Replace Side Effects with Return Values

best_payroll_runner.rb

class PayrollRunner
  def run!
    TaxEngineWrapper.new(tax_engine).apply(commands)
    computed_taxes = tax_engine.calculate_taxes
    payroll.assign_taxes(computed_taxes)
  end

  # This is now a pure function from the view of its callers.
  # Sweet!
  def commands
    tax_engine_spy = TaxEngineSpy.new

    # Commands are recorded against our tax_engine_spy,
    # which pretends to be a perfectly normal `tax_engine`.
    TaxEngineSetter::Factory.for(state).apply_state_info(
      tax_engine_spy,
      build_state_info()
    )

    [SetFederalInfoCommand.new(build_federal_info())] +
      tax_engine_spy.generated_commands
  end
end

class TaxEngineSpy
  def initialize
    @commands = []
  end

  # We did not end up using `method_missing` in on our code,
  # but rather listening for each specific method.
  # `method_missing` is difficult to test and didn’t provide
  # the level of safety we were looking for.
  def method_missing(name, *args)
    @commands << Command.new(name, *args)
  end

  def generated_commands
    @commands
  end
end

payroll_runner.rb

class PayrollRunner
  def run!
    federal_info = build_federal_info()
    state_info = build_state_info()

    tax_engine.set_federal_info(federal_info)
    tax_engine.set_state_info(state_info)

    computed_taxes = tax_engine.calculate_taxes
    payroll.assign_taxes(computed_taxes)
  end
end

payroll_runner_with_commands.rb

class PayrollRunner
  def run!
    federal_info = build_federal_info()
    state_info = build_state_info()

    commands = [
      SetFederalInfoCommand.new(federal_info),
      SetStateInfoCommand.new(state_info),
    ]

    TaxEngineWrapper.new(tax_engine).apply(commands)

    computed_taxes = tax_engine.calculate_taxes
    payroll.assign_taxes(computed_taxes)
  end
end

# This wrapper is designed to not change the API of something
# we don't control (the original tax engine), but allows
# us to apply our custom commands as normal method calls.
class TaxEngineWrapper
  def initialize(tax_engine)
    @tax_engine = tax_engine
  end

  def apply(commands)
    commands.each do |command|
      @tax_engine.public_send(
        command.method_name, *command.arguments
      )
    end
  end
end

real_payroll_runner.rb

class PayrollRunner
  def run!
    federal_info = build_federal_info()
    state_info = build_state_info()

    tax_engine.set_federal_info(federal_info)

    # With 50 states plus Washington, D.C., there are a lot of
    # classes on the other end of this factory.
    #
    # All of them expect to be interacting with the `tax_engine`
    # in an imperative fashion.
    TaxEngineSetter::Factory.for(state).apply_state_info(
      tax_engine, 
      state_info
    )

    computed_taxes = tax_engine.calculate_taxes
    payroll.assign_taxes(computed_taxes)
  end
end