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From: Brian Rogoff <bpr@best.com>
Subject: Re: OO, C++, and something much better!
Date: 1997/02/21
Message-ID: <Pine.SGI.3.95.970221115143.12989C-100000@shellx.best.com>#1/1
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References: <JSA.97Jan16141937@alexandria> <5bphq4$5js@mulga.cs.mu.OZ.AU>
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Newsgroups: 
 comp.lang.eiffel,comp.lang.ada,comp.lang.c++,comp.lang.smalltalk,comp.object
Date: 1997-02-21T00:00:00+00:00
List-Id: <comp.lang.ada>


On 21 Feb 1997, Eric W. Nikitin wrote:
> Chris Bitmead (Chris.Bitmead@Alcatel.com.au) wrote:
> : Not true. There are other definitions of type.
> : 
> : For example, in some languages we could have a "ball" class and a
> : "cat" class, both of which respond to the message "kick", yet there is
> : no explicit "kickable" class.
> 
> In languages like Oberon-2 ( and I am fairly certain like Ada as well ),
> these concerns are modeled by separate language constructs. Encapsulation
> is provided by modules, structure comes from the type mechanism, and 
> behavior is described by dynamically-bound procedures. 
> 
> A language derived from Oberon, Lagoona, goes on to provide an additional
> feature called categories which are distinct from type, but allow 
> different types to be related via similar behaviors. ( Categories
> are similar to protocals in Objective-C and interfaces in Java ).

	Something similar to this is provided in Ada by using generic 
packages with empty bodies. For example

generic
	type Item is private;
	procedure Kick( X : in out Item );
package Kickable_Sig is
end;

with the canonical example being a mathematical group

generic
     type Element is private;
     Identity: in Element;
     with function Op(X, Y: Element) return Element;     
     with function Inverse(X: Element) return Element;  
package Group is end;

Note that the programmer need not have provided a procedure named "Kick" 
for Cat or Ball, any procedure with the specified signature will do in the 
instantiation:

package Kickable_Ball is new Kickable_sig(Element => Soccer_Ball,
Kick => Soccer_Kick);

-- Brian