After a long break, an apple, a cup of coffee, and a beer, I’m back in the Perl track.
The full title of this session is, Scratching the 40-Year Itch of Inheritance with Smalltalk-style Traits.
This is not a tutorial. How to use traits is easy, but why to use them is a more complex discussion.
Inheritance is a very complex problem and an easy one to get wrong. Then people start doing things with multiple inheritance and, even if they’re not doing something deliberately stupid, they end up with diamond inheritance. Not only is this a problem, but it’s been a problem for a very long time—40 years, in fact.
Complex systems can lead to deep class hierarchies. When hierarchies are deep, in particular with a dynamic language like Perl, it becomes difficult to determine where a method came from. Even when its known where a method comes from, undesired behavior may be inherited. This becomes worse when multiple inheritance is used.
As systems grow, the problem becomes two-fold:
- Class responsibility – larger classes are desired
- Class reuse – smaller classes are desired
Inheritance, by itself, cannot solve this problem. So the solution is to
decouple the sub-problems.
Several solutions have been tried:
- Interfaces
- Delegation
- Mixins – incredibly popular
As expected by the name of this session, traits (or roles in the nomenclature of Moose) solve the problem far better than any of the above solutions. Much of the session involved showing real-world application of roles to clean up code at the BBC.