From: Daniel Schierbeck <daniel.schierbeck@...>
Date: 2006-07-15T21:10:12+09:00
Subject: Re: Alternate initializers or alternate class?

Sean O'Halpin wrote:
> 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 ;)

Who said anything about multiple classes?

   class OrderHash
     def initialize(*args)
       options = args.shift.to_hash if args.last.respond_to? :to_hash
       if options and options.has_key? :auto?
         do_something_with{|hsh, key| hsh[key] = self.class.new}
       end
     end
   end

I haven't yet read through the OrderHash/Dictionary code, so it might be 
better to put it in ::new


Cheers,
Daniel