Subject: Re: predicate methods and extensibility are non-orthogonal?
From: Craig Chambers (chambers@cs.washington.edu)
Date: Thu Apr 27 2000 - 17:27:16 PDT
This seems like a plausible analysis and idea. I think that the graphical
environment is orthogonal. The key idea is that the mapping from predicate
expressions to methods might profitably be separated (logically) from the
methods themselves, so that new predicate expressions can be written that map to
existing methods. One issue is how to name methods uniquely. Several
applications exist for such names (at source level and in the intermediate
language), and no good solution is available, other than brute-force attaching
names to each method declaration. Previously the GF and the predicate
expressions uniquely identified each method.
-- Craig Chambers
Mayur Naik wrote:
>
> Hello
>
> Consider the following code:
>
> abstract object X;
> template object A isa X;
>
> method foo(n) when n@A { ... } -- m1
>
> Suppose I wish to extend the above code at a later stage:
>
> template object B isa X;
>
> If I want to invoke method m1 when B is the run-time class of the
> receiver of the 'foo' message, I have to modify the existing code
> and recompile it:
>
> method foo(n) when n@A or n@B { ... } -- m1
>
> However, modifying existing code is non-OO. One way to solve the
> above problem is to make B a subclass of A. But this is a kludge.
>
> I feel a graphical programming environment (discussed in the paper
> "OO Multi-Methods in Cecil") can solve this problem by allowing
> the programmer to add new OR gaurds to existing methods. In this
> case, recompilation of existing code (for instance, the body of
> method m1) is not required.
>
> I think this non-orthogonality between predicate dispatch and
> extensibility arises only in the case of the OR operator and not
> in the case of the AND and NOT operators.
>
> Could you please comment? Thanks for your time.
>
> -- Mayur
>
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