Embracing Functional Programming in Ruby

extracted_slim_taxes.rb

def taxes
  PayrollCalculator::Taxes.calculate(
    @payroll.only_pay_and_location_data
  )
end

extracted_taxes.rb

def taxes
  PayrollCalculator::Taxes.calculate(@payroll)
end

method.rb

def paystubs
  calculate_paystubs(taxes, ...)
end

def debits
  calculate_debits(taxes, ...)
end

def taxes
  @taxes ||= calculate_taxes(@payroll)
end

more_taxes.rb

def taxes
  federal_taxes(@payroll) +
    california_taxes(@payroll) +
    new_york_taxes(@payroll) +
    local_california_taxes(@payroll) +
    local_new_york_taxes(@payroll)
end

payroll.rb

# app/models/payroll.rb
class Payroll < ActiveRecord::Base
end

# app/services/payroll_calculator/payroll.rb
class PayrollCalculator::Payroll < ValueObject
end

payroll_calculator.rb

class PayrollCalculator
  def self.calculate(payroll)
    new(payroll).calculate
  end

  def initialize(payroll)
    @payroll = payroll
  end
  private_class_method :new

  def calculate
    PayrollResult.new(
      payroll: payroll,
      paystubs: paystubs,
      taxes: taxes,
      debits: debits
    )
  end

  def paystubs
    # ...
  end

  def taxes
    # ...
  end

  def debits
    # ...
  end
end

taxes.rb

def taxes
  federal_taxes(@payroll) +
    california_taxes(@payroll) +
    local_taxes(@payroll)
end