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
