Subject: Re: modular statically typed multimethods
From: Todd D Millstein (todd@cs.washington.edu)
Date: Fri Jan 07 2000 - 11:14:49 PST
Hi Mayur,
Thanks for the reference; I hadn't seen that work before. It seems that
what the acyclic visitor pattern does nicely is allow new subclasses to be
added, without requiring existing visitors to handle these new subclasses
(although it needs to use multiple inheritance and a dynamic cast to make
this work). In contrast, in our system, we have to force visitors to write
appropriate default methods to handle any unseen subclasses.
On the other hand, suppose that we would like our existing visitor's
functionality to be extended to handle the new subclass. With the acyclic
visitor pattern, this cannot be done modularly. In particular, this
requires modifying the visitor's class to add the new method, as well as
adding a new abstract class from which the visitor now must inherit. In
our system, we allow the new method to be written without having to modify
any existing code, by simply writing the new method in the same module
where the new subclass was created.
Todd
On Fri, 7 Jan 2000 mnaik@hss.hns.com wrote:
>
>
>
> Hello
>
> As mentioned in the paper "Modular Statically Typed Multimethods",
> the open objects relieve the tension between ease of adding
> operations to existing classes and ease of adding subclasses.
>
> What I understand by "open objects" is the prototype-based model.
> While I appreciate the expressive power of this model, I would
> like to bring to your notice Robert Martin's "Acyclic Visitor"
> (also called RTTI visitor) design pattern
>
> (http://www.objectmentor.com/publications/acv.pdf)
>
> which allows both, adding operations to existing classes and adding
> subclasses, in the class-based model. It is more powerful than the
> visitor design pattern which allows only the former.
>
> Regards
>
> -- Mayur
>
>
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