From: Pete Elmore <pete@...>
Date: 2005-04-02T10:28:12+09:00
Subject: Two Questions about Hash Defaults

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 })