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From: Dennis Hoppe on 14 Jun 2008 16:54
Hi there,

I'm trying to pass an instance of a generic child unit to another
generic package, but the only syntax, I could found specifies a concrete
generic package.
Let's first have a look at my pathological example:

-- GENERIC PARENT 'other' ADS
generic
B : Integer;
package Other is
type Object is tagged null record;
procedure Solve (Obj : Object);
end Other;


-- GENERIC PARENT 'other' ADB
with Ada.Text_IO;

package body Other is
procedure Solve (Obj : Object) is
begin
Ada.Text_IO.Put_Line ("solved");
end;
end Other;


-- GENERIC CHILD of 'other' ADS
with Other;

generic
-- omitted
package Other.Child is
type Instance is new Other.Object with null record;

overriding
procedure Solve (Obj : in Instance);
end Other.Child;


-- GENERIC CHILD of 'other' ADB
with Ada.Text_IO;

package body Other.Child is
procedure Solve (Obj : in Instance) is
begin
Ada.Text_IO.Put ("other child");
end;
end Other.Child;


-- GENERIC PARENT 'parent' ADS
with Other;

generic
A: Integer;
with package XX is new Other (A);
parent Parent is
type Object is abstract tagged null record;
procedure Print (Obj: in Object; X : in XX.Object'Class);
end Parent;


-- GENERIC PARENT 'parent' ADB
with Ada.Text_IO;

package body Parent is
procedure Print (Obj : in Object; X : in XX.Object'Class) is
begin
X.Solve;
Ada.Text_IO.Put (Integer'Image(A));
end;
end Parent;

-- GENERIC CHILD of 'parent' ADS
with Parent;

generic
-- omitted

package Parent.Child is
type Instance is new Parent.Object with null record;

overriding
procedure Print (Obj : in Instance; X : in XX.Object'Class);
end Parent.Child;


-- GENERIC CHILD of 'parent' ADB
with Ada.Text_IO;

package body Parent.Child is
procedure Print (Obj : in Instance; X : in XX.Object'Class) is
begin
Ada.Text_IO.Put_Line ("child");
X.Solve;
end;
end Parent.Child;


-- TESTSUITE
with Parent;
with Other;
with Other.Child;

procedure Test is
A : Integer := 2;
package O is new Other (A);
package E is new O.Child;
package P is new Parent (A, O);

procedure Calls (Obj : P.Object'Class; X : in O.Object'Class) is
begin -- Calls
P.Print (Obj, X);
end Calls;

X : C.Instance;
OO : O.Object;
EE : E.Instance;
begin -- Test
Calls (X, OO);
end Test;



Ok, if I declare 'package P is new Parent (A, O);' and call the Calls
procedure with 'Calls (X, OO);' all went fine, because O is an instance
of Other. I want further to declare my package P with a child unit of
Other, so that:

procedure Test is
A : Integer := 2;
package O is new Other (A);
package E is new O.Child;
package P is new Parent (A, E); -- O is replaced by the child E
.. -- compiler error expected

end Test;

Is it possible to declare a generic package parameter to accept
children, too? Or should I better solve this challenge with another
approach?

generic
A: Integer;
with package XX is new Other (A);
..

.... won't do it.


Thanks in advance,
Dennis
From: Dmitry A. Kazakov on 15 Jun 2008 04:57
On Sat, 14 Jun 2008 22:54:46 +0200, Dennis Hoppe wrote:

> I'm trying to pass an instance of a generic child unit to another
> generic package, but the only syntax, I could found specifies a concrete
> generic package.

Ada does not have interfaces of packages. A generic child package is not an
implementation of the parent's package interface, which could then be
referenced as a formal parameter of some other generic package. So the
answer is no.

But it seems to me that your example does not require that stuff:

> Let's first have a look at my pathological example:
>
> -- GENERIC PARENT 'other' ADS
> generic
> B : Integer;
> package Other is
> type Object is tagged null record;
> procedure Solve (Obj : Object);
> end Other;
>
>
> -- GENERIC PARENT 'other' ADB
> with Ada.Text_IO;
>
> package body Other is
> procedure Solve (Obj : Object) is
> begin
> Ada.Text_IO.Put_Line ("solved");
> end;
> end Other;
>
> -- GENERIC CHILD of 'other' ADS
> with Other;
>
> generic
> -- omitted
> package Other.Child is
> type Instance is new Other.Object with null record;
>
> overriding
> procedure Solve (Obj : in Instance);
> end Other.Child;
>
> -- GENERIC CHILD of 'other' ADB
> with Ada.Text_IO;
>
> package body Other.Child is
> procedure Solve (Obj : in Instance) is
> begin
> Ada.Text_IO.Put ("other child");
> end;
> end Other.Child;
>
> -- GENERIC PARENT 'parent' ADS
> with Other;
>
> generic
> A: Integer;
> with package XX is new Other (A);

type Object is new XX.Object with null record;

> parent Parent is
> type Object is abstract tagged null record;

remove this

> procedure Print (Obj: in Object; X : in XX.Object'Class);
> end Parent;

Now Parent takes an instance of Other, which determines the class of
Objects to use. The class is parametrized by A. The formal type Object is
an instance from the class to deal with.

BTW, you could also remove the parameter A:

generic
with package XX is new Other (<>); -- Brings some A with it
type Object is new XX.Object with null record;

> parent Parent is

[...]

> procedure Test is
> A : Integer := 2;
> package O is new Other (A);
> package E is new O.Child;
> package P is new Parent (A, E); -- O is replaced by the child E
> .. -- compiler error expected

package P is new Parent (A, O, E.Instance);

--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
From: Dmitry A. Kazakov on 15 Jun 2008 05:04
On Sun, 15 Jun 2008 10:57:44 +0200, Dmitry A. Kazakov wrote:

> On Sat, 14 Jun 2008 22:54:46 +0200, Dennis Hoppe wrote:
>
>> I'm trying to pass an instance of a generic child unit to another
>> generic package, but the only syntax, I could found specifies a concrete
>> generic package.
>
> Ada does not have interfaces of packages. A generic child package is not an
> implementation of the parent's package interface, which could then be
> referenced as a formal parameter of some other generic package. So the
> answer is no.
>
> But it seems to me that your example does not require that stuff:
>
>> Let's first have a look at my pathological example:
>>
>> -- GENERIC PARENT 'other' ADS
>> generic
>> B : Integer;
>> package Other is
>> type Object is tagged null record;
>> procedure Solve (Obj : Object);
>> end Other;
>>
>>
>> -- GENERIC PARENT 'other' ADB
>> with Ada.Text_IO;
>>
>> package body Other is
>> procedure Solve (Obj : Object) is
>> begin
>> Ada.Text_IO.Put_Line ("solved");
>> end;
>> end Other;
>>
>> -- GENERIC CHILD of 'other' ADS
>> with Other;
>>
>> generic
>> -- omitted
>> package Other.Child is
>> type Instance is new Other.Object with null record;
>>
>> overriding
>> procedure Solve (Obj : in Instance);
>> end Other.Child;
>>
>> -- GENERIC CHILD of 'other' ADB
>> with Ada.Text_IO;
>>
>> package body Other.Child is
>> procedure Solve (Obj : in Instance) is
>> begin
>> Ada.Text_IO.Put ("other child");
>> end;
>> end Other.Child;
>>
>> -- GENERIC PARENT 'parent' ADS
>> with Other;
>>
>> generic
>> A: Integer;
>> with package XX is new Other (A);
>
> type Object is new XX.Object with null record;

type Object is new XX.Object with private; -- Of course

>> parent Parent is
>> type Object is abstract tagged null record;
>
> remove this
>
>> procedure Print (Obj: in Object; X : in XX.Object'Class);
>> end Parent;
>
> Now Parent takes an instance of Other, which determines the class of
> Objects to use. The class is parametrized by A. The formal type Object is
> an instance from the class to deal with.
>
> BTW, you could also remove the parameter A:
>
> generic
> with package XX is new Other (<>); -- Brings some A with it
> type Object is new XX.Object with null record;
>
>> parent Parent is
>
> [...]
>
>> procedure Test is
>> A : Integer := 2;
>> package O is new Other (A);
>> package E is new O.Child;
>> package P is new Parent (A, E); -- O is replaced by the child E
>> .. -- compiler error expected
>
> package P is new Parent (A, O, E.Instance);


--
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de
From: Dennis Hoppe on 15 Jun 2008 08:04
Hello Dmitry,

thank you for your explanation and for some hints to correct my
implementation. Now, it works just as I planned it.

Best regards,
Dennis


Dmitry A. Kazakov wrote:
> [flipped the long code-snippet]
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