Numeric types

[Mario Amado Alves <maa@di.fct.unl.pt>]

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?
  package Blob is
    -- ...
  end Blob;

  package body Blob is
    -- code using numeric operations ("+", "*", etc.), e.g.:
    function Blib(A, B, C: Numeric_Type) return Numeric_Type is
    begin
      return(A + B * C);      
    end Blib;
  end Blob;

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...

But I am afraid the idiom is just not possible. Please confirm. Perhaps
with class wide programming... I will have to go through Chapter 12 of
Cohen's bookin once more; perhaps I will be able to answer my onw questions
before the end of the year. But from the class hierachy of the types
involved, viz:

    scalar
       discrete
          integer
          ...
       real
       ...

I think it cannot be done. A numeric class should have the interset
fo integer and real operations but leaving out some discrete operations
like 'Pos and 'Val. The manual itself warns:

"The classes ``numeric'' and ``nonlimited'' represent other classification 
dimensions and do not fit into the above strictly hierarchical picture. "
(3.2)

But what "classes" are exactly refered to here? Numeric_IO?

Thanks in advance for any suggestions.

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