From: Sean O'Halpin <sean.ohalpin@...>
Date: 2006-07-15T20:22:25+09:00
Subject: Re: Alternate initializers or alternate class?

On 7/15/06, Daniel Schierbeck <daniel.schierbeck@gmail.com> wrote:
> transfire@gmail.com wrote:
> > Jan Molic recently contacted me about a slighlty modified version of
> > his OrderedHash class that I distribute in Facets, the Dictionary
> > class. There are a few variations of this class initself and Jan
> > originally provided these via a subclass. I, on the other, thought
> > alternate initializers would be better.
> >
> > Which is the best approach?
> >
> > Ex.
> >
> >   class AutoOrderHash < OrderedHash
> >
> >      def new(*args)
> >         super(*args){ |h,k| h[k] = self.class.new }
> >      end
> >
> >   end
> >
> > vs.
> >
> >   class OrderHash
> >
> >      def self.auto(*args)
> >         new(*args){ |h,k| h[k] = self.class.new }
> >      end
> >
> >   end
>
> I actually favor a third approach; keyword argument options.
>
>    OrderHash.new(:auto? => true)
>
> I think Rails has shown us that such option hashes are extremely
> powerful and flexible.
>
>
> Just my $.02
> Daniel
>
I'm a big proponent of keyword arguments (which we implement in the
current version of Ruby using option hashes). However, I'm not sure
it's appropriate for this usage, i.e. selecting which kind of object
is created.

You would have to select which alternative initializer to call in
OrderHash.initialize and extract each different set of arguments
depending on which alternative initializer you're calling. It would
add unnecessary complexity and overhead. It also suffers from the
cognitive load of having to remember three things (class, selector,
which options go with which selector) instead of two (class,
initializer signature). Also, if you add another kind of OrderedHash,
you have to change OrderHash#initialize.

I'm all for keyword ~options~ in initializers which modify some aspect
of the newly created object. I'm just not sure you want to have the
initializer return different classes of object depending on those
options.

BTW, the keyword arguments technique has a long history before Rails
came along ;)

Regards,
Sean