I guess I independently invented this idea a few years back, but instead of the word 'trait' I used the word 'quality.' I use the term 'attach' for adding a quality to a given object instance, because I also support removing (via 'detach') qualities at runtime also. My qualities do support state information, which does constrain the object model to support that.

(I'll be digging into the linked PDF and other folks' ideas here to see how I can improve my own model. Thanks!)

My implementations in Java supported runtime attachment and detachment of qualities, but my draft Perl implementation has been a little heavy and inelegant so far.

These are very handy in design of various roleplaying games; you can add the "edible" quality/trait to an object and it now supports the verb 'eat.'

If classes are nouns, qualities/traits are adjectives.

Once you get used to the idea of qualities/traits, your class hierarchy is a lot different: most classes are really just groups of qualities without much to do directly. For example, a typical player-character in a MUD, just after drinking a magical potion, might look like:

class Human
      is Mammal
      with Bipedal, Sentient, Inventoried, Skilled, Clothed

instance 3453
      is Human
      with Character, Flying, Enchanted
[download]

After the flying-potion enchantment wears off, just remove Enchanted, which in turn can remove Flying.