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RE: supply and demand (was: Roles and Mix-ins?)

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Jonathan Lang
January 7, 2004 16:36
RE: supply and demand (was: Roles and Mix-ins?)
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Austin Hastings wrote:
> Jonathan Lang wrote:
> > OK: when you call $spot.bark, Trog looks for a "bark" method; it finds
> > two: Tree::bark and Dog::bark.  Since both methods have been supplied
> > by roles, $spot has no idea which one to use, and throws an exception 
> > to that effect.  
> I am uncomfortable with this. My understanding has been that the will be
> a compile time error, caught when the class declaration is made. Has 
> that changed recently?

I don't know for certain; but I think so.  Given the ambiguities that can
crop up with multiple dispatches and signatures, it may not be feasable to
catch all of them at compile-time.  

> > As such, the code given above is incorrect.  Adjusting for this error,
> > we rewrite Trog as follows:
> >
> >   class Trog does Tree but no bark does Dog {
> >     method grow() {...};
> >   };
> Modulo syntax, this is more right, yes.

I'm not familiar with modulo.  Also, I'm probably abusing the "but" syntax
like nobody's business here.  

> > But in the case of
> >
> >   role Dog {
> >     method bark() {...}
> >     must bark();
> >     method threaten must bark() { bark; }
> >   }
> >
> >   class Toothless does Dog but no threaten() no bark() {
> >   }
> >
> > You'd still have a problem, because even excluding threaten doesn't
> > exclude Dog's demand to be able to bark.
> "Hit jist haint a dawg ifn hit dont bark."

You've never met my dog.  

> But large roles are going to want to say C<must> just once, since (1)
> that makes for easier, more legible documentation; and (2) if chromatic 
> is correct, most functionality with roles (smaller than classes, recall)
> will likely be pretty incestuous.

The "must" clause in the above "threaten" method is redundant, an artifact
of cut-n-paste coding.  The example would work exactly the same with or
without it.  

Jonathan "Dataweaver" Lang

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