Example of the difference between a side effect free method and a method with side effects.

no_side_effects.rb

class Value
  attr_reader :value
  def initialize(value)
    @value = value 
  end
  
  def add(i)
    class.self.new(i + @value)
  end
end

value = Value.new(1)
value2 = value.add(1)

value.value == 1
value2.value == 2

side_effects.rb

class Value
  def initialize(value)
    @value = value
  end

  def add(i)
    @value = i + @value
  end
end

value = Value.new(1)
value.add(1)
value.value = 2