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Nature of traits

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Jonathan Lang
December 21, 2003 17:08
Nature of traits
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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?  

Jonathan "Dataweaver" Lang

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