What is the best way to over-ride methods in an instance?
class DataWrapper
  attr_reader :foo, :bar
end

case :xml
   @data1.to_xml.foo #foo = 4
case :web_table
   @data1.to_web_table.foo #foo = "four"

array.each{ |o| o.extend(MyModule) }

Nib = Struct.new :val do
  def to_s
    value # Must be implemented by instance/subclass
  end
end

module Nib::Precise
  def value; "%.1f" % val; end
end

module Nib::Rough
  def value; val.round.to_s; end
end

module Nib::Ballpark
  def value; ((val/10).round * 10).to_s; end
end

nibs = [ Nib.new(33.7), Nib.new(29.1) ]

nibs.each{ |n| n.extend(Nib::Precise) }
p nibs.join(", ") #=> "33.7, 29.1"

nibs.each{ |n| n.extend(Nib::Rough) }
p nibs.join(", ") #=> "34, 29"

nibs.each{ |n| n.extend(Nib::Ballpark) }
p nibs.join(", ") #=> "30, 30"

case :xml
  @data1.foo.to_xml #foo = 4
case :web_table
  @data1.foo.to_web_table #foo = "four"
....