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From: antonio_duran@hotmail.com (Antonio Duran)
Newsgroups: comp.lang.ada
Subject: Re: When/Why can a compiler reject an operator but accept a name?
Date: 8 Jul 2002 05:17:14 -0700
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Wes Groleau <wesgroleau@despammed.com> wrote in message news:<3D25D271.ACE6DAC8@despammed.com>...
> In package G I have:
> 
>     type List_Type is array (Natural range <>) of Item;
> 
>     type Order is access function (Left, Right : in Item) return
> Boolean;
> 
>     procedure Sort (List : in out List_Type; Sequence : in Order);
>     
> 
> In procedure P I had:
> 
>     package I is new G (Item);
> 
>     function "<" (Left, Right : in Item) return Boolean is
> 
>     .....
>     
>     I.Sort (List => List, Sequence => "<"'Access);
> 
>     
> Compiler rejected it, saying:
> 
>    "<" has no definition that matches function (Left, Right : in Item)
> return Boolean [RM_95 3.10.2(32)]
>    
> I replaced "<" with Ord_Check (no other changes), and
> the compiler accepted it.
> 
> Does using an operator instead of a name make the
> function Intrinsic?
> 
> If not, there is no support for rejection in 3.10.2(32)
> Is there any reason elsewhere in the RM to justify this
> odd behavior?
> 
> (Certainly either way, the message could be improved!)

ARM 3.10.2 1 says:

"The attribute Access is used to create access values designating
aliased objects and non-intrinsic subprograms. The
&#8220;accessibility&#8221; rules prevent dangling references (in the
absence of uses of certain unchecked features &#8212; see Section
13)."

So what I think is happening is that you're instanciating G with a
type that has an intrinsic "<" operator. For example, if you use
Integer, in your P procedure you must write a customized "<" operator
like:

with G;

procedure P is
   package I is new G(Integer);

   function "<"(Left, Right: in Integer) return Boolean
   is
   begin
      return Standard."<"(Left, Right);
   end "<";

   -- More stuff here.

begin
   -- More stuff here.
end P;

Regards,
    Antonio Duran.