gistfile1.rb

class Hash
  def hash_map()
    result = self.inject({}) do |hash, (k, v)|
      hash.merge(yield(k, v))
    end
  end
end

{:a => 1, :b => 2}.hash_map {|k,v| {k.to_s*2, v*2}} # => {"aa"=>2, "bb"=>4}