double_dispatch.rb

#!/usr/bin/ruby

OPS = {}

def OPS.[](cmd, *arg)
  self.fetch([cmd, arg.first.class])[*arg]
end

Constant = Struct.new(:value) do
  def initialize(v)
    self[:value] = v.to_f
  end
end

BinaryPlus = Struct.new :lhs, :rhs

OPS[[:stringify, Constant]] = lambda {|c| v.value.to_s}
OPS[[:stringify, BinaryPlus]] = lambda {|c| "#{OPS[:stringify, lhs]} + #{OPS[:stringify, c.rhs]}"}

OPS[[:evaluate, Constant]] = lambda {|c| c.value}
OPS[[:evaluate, BinaryPlus]] = lambda {|c| OPS[:evaluate, c.lhs] + OPS[:evaluate, c.rhs]}

tree = BinaryPlus[Constant[1.2], Constant[2.5]]

p tree

puts OPS[:evaluate, tree]