From: "David A. Black" <dblack@...>
Date: 2009-10-27T00:45:22+09:00
Subject: Re: Setting instance variables from hash parameters (with defaults)

Hi --

On Tue, 27 Oct 2009, Leslie Viljoen wrote:

> Hi!
>
> I'm sure I might be reinventing the wheel here.
> I was writing the following ugly code in order to set options (with
> defaults) from a hash:
>
> 	def initialize(opts = {})
> 		@select_max = opts.has_key?(:select_max) ? opts[:select_max] : 10000
> 		@select_try = opts.has_key?(:select_try) ? opts[:select_try] : 1000
> 		@min_sleep_sec = opts.has_key?(:min_sleep_sec) ? opts[:min_sleep_sec] : 5
> 		@max_sleep_sec = opts.has_key?(:max_sleep_sec) ? opts[:max_sleep_sec] : 1800
> 		@default_sleep = opts.has_key?(:default_sleep) ? opts[:default_sleep] : 10
> ....
>
> So to avoid that I wrote:
>
> module Defaulting
> 	def set_params(defs, params)
> 		defs.each do |name, val|
> 			if params.has_key?(name)
> 				eval "@#{name} = params[name]"
> 			else
> 				eval "@#{name} = val"
> 			end
> 		end
> 	end
> end
>
> So that I could do the much nicer:
>
> 	include Defaulting
> 	def initialize(opts = {})
> 		defs = {
> 			:select_max => 10000,
> 			:select_try => 1000,
> 			:min_sleep_sec => 5,
> 			:max_sleep_sec => 1800,
> 			:default_sleep => 10
> 		}
> 		set_params(defs, opts)
>
> ....
>
> Note that this allows me to pass options that are explicitly set to nil.
>
> Now:
> 1. Is this functionality already tucked away somewhere else?
> 2. How can I get rid of those nasty evals?

Starting with #2: you can always do:

   instance_variable_set("@#{name}", value)

For the initialize thing, I would probably do something like this:

   class Whatever
     DEFAULTS = {
                  :select_max => 10000,
                  :select_try => 1000,
                  :min_sleep_sec => 5,
                  :max_sleep_sec => 1800,
                  :default_sleep => 10
                }

     def initialize(opts)
       DEFAULTS.update(opts).each do |name, value|
         instance_variable_set("@#{name}", value)
       end
     end
   end

Another thing to keep in mind for similar cases is that hashes return
nil (unless you override the default) for non-existent keys. So,
unless you have a hash where nil might be a valid value, you can do:

   h[x] ||= y

rather than checking for a key.


David

-- 
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