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From: Shark8 <onewingedshark@gmail.com>
Newsgroups: comp.lang.ada
Subject: Generic-Package Elaboration Question / Possible GNAT Bug.
Date: Sat, 19 Nov 2011 13:14:44 -0800 (PST)
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Date: 2011-11-19T13:14:44-08:00
List-Id: <comp.lang.ada>

I was working on some simple little array manipulation functions when
I encountered an interesting behavior in the compiler; it said that
trying to instantiate a function of the form:
   Generic
      Type Element is Private;
      Type Index_Type is (<>);
      Type Array_Type is Array (Index_Type Range <>) of Element;
   Function Generic_Delete( Index : In Index_Type; Data : In
Array_Type ) Return Array_Type;

When given the following body, the compiler complained about the
Index_Range'First/Last not being static:

   Function Generic_Delete( Index : In Index_Type; Data : In
Array_Type ) Return Array_Type is
   begin
      if Index not in Data'Range then
         Raise Constraint_Error;
      else
           case Index is
           when Index_Type'First => Return
Data( Index_Type'Succ(Index)..Data'Last );
           when Index_Type'Last => Return
Data( Data'First..Index_Type'Pred(Index) );
           when others =>
                  Return Data( Data'First..Index_Type'Pred(Index) )
  		 & Data( Index_Type'Succ(Index)..Data'Last );
           end case;
      end if;
   end Generic_Delete;

Now, I know that the following works as a fix:

     Function Generic_Delete( Index : In Index_Type; Data : In
Array_Type ) Return Array_Type is
     begin
        if Index not in Data'Range then
           Raise Constraint_Error;
        else
           if Index = Data'First then
              Return Data( Index_Type'Succ(Index)..Data'Last );
           elsif Index = Data'Last then
              Return Data( Data'First..Index_Type'Pred(Index) );
           else
              Return Data( Data'First..Index_Type'Pred(Index) )
  		 & Data( Index_Type'Succ(Index)..Data'Last );
           end if;
        end if;
     end Generic_Delete;

But the question that came to my mind after reading the ARM section on
the error (sorry, I forgot to write it down) is, why is it not static?
Is it because though the elaboration-parameters *may* be known [and
static] at compile-time that some instantiation might NOT be
[guaranteed] to be known at compile-time? (i.e. because the compiler
cannot guarantee that you won't throw 1..N where N is something pulled
from the user-input?)