Re: attributes in generic procedure

["Matthew Heaney" <matthew_heaney@acm.org>]

In article <818esd$t6l$1@nnrp1.deja.com> , daelen@my-deja.com  wrote:

> I would like to create a generic procedure as shown below which will only be
> used for scalar types. Does anybody have a hint how to accomplish this?

The issue is that there are different classes of scalar type, and
generic formal types differentiate the classes:

generic
  type Integer_Type is range <>;
  type Enum_Or_Int_Type is (<>);
  type Float_Type is digits <>;
  type Fixed_Type is delta <>;
  type Mod_Type is mod <>;



> generic
>    type A_Type is private;
> procedure Update (Par : in out A_Type; Value : String);

It looks like you need First, Last, Pred, Succ.  Import the type as
private (like you already have), but also import constants and
operations on that type:

generic
  type T is private;
  First : in T;
  Last  : in T;
  with function Pred (O : T) return T is <>;
  with function Succ (O : T) return T is <>;
  with function Value (S : String) return T is <>;
procedure Generic_Update (O : in out T; Image : in String);

Now, when a client instantiates the generic, he supplies the attribute
functions defined for the type as generic actuals:

function Update is new Generic_Update
  (T => Integer,
  First => Integer'First,
  Last => Integer'Last,
  Pred => Integer'Pred, ...);

This will work for any scalar type.


> procedure Update (Par : in out A_Type; Value : String) is
> begin
>    if Value = "+" then
>       if Par = A_Type'Last then
>          Par := A_Type'First;
>       else
>          Par := A_Type'Succ (Par);
>       end if;

For example:

  if Value = "+" then
    if Par = Last then
      Par := First;
    else
      Par := Succ (Par);
    end if;



--
Yeah, well, no one was present when the dinosaurs lived, so I guess
their presence in the fossil record is "only a theory." No one was
present when the Grand Canyon was formed either, or the creation of the
solar system, or, or, or. Of course evolution is "only a theory." What
else would it be as a scientific concept?

Skeptic magazine publisher Micheal Shermer, commenting on the recent
decision by the Oklahoma board of education to include a disclaimer on
school science textbooks warning that evolution is "only a theory."

Skeptic web page: http://www.skeptic.com/