From: Tomasz Wegrzanowski <taw@...>
Date: 2001-12-30T13:25:25+09:00
Subject: [ruby-talk:29700] Re: extending method of class A to support arguments of class B by promoting `self' to class B

On Sat, Dec 29, 2001 at 06:08:49PM +0900, Yukihiro Matsumoto wrote:
> Hi,
> 
> In message "[ruby-talk:29610] Re: extending method of class A to support arguments of class B by promoting `self' to class B"
>     on 01/12/29, Tomasz Wegrzanowski <taw@users.sf.net> writes:
> 
> |> Otherwise, if you mean generic classes, there's no such
> |> framework now.
> |
> |It would be really nice if such thing were added.
> 
> Define "such thing".  And convince me it's nice.
> 
> 							matz.

<problem>
	Such-thing is defined more by problem it tries to solve than by solution.

	Not every relation can be put into "object-oriented" form,
	where one of involved objects logically-dominates all others.
	Often, relation involve more than one object at logically-equal-rights.
	The most obvious example are mathematical relations, like addition and substraction, where none
	of objects is logically-more-important than others.
	It doesn't happen only in case of symmetrical relations - substraction is obviously asymmetrical,
	but none of the objects dominates another.
	Such-thing would allow one to provide rules of interaction on any side of relation.
	Now, in most object-oriented-languages, including Ruby,
	NewClass.relation(OldClass) is much easier to define than OldClass.relation(NewClass),
	although their nature is alike.
</problem>

<proposed_solution>
	Such-thing could be based on TypeError exception mechanism.
	It would be possible to registere catching functions:
		register_catch_typeerror_function(ourclass, savingmethod, savedclass, savedmethod)
	that would be called in case of such exceptions. Example:
		register_catch_typeerror_function(Complex, :real_plus_complex, Real, :+)

	:real_plus_complex would be method of real that is called if :+ fails with
	typeerror and one of its arguments is Complex.
</proposed_solution>

Convincing anyone that this particular solution is good will be hard,
but I hope you agree that there is a problem.