From: Phrogz <phrogz@...>
Date: 2007-11-09T06:40:09+09:00
Subject: Re: standard, pretty, and flexible meta-methods?

On Nov 8, 2:29 pm, Phrogz <phr...@mac.com> wrote:
> On Nov 8, 1:44 pm, Greg Weeks <gregwe...@shaw.ca> wrote:
>
> > I'm still curious about the most common practice, if there is one,
> > specifically about creating little "mini-languages" that can be used
> > inside class definitions.  (Yes, I'm wondering about Rails.)  My example
> > with explicit class methods now smells pretty bad to me, for example.
> > Could modules (using "define_method" and other magic) "extended" into
> > class objects be considered the norm?  This is a more vague question,
> > but I'm putting it out FWIW.
>
> It's what Rails and its component libraries mostly do. For example:

Actually, I would like to change my answer:

The various Rails libraries generally define class methods as
'instance' methods in modules and extend classes with those modules to
add the class methods to various classes.

However, as you've no doubt seen in rails, most of the end user
functionality of the framework is achieved by subclassing (as with my
example based on ActiveRecord). The Rails programmer does not write:

  class Person
    include ActiveRecord::Validations
    include ActiveRecord::Locking::Optimistic
    include ActiveRecord::Locking::Pessimistic
    include ActiveRecord::Callbacks
    include ActiveRecord::Observing
    include ActiveRecord::Timestamp
    include ActiveRecord::Associations
    include ActiveRecord::Aggregations
    include ActiveRecord::Transactions
    include ActiveRecord::Reflection
    include ActiveRecord::Acts::Tree
    include ActiveRecord::Acts::List
    include ActiveRecord::Acts::NestedSet
    include ActiveRecord::Calculations
    include ActiveRecord::XmlSerialization
    include ActiveRecord::AttributeMethods

    # phew! time to actually code stuff up
  end

So perhaps you don't care *how* the class methods were placed in the
parent class.

I would say:

* Define the class methods in the class itself if you only need them
for that class.

* Define the class methods in a common parent class if a strict
hierarchy suits your needs.

* Define the class methods in a module if you want to be able to pick
and choose which sets of class methods to bring into a particular
class.