From: David Masover <ninja@...>
Date: 2009-10-29T23:19:21+09:00
Subject: Re: Setting instance variables from hash parameters (with defaults)

On Monday 26 October 2009 10:42:01 am Leslie Viljoen wrote:
> I see this is a shorter way:
>
> module Defaulting
> 	def set_params(defs, params)
> 		defs.merge(params).each {|name, val| eval "@#{name} = val"}
> 	end
> end
>
> ..still, any way to get rid of that eval?

You want instance_variable_set:

module Defaulting
  def set_params def, params
    defs.merge(params).each {|name, val| instance_variable_set name, val}
  end
end

I can think of a few ways to make it easier to use, and I'd suggest actually 
calling the name= method, rather than setting the instance variable directly. 
Here's a rough sketch:

module Defaulting
  def init_with_vars *vars, &init_block
    defaults = vars.last.kind_of?(Hash) ? vars.pop : {}
    vars = (vars + defaults.keys).uniq
    attr_accessor *vars
    define_method :initialize do |*args, &block|
      if args.last.kind_of? Hash
        defaults.merge(args).each_pair {|name, val| self.send("#{name}=", val) 
}
      end
      if init_block
        init_block.call *args, &block
      end
    end
  end
end

I'm fairly sure there's something like this already, some combination of 
something like Struct could work. I do like this usage, though:

class Foo
  include Defaulting
  init_with_vars :foo, :bar, :baz => 'default baz'
  def bar
    'overriding default bar reader'
  end
end

Mostly because for so many classes, I don't need an initialize method at all, 
except as a convenience to set up variables I know I'll need.