[Xotcl] Precedence Order

[Uwe Zdun]

well, why do you think the behavior breaks encapsulation? Rather a 
behavior where mixin are not added in precedence before the class 
hierarchy, requires you to know about the interceptor and might break 
encapsulation. Consider you are omitting to use "next" in a method of 
Derived, for instance simply because this method exists nowhere else in 
the class hierarchy. If the mixin would be introduced after Derived, you 
would need to modify Derived to introduce a method of the same name on 
the mixin. If the mixin is always before the class hierarchy, as a base 
hierarchy developer you don't need to care for how a potential mixin is 
designed, because you can be sure once a call has reached your class, 
you can be sure, you cannot accidently introduce side-effects on mixins 
(maybe designed later by other developers).

Scott Gargash wrote:

>
> As I understand it, precedence order is computed as filters then 
> mixins the superclasses, and this sort order is applied "globally" to 
> an object.  
>
> In other words:
>
> Class create Base
> Class create Derived -superclass Base
> Derived d
> d info precedence ==> ::Derived ::Base ::xotcl::Object
>
> Class create BaseMixin
> Base instmixin add BaseMixin
> d info precedence  ==> ::BaseMixin ::Derived ::Base ::xotcl::Object
>
> This behavior seems to violate encapsulation. BaseMixin is intended to 
> intercept messages to Base. Derived doesn't know about BaseMixin and 
> BaseMixin doesn't know about Derived, yet BaseMixin ends up being the 
> first interceptor of messages to Derived.
>
> Since BaseMixin is intended to modify the behavior of Base, it would 
> be better BaseMixin preceeded Base (and only Base) in the sort order.  
>
> d info precedence  ==> ::Derived ::BaseMixin ::Base ::xotcl::Object
>
> The current behavior means you can't really add a mixin without 
> understanding everywhere that the object being extended is used. Is 
> there some benefit to the current behavior that I don't understand?
>
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