From: Joel VanderWerf <vjoel@...>
Date: 2004-11-14T11:47:16+09:00
Subject: Re: [ANN] Copland to Needle article on RubyGarden (LONG)

Jamis Buck wrote:
> Joel VanderWerf wrote:
...
>> There are other service models that are very easy to construct with
>> methods, but I'm not aware of an equivalent construct in Needle:
>>
>>   def printer(kind)
>>     @printers ||= {}
>>     @printers[kind] = Printer.new(kind)
>>   end
>>
>>   def printer_for_code_files
>>     @printer_for_code_files ||= printer(:monochrome)
>>   end
>>
>>   def printer_for_images
>>     @printer_for_images ||= printer(:color)
>>   end
>>
>> It might be possible to do the above in Needle using Pipelines.[2]
> 
> 
> Hmm. I wouldn't call this a service model so much as a factory service. 
> In fact, I have a new favorite way of doing this kind of thing, as of 
> this morning. To complicate things a bit, suppose that the Printer class 
> needs to be dependency injected (with a logger instance, for example), 
> and the following approach really shines:
> 
>   reg.define do |b|
>     b.printer do |ctr,info|
>       logger = ctr[:logs].get( info )
>       lambda { |kind| Printer.new( logger, kind ) }
>     end
> 
>     b.code_printer { |ctr,| ctr.printer.call( :monochrome ) }
>     b.image_printer { |ctr,| ctr.printer.call( :color ) }
>   end
> 
> I've actually incorporated this approach into various places in the 
> upcoming new release of Net::SSH, and it works beautifully.

That captures the parameterization, but it doesn't capture the 
multiton[1] service model of my code. A unique instance is generated for 
each distinct argument list (distinct in the sense of hash keys, which 
is to say #eql?). What's the most natural way to do that with Needle?

--

[1] http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-talk/67899
  (ruby-talk.org is down right now).