uc-simple.rb

module Guillemets
  def inspect; "<<#{self}>>"; end
end

Value = Struct.new(:value) do
  include Guillemets
  def reducible?; false; end
end

Operation = Struct.new(:left, :right) do
  include Guillemets
  def self.operator(symbol = nil); @operator ||= symbol end
  def operator; self.class.operator; end
  def reducible?; true; end
  def to_s; "#{left} #{operator} #{right}"; end
  def reduce
    if left.reducible?
      self.class.new(left.reduce, right)
    elsif right.reducible?
      self.class.new(left, right.reduce)
    else
      Number.new(left.value.send(operator, right.value))
    end
  end
end

class Number < Value
  def to_s; "#{value}"; end
end

class Add < Operation
  operator :+
end

class Multiply < Operation
  operator :*
end

Machine = Struct.new(:expression) do
  def step
    self.expression = expression.reduce # => <<2 + 3 * 4>>, <<2 + 12>>, <<14>>
  end

  def run
    step while expression.reducible?
    expression
  end
end

expression = Add.new(
  Multiply.new(Number.new(1), Number.new(2)),
  Multiply.new(Number.new(3), Number.new(4))
) # => <<1 * 2 + 3 * 4>>

machine = Machine.new(expression)

machine.run # => <<14>>