" in Java so
> that whatever a "Method" is, it will always return an "Object"? because
> all classes are inherited from "Object", so an Integer is an Object, a
> Float is an Object, ...........

Java does not solve this problem any better than Ada.
The Java primitive types do not inherit from Object. Java provides
wrapper classes for primitive types, but they cannot be used
interchageably with the primitive types themselves, there is always
a type conversion required.

Ada allows you to create a tagged type, either concrete or abstract,
as the root of an inheritance hierarchy. You can then deal with
types such as T'Class and "access T'Class".

Java requires reflection because it provides the Object class. One
result of using a universal ancestor class is the confusion of having
a reference possibly point to either a Float (the wrapper class for the
float primitive type) or a Button (representing a GUI component). You
must then have reflection capabilities so that you can determine what
kind of Object you are pointing to. The methods inherited from Object
are useful in a general sense, but not much help when trying to deal
with characteristics of different classes that do not have anything
in common except the Object class.

Many Java designs include sloppy inheritance hierarchies encouraged by
the availability of the Object references. If you are converting a
program from Java to Ada you should try to do more than simply
recreate a Java inheritance hierarchy using Ada syntax. You should
re-design the program to use Ada as Ada.

Consider converting an Ada program to Java. What if the Ada program
used tasks with entries and the select clause in at least one of the
tasks included a terminate option? How would you express that in Java
syntax? Java has no direct equivalent to the Ada select clause. Java
has no direct equivalent to the terminate option. Even more
difficult, what if one task called the entry in another task using a
selective entry call? Java provides no direct equivalent for a
selective entry call.  You would need to re-design the Ada program
so that you could get equivalent functionality using Java syntax.
You might not be able to implement some Ada features directly in Java.
You might need to settle for similar, if not fully equivalent
functionality.

Jim Rogers

Re: Universal type in Ada

[Dmitry A. Kazakov]

On 16 Jun 2005 06:23:51 -0700, zw@cs.man.ac.uk wrote:

> Hi, thank you very much. This is a great idea to create pointers to an
> Object.

No, you don't need pointers. Create can return Object'Class which is much
safer and easier to use.

The example I gave, was to illustrate a language problem. In a real case
the code looks different:

function Create_My_Object (...) return Object_Handle is
   Result  : Object_Ptr;
begin
   Result := new My_Object (...);
   declare
      This : My_Object renames  M_Object (Result.all);
   begin
      ...
   end;
   return Ref (Result); -- The result is a handle (for GC)
exception
   when others =>
       Free (Result); -- Don't want it leaking
end Create_My_Object;

Note the result is a handle instead of a raw pointer. This is closer to
Java spirit, but not necessarily a universal solution. Not everything needs
smart pointers. I wouldn't program a hard real-time application this way...
(:-))

> I also have the same idea, but my problem is a pointer must be
> declared with a specific type such as a pointer to an Integer. However,
> in Ada, when we declared Object as an abstract superclass, the existing
> types like Integer, Float, String are not an Object, we could easily
> define a superclass called "Object" and define all of our types
> inherited from the "Object", this way we will get roughly same
> functionalities like Object as the root type. However, there is no way
> to cope with existing types like Integer.

Yes. You have to aggregate:

type Integer_Object is new Object with record
   Value : Integer;
end record;

But, what are you looking for? Often people with a baggage of other
languages are asking for a language-X solution in Ada. It is not the best
way...

> I would like to know that
> whether this is possible to define a root type like "Object" in Java so
> that whatever a "Method" is, it will always return an "Object"? because
> all classes are inherited from "Object", so an Integer is an Object, a
> Float is an Object, ...........

Technically, this requires supertyping, which neither Ada nor Jave have.
Though normally types like Integer, Float, Boolean etc need not to be
"objects" in the sense that they all would have "Method" as a primitive
operation. Also "objects" are usually thought as something large having
identity. Well, this is wrong in general, but often right in particular.
(:-))

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de

Re: Universal type in Ada

[Preben Randhol]

Organization: PVV 

zw@cs.man.ac.uk wrote on 16/06/2005 (13:17) :
> Yes, I am trying to transfer a program in Java to Ada. Because as far
> as I know there is no reflection mechanisms in Ada, so what I do is to
> try to create a new API for reflection in Ada that would do someting
> similar to java.lang.reflect

If you are transfering, why not improve the program at the same time by
using Ada's powers? Doesn't make sense to me to transfer the program
unless there is a benifit from it. But I don't know the program, so
there may be other reasons that one need to cheat the type system.

Preben

Re: Universal type in Ada

[Stephen Leake]

zw@cs.man.ac.uk writes:

> Yes, I am trying to transfer a program in Java to Ada. Because as far
> as I know there is no reflection mechanisms in Ada, 

Reflection in Ada is provided by ASIS.

It's a separate pass at compile time, rather than integrated at run
time. So it is a different flavor.

> so what I do is to try to create a new API for reflection in Ada
> that would do someting similar to java.lang.reflect

Depending on what you want to do with reflection, ASIS might be the
right answer.

-- 
-- Stephe

Re: Universal type in Ada

[Martin Krischik]

zw wrote:

> Hi, I am trying to create a Universal type in Ada such as "Object" in
> Java so that I could define a function that returns a value in this
> Universal type, then I could lower-cast value in this type to its
> specific type such as String, Float or any user-defined types. Because
> in Java if a method returns an Object, it could be casted to any other
> types that are subtypes of Object.

In Ada you don't usually "cast" types you convert them. Read the article
about type conversions:

http://en.wikibooks.org/wiki/Programming:Ada:Subtypes#Converting_Data

> Is there a Universal type in Ada? 
> Could anyone tell me how to create such types in Ada, please?

A tagged type will do:

http://en.wikibooks.org/wiki/Programming:Ada:OO

Martin

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com

Re: Universal type in Ada

[Martin Krischik]

zw@cs.man.ac.uk wrote:

> his is a great idea to create pointers to an
> Object.

Well, your don't use pointers to objects in Ada - you usually use class wide
types - and *sometimes* pointers to class wide types.

http://en.wikibooks.org/wiki/Programming:Ada:OO#The_class-wide_type

Martin
-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com

Re: Universal type in Ada

[Martin Krischik]

zw@cs.man.ac.uk wrote:

> Yes, I am trying to transfer a program in Java to Ada. Because as far
> as I know there is no reflection mechanisms in Ada, so what I do is to
> try to create a new API for reflection in Ada that would do someting
> similar to java.lang.reflect

Not quite true. Ada does have some form of reflection via the tag. You could
use the external tag or the expanded name to determine the type of an
object. 

http://www.adaic.com/standards/rm-amend/html/RM-3-9.html

Also the operator "in" can be helpfull.

And just like Java Ada will issue an exception when you convert objects
which should not be converted.

Martin 

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com