Chain-of-responsibility pattern in Ruby

CORP.rb

module PurchasePower
  BASE = 500

  private

  attr_accessor :successor
end

class Manager
  include PurchasePower

  ALLOWABLE = 10 * BASE

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("Manager will approve $#{request.amount}") 
      return
    elsif !successor.nil?
      successor.process_request(request)
    end
  end
end

class Director
  include PurchasePower

  ALLOWABLE = 20 * BASE

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("Director will approve $#{request.amount}")
      return
    elsif !successor.nil?
      successor.process_request(request)
    end
  end
end

class VicePresident
  include PurchasePower

  ALLOWABLE = 40 * BASE

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("VicePresident will approve $#{request.amount}")
      return
    elsif !successor.nil?
      successor.process_request(request)
    end
  end
end

class President
  include PurchasePower

  ALLOWABLE = 60 * BASE

  def process_request(request)
    if request.amount < ALLOWABLE
      puts("President will approve $#{request.amount}")
      return
    else
      puts "Your request for $#{request.amount} needs a board meeting!"
    end
  end
end

class PurchaseRequest
  def initialize(number, amount, purpose)
    @number = number
    @amount = amount
    @purpose = purpose
  end

  def amount
    @amount
  end
end

class Approvers
  def initialize(*approvers)
    @approvers = approvers.map(&:new)
  end

  def add_approver(approver)
    @approvers << approver
  end

  def delete_approver(approver)
    @approvers.delete(approver)
  end

  def approvers_list
    @approvers
  end
end

class CheckAuthority
  def initialize(approvers)
    @approvers         = approvers
    @starting_approver = build_chains.first
  end

  def checking
    loop do
      $stdout.flush
      puts "Enter the amount to check who should approve your expenditure."
      puts ">>"
      @starting_approver.process_request PurchaseRequest.new(0, gets.to_i, 'General')
    end
  end

  private

  def build_chains
    @approvers.each_cons(2).map { |prev_approver, next_approver|  prev_approver.send :successor=, next_approver }
  end
end

approvers = Approvers.new(Manager, Director, VicePresident, President)
CheckAuthority.new(approvers.approvers_list).checking

output

Arup-iMac:arup_ruby shreyas$ ruby a.rb
Enter the amount to check who should approve your expenditure.
>>
1200
Director will approve $1200
Enter the amount to check who should approve your expenditure.
>>
2000
Director will approve $2000
Enter the amount to check who should approve your expenditure.
>>
^[[A^[[B
Director will approve $0
Enter the amount to check who should approve your expenditure.
>>
3000
Director will approve $3000
Enter the amount to check who should approve your expenditure.
>>
40000
Your request for $40000 needs a board meeting!
Enter the amount to check who should approve your expenditure.
>>
4000
Director will approve $4000
Enter the amount to check who should approve your expenditure.
>>
30000
Your request for $30000 needs a board meeting!
Enter the amount to check who should approve your expenditure.
>>
10000
VicePresident will approve $10000
Enter the amount to check who should approve your expenditure.
>>

# this is written by @jhass. http://paste.mrzyx.de/pmpv58f1t/un5vrr/raw
module PurchasePower
  BASE = 500
  attr_reader :successor

  def initialize successor
    @successor = successor
  end

  def process_request request
    if request.amount < self.class::ALLOWABLE
      approve_request request
    elsif successor
      successor.process_request request
    else
      deny_request request
    end
  end
end

class Manager
  include PurchasePower

  ALLOWABLE = 10 * BASE

  def approve_request request
    puts "Manager will approve $#{request.amount}"
  end
end

class Director
  include PurchasePower

  ALLOWABLE = 20 * BASE

  def approve_request request
    puts "Director will approve $#{request.amount}"
  end
end

class VicePresident
  include PurchasePower

  ALLOWABLE = 40 * BASE

  def approve_request request
    puts "VicePresident will approve $#{request.amount}"
  end
end

class President
  include PurchasePower

  ALLOWABLE = 60 * BASE

  def approve_request request
    puts "President will approve $#{request.amount}"
  end

  def deny_request request
    puts "Your request for $#{request.amount} needs a board meeting!"
  end
end

class PurchaseRequest
  attr_reader :amount

  def initialize(number, amount, purpose)
    @number = number
    @amount = amount
    @purpose = purpose
  end
end

class CheckAuthority
  def initialize(starting_approver)
    @starting_approver = starting_approver
  end

  def check request
    @starting_approver.process_request request
  end
end

class CLI
  def initialize
    approvers = build_approvers(Manager, Director, VicePresident, President)
    @authority = CheckAuthority.new(approvers.first)
  end

  def run
    loop do
      $stdout.flush
      puts "Enter the amount to check who should approve your expenditure."
      print ">> "
      amount = gets.to_i
      @authority.check PurchaseRequest.new(0, amount, 'General')
    end
  end

  private def build_approvers(*approver_classes)
    [].tap do |approvers|
      approver_classes.reverse.inject(nil) {|successor, approver|
        approver.new(successor).tap {|approver| approvers.unshift approver }
      }
    end
  end
end

CLI.new.run