From: "David A. Black" <dblack@...>
Date: 2005-04-12T07:22:56+09:00
Subject: Re: Factory design

Hi --

On Tue, 12 Apr 2005, Claus Spitzer wrote:

> Greetings!
> A friend of mine has recently started using Ruby, and has run into a
> little problem. He is trying to create different objects depending on
> the contents of a string. His intuition is to use factory design for
> this, and he'd like to know if there is a Ruby Way to do this.
>
> Below is his original e-mail.
> Regards...
> -CWS
>
> ----8<----
>
> In any case, I have an identifier on my log file that looks like "2005-01-04
> 23:34:12 begin_phase".  If I see this, I want to create a "Phase" object.  If
> it says "2005-01-04 23:52:16 begin_interruption", then I want to create an
> interruption instead.
>
> My intuition is that this is a typical Factory Design Pattern, so I would
> create a Ruby class method that creates the appropriate type based on the
> results of  the string and returns it.  However, someone familiar with Ruby
> told me that Ruby's dynamic typing and duck typing is such that this kind of
> solution "feels incorrect".
>
> Does anyone have suggestions as to what I can do?  Thanks!

I wouldn't worry about the matter of duck typing here.  The concept of
duck typing, as I understand it, is essentially an explanatory tool
for getting people to understand, visualize, and make use of the fact
that class and type are not the same as each other in Ruby (a fact
that can be summed up very quickly but that in fact has huge
ramifications).  Duck typing doesn't mean you never instantiate
objects of a particular class, nor that you never parse a string :-)

For this project, I don't think any (webbed) toes will be stepped on
if you do something like:

   require 'scanf'

   File.open("input.dat") do |fh|
     fh.scanf("%d-%d-%d %d:%d:%d begin_%s") do |y,mo,d,h,mi,s,klass|
       c = Object.const_get(klass.capitalize).new
       # etc.
     end
   end

(to illustrate with a simple line-by-line treatment).


David

-- 
David A. Black
dblack@wobblini.net