Example of creaing an object oriented calculator in ruby

calculator.rb

require 'rubygems'
require 'pry'

class Calculation
  attr_reader :input
  
  def initialize(*args)
    @input = args
    @result = calculate
  end
  
  def result
    @result
  end
end

class Add < Calculation
  OPERAND = :+
  
  def calculate
    @input.reduce(&OPERAND)
  end
end

# Add.new(1,2,3,4).result
# => 10

class Subtract < Calculation
  OPERAND = :-
  
  def calculate
    @input.reduce(&OPERAND)
  end
end

# Subtract.new(100, 10, 10).result
# => 80

class Divide < Calculation
  OPERAND = :/
  
  def calculate
    @input.reduce(&OPERAND)
  end
end

# Divide.new(100, 2, 2).result
# => 25

class Multiply < Calculation
  OPERAND = :*
  
  def calculate
    @input.reduce(&OPERAND)
  end
end

# Multiply.new(100, 2, 2).result
# => 400

# now we can define a calculator class to encapsulate the responsibility for performing
# the calculations, much like how a real calculator works

class Calculator
  def initialize
    clear!
  end
  
  def add(*args)
    @calculations << Add.new(@result, *args)
    @result = @calculations.last.result
    self
  end
  
  def subtract(*args)
    @calculations << Subtract.new(@result, *args)
    @result = @calculations.last.result
    self
  end
  
  def divide(*args)
    @calculations << Divide.new(@result, *args)
    @result = @calculations.last.result    
    self
  end
  
  def multiply(*args)
    @calculations << Multiply.new(@result, *args)
    @result = @calculations.last.result    
    self
  end
  
  def clear!
    @calculations = []
    @result = 0
  end
  
  def result
    @result
  end
  
  def print!
    @calculations.each do |calculation|
      str = calculation.input.map(&:to_s).join(" #{calculation.class::OPERAND} ")
      puts str
    end
  end
end

# screen = Calculator.new.add(1,2,3,4).add(1,2,3,4)
# screen.result
# => 20
# screen.multiply(2)
# => 40
# screen.multiply(2, 2)
# => 160
# screen.divide(16)
# => 10

#Now what if we want to see the trail of commands we have ran?
# 0 + 1 + 2 + 3 + 4
# 10 + 1 + 2 + 3 + 4
# 20 * 2 * 2
# 80 * 2
# 160 / 16

binding.pry