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From: Brian Rogoff <bpr@shell5.ba.best.com>
Subject: Re: [student help] Won't allow package body.
Date: 1998/05/13
Message-ID: <Pine.BSF.3.96.980513114052.23257A-100000@shell5.ba.best.com>#1/1
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References: <35514AAC.D46B535C@iinet.net.au> <6jcfln$ft$1@news.eclipse.net>
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Newsgroups: comp.lang.ada
Date: 1998-05-13T00:00:00+00:00
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On Wed, 13 May 1998, MSG wrote:

> Besides the fact that Davic C Hoos, Sr. is correct and that you need to
> provide a more complete example...
> 
> Assuming that your spec is compilable but doesn't require a body (a generic
> with no body?...I'll have to think about that later)...you can force need

Bodiless generics are part of a very powerful Ada programming idiom. Using
them we can decouple interfaces (signatures) from implementations. For
example, here is a pseudo-Ada outline of a sequence type, showing roughly
how it might be used.

generic
   type Item_Type is private;
   type Position_Type is private;
   type Sequence_Type is private;
   with procedure Initialize (Seq : out Sequence_Type);
   with function  Is_Empty (Seq : Sequence_Type) 
			    return Boolean is private;
   with function  Is_Done (Seq : Sequence_Type; Pos : Position_Type) 
			   return Boolean is private;
package Sequence_Sig is 
-- Here is your bodiless generic!
end;


generic
   type Item_Type is private;
package Lists is
   type Position_Type is private;
   type List_Type is private;

   with procedure Initialize (Seq : out Sequence_Type);
   with function  Is_Empty (Seq : Sequence_Type) 
			    return Boolean;
   with function  Is_Done (Seq : Sequence_Type; Pos : Position_Type) 
			   return Boolean;
   
   with function  Get_Value_At (Seq : Sequence_Type; Pos : Position_Type) 
			   return Item_Type;
   
   with function  Succ (Pos : Position_Type) return Position_Type;
   
private
   -- Singly linked list implementation
end;

package Integer_Lists is new Lists(Item_Type => Integer);

package Integer_Seqs is 
  new Sequences(Item_Type     => Integer_Lists.Item_Type,
		Position_Type => Integer_Lists.Position_Type,
		Sequence_Type => Integer_Lists.List_Type,
		Initialize    => Integer_Lists.Initialize
                ... etc...
                );

generic 
   with Concrete_Seqs is new Sequences(<>);
package Concrete_Algorithms is 
...
end Concrete_Algorithms;

A client package like Concrete_Algorithms will only depend on the
Signature and so you can substitute arrays or lists or vectors or 
whatever as your sequence. 

-- Brian

> for a body by adding a dummy procedure with no code OR you can specify
> "pragma Elaborate_Body;" in the spec of the generic pacakge.
> 
> Michael Garnett
> 
> Oliver White <ojw@iinet.net.au> wrote in message
> 35514AAC.D46B535C@iinet.net.au...
> I'm compiling a package, the ads file begins as so -
> 
> 
> generic
> 
>    type ObjectType is private;
> 
> package LinkedListPackage is
> but when I compile, I get this message -
> "generic package LinkedListPackage does not allow a body"
> 
> I'm sure there's an obvious error, could you help me?
> 
> Oliver White.
> 
> 
> 
> 
>