Generics question

Generics question

[Skip Carter]

	I have a computational algorithm that I've implemented that has
	the (admitedly unusual) property that it is perfectly valid for either
	arrays of Integers or arrays of Floats.    As far as I can see I
	CANNOT make a generic version of the package that will work for
	both, e.g.:

	Generic
                Type Element is <something meaning INTEGER or FLOAT subtypes>
                Type DataArray  is Array(Natural  Range<>) of Element;

	.....


	Is there a way to do this ?   or do I have to actually make two different copies
	(which differ only in the definition of ELEMENT).


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Re: Generics question

[W. Wesley Groleau x4923]

>         I have a computational algorithm that I've implemented that has
>         the (admitedly unusual) property that it is perfectly valid for either
>         arrays of Integers or arrays of Floats.    As far as I can see I
>         CANNOT make a generic version of the package that will work for
>         both, e.g.:
> 
>         Generic
>                 Type Element is <something meaning INTEGER or FLOAT subtypes>
>                 Type DataArray  is Array(Natural  Range<>) of Element;
> 
>         .....
> 
>         Is there a way to do this ?   or do I have to actually make two different copies
>         (which differ only in the definition of ELEMENT).

Does it use attributes of type Element?  If not, say
   generic
      type Element is private;
      type Data_Array is Array(Natural  Range<>) of Element;
   ....

when you try to compile, you'll get error messages about undefined
operators.  Something along the lines of "*"(Element,element)return
element is not defined.  When that happens, add

    function "*" ( L, R : Element ) return Element is <>;

to your list of formal parameters.

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Re: Generics question

[Robert Dewar]

Skip Carter says

<<        I have a computational algorithm that I've implemented that has
        the (admitedly unusual) property that it is perfectly valid for either
        arrays of Integers or arrays of Floats.    As far as I can see I
        CANNOT make a generic version of the package that will work for
        both, e.g.:

        Generic
                Type Element is <something meaning INTEGER or FLOAT subtypes>
                Type DataArray  is Array(Natural  Range<>) of Element;
>>

No problem, just make the generic formal type private, and then require
the user (by default if convenient) to pass in the set of operators you
require, i.e. put statements like

   with "+"(L,R : Element) return Element

That will work fine.

Re: Generics question

[Brian Rogoff]

On 22 Jul 1997, Skip Carter wrote:
> 	I have a computational algorithm that I've implemented that has
> 	the (admitedly unusual) property that it is perfectly valid for either
> 	arrays of Integers or arrays of Floats.    As far as I can see I
> 	CANNOT make a generic version of the package that will work for
> 	both, e.g.:

To do a general numerical algorithm like you describe the Ada 95 idiom is: 

generic 
	type Numeric_Type is private;
	with function "+" (X,Y : Numeric_Type) return Numeric_Type is <>; 
	with function "-" (X,Y : Numeric_Type) return Numeric_Type is <>; 
	with function "*" (X,Y : Numeric_Type) return Numeric_Type is <>; 
	... etc ...
package Numerics is 
-- no body
end;

generic 
	with package Numbers is new Numerics(<>); 
        -- generic formal package parameters
package My_Numerical_Algorithms is 
... insert your stuff here ...
end;

I suppose someone will suggest making the numbers tagged types too, but I 
doubt you want to dispatch and if you make it a tagged type you get
probably double the size of your matrices. 

> 	Is there a way to do this ?   or do I have to actually make two different copies
> 	(which differ only in the definition of ELEMENT).

Note that what I describe easily handles any numeric type you have, with
the caveat that if you end up heap allocating numbers (say bignums) you 
will probably get yourself into trouble unless you deal with finalization.
But Integers, Floats, Complex, Interval, all work fine.

-- Brian

Re: Generics question

[Michael F Brenner]

    > To do a general numerical algorithm like you describe the Ada 95 idiom is:
    >   generic
    >   type Numeric_Type is private;
    >   with function "+" (X,Y : Numeric_Type) return Numeric_Type is <>;
    >   with function "-" (X,Y : Numeric_Type) return Numeric_Type is <>;
    >   with function "*" (X,Y : Numeric_Type) return Numeric_Type is <>;
    >   ... etc ...
    >   package Numerics is

The etc is not too bad in an unnested generic package, but there is no limit
to how big it can get in nested packages, which is why Ada-200X should
consider passing packages to packages.

Re: Generics question

[Jon S Anthony]

In article <5r7cvb$bfb@top.mitre.org> mfb@mbunix.mitre.org (Michael F Brenner) writes:

>     > To do a general numerical algorithm like you describe the Ada 95 idiom is:
>     >   generic
>     >   type Numeric_Type is private;
>     >   with function "+" (X,Y : Numeric_Type) return Numeric_Type is <>;
>     >   with function "-" (X,Y : Numeric_Type) return Numeric_Type is <>;
>     >   with function "*" (X,Y : Numeric_Type) return Numeric_Type is <>;
>     >   ... etc ...
>     >   package Numerics is
> 
> The etc is not too bad in an unnested generic package, but there is no limit
> to how big it can get in nested packages, which is why Ada-200X should
> consider passing packages to packages.

?!?!??  Ada95 supports passing packages to packages (generically) and
one of its intended uses is for just this kind of thing (signatures
with boatloads of operations).

/Jon

-- 
Jon Anthony
OMI, Belmont, MA 02178
617.484.3383
"Nightmares - Ha!  The way my life's been going lately,
 Who'd notice?"  -- Londo Mollari

Re: Generics question

[Brian Rogoff]

On 24 Jul 1997, Michael F Brenner wrote:
>     > To do a general numerical algorithm like you describe the Ada 95 idiom is:
>     >   generic
>     >   type Numeric_Type is private;
>     >   with function "+" (X,Y : Numeric_Type) return Numeric_Type is <>;
>     >   with function "-" (X,Y : Numeric_Type) return Numeric_Type is <>;
>     >   with function "*" (X,Y : Numeric_Type) return Numeric_Type is <>;
>     >   ... etc ...
>     >   package Numerics is
> 
> The etc is not too bad in an unnested generic package, but there is no limit
> to how big it can get in nested packages, which is why Ada-200X should
> consider passing packages to packages.

I don't understand. Ada 95 allows you to pass an instantiation of the
package Numerics to another generic package. The "etc" has to be done
because Ada generics must be explicitly instantiated. Could you describe 
your proposal, in "pseudo-Ada", and show how it helps with this problem?

I think that generic package parameters are pretty powerful, especially 
in combination with those signature packages. 

-- Brian

RE: Generics Question

[Jay Labhart]

Hello,


I am working on a project trying to utilize the generics portion
of Ada.  Here is my problem.

I need to have the user instantiate a package from his procedure
which inturn instantiates many more packages using the users
inputed type.

Here a sample


 generic
  type x is (<>);

  package foo is
   procedure start_it ( s : x);
  end foo;

  package body foo is
   procedure start_it
     ****
      *** here is where I would like to instaniate more packages
  end foo;



I am trying to allow a tool to accept different types within
Ada, rather than utilizing only integer, float...


Your help is greatly appreciated



Jay Labhart
jay@snow-white.merit-tech.com