From: "Ara.T.Howard" <Ara.T.Howard@...>
Date: 2005-05-05T05:54:51+09:00
Subject: traits (the other ones) vs. mixins


from these:

   http://www.iam.unibe.ch/~scg/cgi-bin/oobib.cgi?query=nathanael+traits+composable+units+ecoop
   http://www.cantrip.org/traits.htmlhttp://www.cantrip.org/traits.html

and snippets like:

   - A trait provides a set of methods that implement behaviour

   - A trait requires a set of methods that parameterize the provided behaviour

   - Traits do not specify any state variables, and the methods provided by traits
   never directly access state variables

   - Traits can be composed: trait composition is symmetric and conflicting
   methods are excluded from the composition

   - Traits can be nested, but the nesting has no semantics for classes --
   nested traits are equivalent to flattened traits

i cannot see how this differs from mixins?  for example:

   jib:~ > cat a.rb
   module DoubleSizeTrait
     def double_size
       2 * size
     end
   end
   module QuadrupleSizeTrait
     include DoubleSizeTrait
     def quadruple_size
       2 * double_size
     end
   end

   class Array
     include QuadrupleSizeTrait
   end

   p(Array::new(10).quadruple_size + 2)


   jib:~ > ruby a.rb
   42

and ruby itself already ships with many mixins that meet these criteria.
except for part 4 above, which i don't really get - mixins sure be, at least, a
super-set of traits aren't they?  they would cease to be traits if you set and
instance var or something... but that's quibbling no?

i guess you could do someting with the 'parameterize' bit by requiring the
methods used within traits to be specified in some ctor in a factory type setup
- but why bother to confine them that way instead of just using duck typing on
the object the trait is used in?

-a
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