Jonathan Lang writes: > I've just been rereading the recent discussions, and I noticed something > that I missed the first time: > > Larry Wall wrote: > > Traits are not definitional but rather operationally defined in the > > worst way. That's why traits are renegade roles. They don't play by > > the rules. > > In other words, a trait effectively has an "installer" routine which > gets run whenever you apply it to a particular object (that is, "is > const" calls const::INSTALL or something of the sort). > > I've been trying to puzzle out what the distinction between roles and > traits is so that I can understand why traits need to be handled > differently than roles are - in particular, I was thinking in terms of > why traits don't fit into the "order of precedence" for method calls > (that is, class || role || superclass). If I'm right about the > installer, the answer would seem to be that the trait gets to choose > (by means of the installer) where it fits into said chain, if at all - > it could insert itself into the class's ISA list, tell the class to > treat it as a role, or maybe even wrest control from the class, > effectively treating the class definition as a role or superclass of > its own and then masquerading as the class to outsiders (effectively > putting itself ahead of the class in the order of precedence chain). > Or it could do none of the above, thus requiring its namespace to be > explicitly referenced if you want access to any of its methods or > attributes. > > The point is that - if I'm understanding the current thoughts on the > subject - the difference between a trait and a role is that a trait's > interaction with a class is programmable. How am I faring?
That seems to be it. A trait, from the user's perspective, is something declarative that happens other than inheritance or role composition. If you're wondering why roles aren't unified with traits, it looks like a linguistic thing. We only have so many naming conventions. Traits are the catch-all of encapsulated behavior. You put something that happens behind a name, and it does what it needs to in order to fulfill its duty. For the things more specific, there are concepts more specific. I think traits are a brilliant idea though. If you're going to hack, it's nice to have a standard way to encapsulate the hack. Luke
