On Fri, 16 May 2008 10:52:57 +0200, Ryan Mitchley wrote:

>
> I am genuinely curious to see a simple example that clearly demonstrates  
> the
> advantages of traits.
>
> It is my perception (or misperception) that traits are an attempt  
> (perhaps
> unknowingly) to bring multiple inheritance into the language.

I think you can easily judge for yourself whether or not a system gives  
you multiple inheritance (MI):

o in Smalltalk every class has at most one superclass
o every MI class has at least two superclasses
o in Smalltalk a method can send to self or super
o a method in an MI class can choose to which super it sends

Compare Squeak's traits to these statements ...

/Klaus

> As I see it,
> this confuses
> Y Is-A X (i.e. Y subclasses X)
> with
> Y Has-A-Part X (i.e. Y has a member variable / subpart / trait X)
>
> I've seen this in GUI toolkits - e.g. someone has a Bitmap class and a
> ClickableArea class. They want to make a Button - i.e. something that has
> both Bitmap and ClickableArea traits. So they create Button by inheriting
> from both Bitmap and ClickableArea classes (usually C++). The better
> solution is to say that Button has both Bitmap and ClickableArea parts  
> (i.e.
> member variables). This confusion becomes ridiculously clear if we  
> attempt
> to create e.g. a Human class by inheriting from Eye, Leg, Arm etc. There  
> is
> very likely much confusion between all of the members that get dragged in
> and try to compete with each other.
>
> So, if I have a mistaken view of traits, I would in all honesty love to  
> see
> an example that shows this to me clearly.
>
> I have a strong view that software should be designed with a very high  
> view
> of orthogonality. I will often reject non-orthogonal approaches in my own
> software without much thought, simply because I have been down that road
> before and seen the trouble that ensues. That's why I would suggest that
> traits should be rejected IFF they turn out to be a non-orthogonal design
> (i.e. their utility is duplicated by e.g. member variables). I am not yet
> convinced one way or the other.
>