From: Saynatkari <ruby-ml@...>
Date: 2005-04-02T11:23:45+09:00
Subject: Re: Two Questions about Hash Defaults


Con fecha 2/4/2005, "Pete Elmore" <pete@petta-tech.com> escribi�:
>I have a couple of questions about hashes.  The first is fairly simple:
>is there a way to set a default so that when the value for a key is
>changed, the object it points to is default.dup instead of default?  So
>that if I do something like this:
>
>lists = Hash.new([])
>lists[:a].push 1
>lists[:b].push 2
>#  Then doing the following
>p lists[:a]
>#  Gets me
>=> [1]
>#  Instead of
>=> [1, 2]
>
>And 'lists' is '{:a => [1], :b => [2]}' instead of '{}' with a default
>of '[1, 2]'.
>
>I know why it happens this way, but is it possible to get it to behave
>differently?  So that instead of
>	lists[key] = [] if lists[key].nil?  #  Assuming default is nil
>	lists[key].push value
>or
>	lists[key] = lists[key].dup.push value # default is []
>I can write simply
>	lists[key].push value # default is []
>
>The second question is about setting defaults from one hash based on
>another.  This question is more along the lines of 'Does this code
>violate Ruby sensibilities?'
>
>class Hash
>	def add_defaults(default_hash)
>		raise TypeError unless default_hash.kind_of? Hash
>		default_hash.each { |k,v|
>			self[k] = v unless self.has_key? k
>		}
>		self
>	end
>end
>
>def some_function(str, options=Hash.new(nil))
>	#  Set the defaults
>	defaults = {
>		:preserve_breaks => true,
>		:indent => 0,
>		:columns => 79,
>	}
>	options.add_defaults defaults
>end
>
>some_function("asdf", { :preserve_breaks => false })

Looks like matz already provided you with a good answer,
but here's a different take since I am not sure if you
are indeed referring to default values as the language
defines them or 'just' default values for your application
logic:

def some_function(str, options = {})
  defaults = {:preserve => true, :indent => 0, :columns =>79}
  options  = defaults.merge options
end