demeter-after-two.rb

dog.legs.walk! if dog.normal?
dog.hover_craft.hover! if dog.robot?

demeter-after.rb

class Dog
  def move!
    legs.walk!
  end
end

dog.move!

demeter-before.rb

dog.legs.walk!

if-must.rb

def sales_tax
  return 0 if purchaser.exempt_from_taxes?
  price * .10
end

def car.brake
  switch_on_brake_light
  unless stopped?
    slow_down
  end
end

law-of-7-bad.rb

def self.import_from_csv(csv)
  csv.each_line do |line|
    if line["type"] == "distributer"
      Document.new(line["foo"], line["retail_price"])
    else
      Document.new(line["foo"], line["wholesale_price"])
    end
  end
end

law-of-7-good.rb

def self.import_line(line)
  if line["type"] == "distributer"
    Document.new(line["foo"], line["retail_price"])
  else
    Document.new(line["foo"], line["wholesale_price"])
  end
end

def self.import_from_csv(csv)
  csv.each_line do |line|
    import_line(line)
  end
end

polymorphism-bad.rb

def expiry_date
  case plan_type
  when 'subscription' then next_month
  when 'prepaid' then never
  end
end

def monthly_charge
  case plan_type
  when 'subscription' then 300
  when 'prepaid' then 0
  end
end

def charge_per_rental
  case plan_type
  when 'subscription' then 0
  when 'prepaid' then 100
  end
end

polymorphism-good.rb

class SubscriptionPlan
  attributes :expiration_date => never, :monthly_charge => 300, :charge_per_rental => 0
end

class PrepaidPlan
  attributes :expiration_date => never, :monthly_charge => 300, :charge_per_rental => 0
end

def plan_type
  case plan_type
  when 'subscription' then SubscriptionPlan.new
  when 'prepaid' then PrepaidPlan.new
  end
end

def expiry_date
  plan_type.expiry_date
end

def monthly_charge
  plan_type.monthly_charge
end

def charge_per_rental
  plan_type.charge_per_rental
end

tell-dont-ask-bad.rb

if account.balance >= amount
  account.deduct(amount)
else
  raise NotEnoughBalance
end

class Account
  def deduct(amount)
    balance -= amount
  end
end

tell-dont-ask-good.rb

account.deduct(amount)

class Account
  def deduct(amount)
    raise NotEnoughBalance unless balance >= amount
    balance -= amount
  end
end