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From: Tucker Taft <stt@averstar.com>
Subject: Re: Numeric types
Date: 1999/12/08
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Date: 1999-12-08T00:00:00+00:00
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Mario Amado Alves wrote:
> 
> The manual defines "numeric types" as the set of integer and real types
> (cf. e.g. 3.5).
> 
> Now, is it a codeable type, viz. is it possible to have a generic formal
> parameter of numeric type, meaning inheriting all numeric operations ("+",
> "*", ...)? E.g.:
> 
>   generic
>     type Numeric_Type is ...; -- what here?

generic
  type Numeric_Type is private;
  with function "+"(Left, Right : Numeric_Type) return Numeric_Type is <>;
  with function "*"(...
  ...
  Zero : in Numeric_Type;
package blob is ...
 
> Is this possible to code properly? And how would we write "zero" in this
> idiom (or any other literal for that matter)? It must be a commonality of
> "0" and "0.0". This would perhaps work:
> 
>   function Zero return Numeric_Type is
>   begin
>     return(Numeric_Type'Value("0"));
>   exception
>     when others =>
>       return(Numeric_Type'Value("0.0"));
>   end Zero;
> 
> but it sounds like a dirty trick. An attribute 'Zero would be nice...

Just make zero a generic formal parameter.
You could also create a generic "signature" for a numeric type,
and then take any instantiation of that signature.  E.g.:

   generic
       type Num is private;
       with function "+"(...   -- as above
       Zero : in Num;
       One : in Num;
       with function Image(N : Num) return String;
       with function Value(S : String) return Num;
   package Numeric_Signature is end;

   generic
       with package Num_Pkg is new Numeric_Signature(<>);
   package Blob is
       use Num_Pkg;
       ...

   package Integer_Num is new Numeric_Signature(Integer, Zero => 0, One => 1,
     Integer'Image, Integer'Value);

   package Integer_Blob is new Blob(Num_Pkg => Integer_Num);

> But I am afraid the idiom is just not possible. Please confirm. Perhaps
> with class wide programming... 

Generics are pretty powerful.  The concept of a generic
signature can help reduce the number of parameters
passed to each generic.

> ...
> Thanks in advance for any suggestions.
> 
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> |M|A|R|I|O|                                                  mob 939354005
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-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Distributed IT Solutions  (www.averstar.com/tools)
AverStar (formerly Intermetrics, Inc.)   Burlington, MA  USA