Clean Code concepts adapted for Ruby.

Inspired by clean-code-javascript.

Note: The examples are largely ported over from JavaScript so they may not be idiomatic. Feel free to point out any non-idiomatic Ruby code by submitting an issue and I'll correct it right away. Also, pull requests are always welcome!

1. Introduction
2. Variables
3. Methods
4. Objects and Data Structures
5. Classes
6. SOLID
7. Testing
8. Error Handling
9. Formatting
10. Comments
11. Translations

Software engineering principles, from Robert C. Martin's book Clean Code, adapted for Ruby. This is not a style guide. It's a guide to producing readable, reusable, and refactorable software in Ruby.

Not every principle herein has to be strictly followed, and even fewer will be universally agreed upon. These are guidelines and nothing more, but they are ones codified over many years of collective experience by the authors of Clean Code.

Our craft of software engineering is just a bit over 50 years old, and we are still learning a lot. When software architecture is as old as architecture itself, maybe then we will have harder rules to follow. For now, let these guidelines serve as a touchstone by which to assess the quality of the Ruby code that you and your team produce.

One more thing: knowing these won't immediately make you a better software developer, and working with them for many years doesn't mean you won't make mistakes. Every piece of code starts as a first draft, like wet clay getting shaped into its final form. Finally, we chisel away the imperfections when we review it with our peers. Don't beat yourself up for first drafts that need improvement. Beat up the code instead!

Bad:

yyyymmdstr = Time.now.strftime('%Y/%m/%d')

Good:

current_date = Time.now.strftime('%Y/%m/%d')

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Pick one word for the concept and stick to it.

Bad:

user_info
user_data
user_record

starts_at
start_at
start_time

Good:

user

starts_at

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We will read more code than we will ever write. It's important that the code we do write is readable and searchable. By not naming variables that end up being meaningful for understanding our program, we hurt our readers. Make your names searchable.

Also, instead of hardcoding values and using "magic numbers", create constants.

Bad:

# What the heck is 86400 for?
status = Timeout::timeout(86_400) do
  # ...
end

Good:

# Declare them as capitalized globals.
SECONDS_IN_A_DAY = 86_400

status = Timeout::timeout(SECONDS_IN_A_DAY) do
  # ...
end

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Bad:

address = 'One Infinite Loop, Cupertino 95014'
city_zip_code_regex = /^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$/
save_city_zip_code(city_zip_code_regex.match(address)[1], city_zip_code_regex.match(address)[2])

Good:

address = 'One Infinite Loop, Cupertino 95014'
city_zip_code_regex = /^[^,\\]+[,\\\s]+(.+?)\s*(\d{5})?$/
_, city, zip_code = city_zip_code_regex.match(address).to_a
save_city_zip_code(city, zip_code)

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Explicit is better than implicit.

Bad:

locations = ['Austin', 'New York', 'San Francisco']
locations.each do |l|
  do_stuff
  do_some_other_stuff
  # ...
  # ...
  # ...
  # Wait, what is `l` for again?
  dispatch(l)
end

Good:

locations = ['Austin', 'New York', 'San Francisco']
locations.each do |location|
  do_stuff
  do_some_other_stuff
  # ...
  # ...
  # ...
  dispatch(location)
end

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If your class/object name tells you something, don't repeat that in your variable name.

Bad:

car = {
  car_make: 'Honda',
  car_model: 'Accord',
  car_color: 'Blue'
}

def paint_car(car)
  car[:car_color] = 'Red'
end

Good:

car = {
  make: 'Honda',
  model: 'Accord',
  color: 'Blue'
}

def paint_car(car)
  car[:color] = 'Red'
end

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Default arguments are often cleaner than short circuiting. Be aware that if you use them, your method will only provide default values for undefined arguments. Other "falsy" values such as false and nil will not be replaced by a default value.

Bad:

def create_micro_brewery(name)
  brewery_name = name || 'Hipster Brew Co.'
  # ...
end

Good:

def create_micro_brewery(brewery_name = 'Hipster Brew Co.')
  # ...
end

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Limiting the amount of method parameters is incredibly important because it makes testing your method easier. Having more than three leads to a combinatorial explosion where you have to test tons of different cases with each separate argument.

One or two arguments is the ideal case, and three should be avoided if possible. Anything more than that should be consolidated. Usually, if you have more than two arguments then your method is trying to do too much. In cases where it's not, most of the time a higher-level object will suffice as an argument. Or you can pass data to the method by instance variables.

Since Ruby allows you to make objects on the fly, without a lot of class boilerplate, you can use an object if you are finding yourself needing a lot of arguments. The prevailing pattern in Ruby is to use a hash of arguments.

To make it obvious what properties the method expects, you can use the keyword arguments syntax (introduced in Ruby 2.1). This has a few advantages:

1. When someone looks at the method signature, it's immediately clear what properties are being used.
2. If a required keyword argument is missing, Ruby will raise a useful ArgumentError that tells us which required argument we must include.

Bad:

def create_menu(title, body)
  # ...
end

create_menu('Foo', 'Bar')

Good:

def create_menu(title:, body:)
  # ...
end

create_menu(title: 'Foo', body: 'Bar')

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This is by far the most important rule in software engineering. When methods do more than one thing, they are harder to compose, test, and reason about. When you can isolate a method to just one action, they can be refactored easily and your code will read much cleaner. If you take nothing else away from this guide other than this, you'll be ahead of many developers.

Bad:

def email_clients(clients)
  clients.each do |client|
    client_record = database.lookup(client)
    email(client) if client_record.active?
  end
end

email_clients(clients)

Good:

def email_clients(clients)
  clients.each { |client| email(client) }
end

def active(objects)
  objects.select { |object| active?(object) }
end

def active?(object)
  record = database.lookup(object)
  record.active?
end

email_clients(active(clients))

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Poorly named methods add to the code reviewer's cognitive load at best, and mislead the code reviewer at worst. Strive to capture the precise intent when naming methods.

Bad:

def add_to_date(date, month)
  # ...
end

date = DateTime.now

# It's hard to tell from the method name what is added
add_to_date(date, 1)

Good:

def add_month_to_date(date, month)
  # ...
end

date = DateTime.now
add_month_to_date(date, 1)

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When you have more than one level of abstraction your method is usually doing too much. Splitting up methods leads to reusability and easier testing. Furthermore, methods should descend by the level of abstraction: one very abstract method should call methods that are less abstract and so on.

Bad:

def interpret(code)
  regexes = [
    # ...
  ]
  statements = code.split(' ')

  tokens = regexes.each_with_object([]) do |regex, memo|
    statements.each do |statement|
      # memo.push(...)
    end
  end

  ast = tokens.map do |token|
    # ...
  end

  ast.map do |node|
    # ...
  end
end

Good:

def interpret(code)
  tokens = tokenize(code)
  ast = lex(tokens)
  parse(ast)
end

def tokenize(code)
  regexes = [
    # ...
  ]
  statements = code.split(' ')

  regexes.each_with_object([]) do |regex, tokens|
    statements.each do |statement|
      # tokens.push(...)
    end
  end
end

def lex(tokens)
  tokens.map do |token|
    # ...
  end
end

def parse(ast)
  ast.map do |node|
    # ...
  end
end

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Do your absolute best to avoid duplicate code. Duplicate code is bad because it means that there's more than one place to alter something if you need to change some logic.

Imagine if you run a restaurant and you keep track of your inventory: all your tomatoes, onions, garlic, spices, etc. If you have multiple lists that you keep this on, then all have to be updated when you serve a dish with tomatoes in them. If you only have one list, there's only one place to update!

Oftentimes you have duplicate code because you have two or more slightly different things, that share a lot in common, but their differences force you to have two or more separate methods that do much of the same things. Removing duplicate code means creating an abstraction that can handle this set of different things with just one method/module/class.

Getting the abstraction right is critical, that's why you should follow the SOLID principles laid out in the Classes section. Bad abstractions can be worse than duplicate code, so be careful! Having said this, if you can make a good abstraction, do it! Don't repeat yourself, otherwise you'll find yourself updating multiple places anytime you want to change one thing.

Bad:

def show_developer_list(developers)
  developers.each do |developer|
    data = {
      expected_salary: developer.expected_salary,
      experience: developer.experience,
      github_link: developer.github_link
    }

    render(data)
  end
end

def show_manager_list(managers)
  managers.each do |manager|
    data = {
      expected_salary: manager.expected_salary,
      experience: manager.experience,
      portfolio: manager.mba_projects
    }

    render(data)
  end
end

Good:

def show_employee_list(employees)
  employees.each do |employee|
    data = build_data(employee)
    render(data)
  end
end

def build_data(employee)
  general_data = {
    expected_salary: employee.expected_salary,
    experience: employee.experience
  }

  general_data.merge(position_specific_data(employee))
end

def position_specific_data(employee)
  case employee.type
  when 'manager'
    { portfolio: employee.mba_projects }
  when 'developer'
    { github_link: employee.github_link }
  end
end

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Flags tell your user that this method does more than one thing. Methods should do one thing. Split out your methods if they are following different code paths based on a boolean.

Bad:

def create_file(name, temp)
  if temp
    fs.create("./temp/#{name}")
  else
    fs.create(name)
  end
end

Good:

def create_file(name)
  fs.create(name)
end

def create_temp_file(name)
  create_file("./temp/#{name}")
end

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A method produces side effects if it does anything more than take values and/or return values. A side effect could be writing to a file, modifying some global variable, or accidentally wiring all your money to a stranger.

Now, you do need to have side effects in a program on occasion. Like the previous example, you might need to write to a file. What you want to do is to centralize where you are doing this. Don't have several methods and classes that write to a particular file. Have one service that does it. One and only one.

The main point is to avoid common pitfalls like sharing state between objects without any structure, using mutable data types that can be written to by anything, and not centralizing where your side effects occur. If you can do this, you will be happier than the vast majority of other programmers.

Bad:

# Global variable referenced by following method.
# If we had another method that used this name, now it'd be an array and it could break it.
$name = 'Ryan McDermott'

def split_into_first_and_last_name
  $name = $name.split(' ')
end

split_into_first_and_last_name()

puts $name # ['Ryan', 'McDermott']

Good:

def split_into_first_and_last_name(name)
  name.split(' ')
end

name = 'Ryan McDermott'
first_and_last_name = split_into_first_and_last_name(name)

puts name # 'Ryan McDermott'
puts first_and_last_name # ['Ryan', 'McDermott']

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In Ruby, everything is an object and everything is passed by value, but these values are references to objects. In the case of objects and arrays, if your method makes a change in a shopping cart array, for example, by adding an item to purchase, then any other method that uses that cart array will be affected by this addition. That may be great, however it can be bad too. Let's imagine a bad situation:

The user clicks the "Purchase", button which calls a purchase method that spawns a network request and sends the cart array to the server. Because of a bad network connection, the purchase method has to keep retrying the request. Now, what if in the meantime the user accidentally clicks "Add to Cart" button on an item they don't actually want before the network request begins? If that happens and the network request begins, then that purchase method will send the accidentally added item because it has a reference to a shopping cart array that the add_item_to_cart method modified by adding an unwanted item.

A great solution would be for the add_item_to_cart to always clone the cart, edit it, and return the clone. This ensures that no other methods that are holding onto a reference of the shopping cart will be affected by any changes.

Two caveats to mention to this approach:

1. There might be cases where you actually want to modify the input object, but when you adopt this programming practice you will find that those cases are pretty rare. Most things can be refactored to have no side effects!

2. Cloning big objects can be very expensive in terms of performance. Luckily, this isn't a big issue in practice because there are great gems that allow this kind of programming approach to be fast and not as memory intensive as it would be for you to manually clone objects and arrays.

Bad:

def add_item_to_cart(cart, item)
  cart.push(item: item, time: Time.now)
end

Good:

def add_item_to_cart(cart, item)
  cart + [{ item: item, time: Time.now }]
end

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Ruby isn't a functional language in the way that Haskell is, but it has a functional flavor to it. Functional languages are cleaner and easier to test. Favor this style of programming when you can.

Bad:

programmer_output = [
  {
    name: 'Uncle Bobby',
    lines_of_code: 500
  }, {
    name: 'Suzie Q',
    lines_of_code: 1500
  }, {
    name: 'Jimmy Gosling',
    lines_of_code: 150
  }, {
    name: 'Grace Hopper',
    lines_of_code: 1000
  }
]

total_output = 0

programmer_output.each do |output|
  total_output += output[:lines_of_code]
end

Good:

programmer_output = [
  {
    name: 'Uncle Bobby',
    lines_of_code: 500
  }, {
    name: 'Suzie Q',
    lines_of_code: 1500
  }, {
    name: 'Jimmy Gosling',
    lines_of_code: 150
  }, {
    name: 'Grace Hopper',
    lines_of_code: 1000
  }
]

INITIAL_VALUE = 0

total_output = programmer_output.sum(INITIAL_VALUE) { |output| output[:lines_of_code] }

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Bad:

if params[:message].present? && params[:recipient].present?
  # ...
end

Good:

def send_message?(params)
  params[:message].present? && params[:recipient].present?
end

if send_message?(params)
  # ...
end

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Bad:

if !genres.blank?
  # ...
end

Good:

unless genres.blank?
  # ...
end

# or

if genres.present?
  # ...
end

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This seems like an impossible task. Upon first hearing this, most people say, "how am I supposed to do anything without an if statement?" The answer is that you can use polymorphism to achieve the same task in many cases. The second question is usually, "well that's great but why would I want to do that?" The answer is a previous clean code concept we learned: a method should only do one thing. When you have classes and methods that have if statements, you are telling your user that your method does more than one thing. Remember, just do one thing.

Bad:

class Airplane
  # ...
  def cruising_altitude
    case @type
    when '777'
      max_altitude - passenger_count
    when 'Air Force One'
      max_altitude
    when 'Cessna'
      max_altitude - fuel_expenditure
    end
  end
end

Good:

class Airplane
  # ...
end

class Boeing777 < Airplane
  # ...
  def cruising_altitude
    max_altitude - passenger_count
  end
end

class AirForceOne < Airplane
  # ...
  def cruising_altitude
    max_altitude
  end
end

class Cessna < Airplane
  # ...
  def cruising_altitude
    max_altitude - fuel_expenditure
  end
end

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Ruby is dynamically typed, which means your methods can take any type of argument. Sometimes you are bitten by this freedom and it becomes tempting to do type-checking in your methods. There are many ways to avoid having to do this. The first thing to consider is consistent APIs.

Bad:

def travel_to_texas(vehicle)
  if vehicle.is_a?(Bicycle)
    vehicle.pedal(@current_location, Location.new('texas'))
  elsif vehicle.is_a?(Car)
    vehicle.drive(@current_location, Location.new('texas'))
  end
end

Good:

def travel_to_texas(vehicle)
  vehicle.move(@current_location, Location.new('texas'))
end

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If you are working with basic values like strings and integers, and you can't use polymorphism but you still feel the need to type-check, you should consider using contracts.ruby. The problem with manually type-checking Ruby is that doing it well requires so much extra verbiage that the faux "type-safety" you get doesn't make up for the lost readability. Keep your Ruby clean, write good tests, and have good code reviews.

Bad:

def combine(val1, val2)
  if (val1.is_a?(Numeric) && val2.is_a?(Numeric)) ||
     (val1.is_a?(String) && val2.is_a?(String))
    return val1 + val2
  end

  raise 'Must be of type String or Numeric'
end

Good:

def combine(val1, val2)
  val1 + val2
end

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Dead code is just as bad as duplicate code. There's no reason to keep it in your codebase. If it's not being called, get rid of it! It will still be safe in your version history if you still need it.

Bad:

def old_request_module(url)
  # ...
end

def new_request_module(url)
  # ...
end

req = new_request_module(request_url)
inventory_tracker('apples', req, 'www.inventory-awesome.io')

Good:

def new_request_module(url)
  # ...
end

req = new_request_module(request_url)
inventory_tracker('apples', req, 'www.inventory-awesome.io')

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Using getters and setters to access data on objects could be better than simply looking for a property on an object. "Why?" you might ask. Well, here's an unorganized list of reasons why:

• When you want to do more beyond getting an object property, you don't have to look up and change every accessor in your codebase.
• Makes adding validation simple when doing a set.
• Encapsulates the internal representation.
• Easy to add logging and error handling when getting and setting.
• You can lazy load your object's properties, let's say getting it from a server.

Bad:

def make_bank_account
  # ...

  {
    balance: 0
    # ...
  }
end

account = make_bank_account
account[:balance] = 100
account[:balance] # => 100

Good:

class BankAccount
  def initialize
    # this one is private
    @balance = 0
  end

  # a "getter" via a public instance method
  def balance
    # do some logging
    @balance
  end

  # a "setter" via a public instance method
  def balance=(amount)
    # do some logging
    # do some validation
    @balance = amount
  end
end

account = BankAccount.new
account.balance = 100
account.balance # => 100

Alternatively, if your getters and setters are absolutely trivial, you should use attr_accessor to define them. This is especially convenient for implementing data-like objects which expose data to other parts of the system (e.g., ActiveRecord objects, response wrappers for remote APIs).

Good:

class Toy
  attr_accessor :price
end

toy = Toy.new
toy.price = 50
toy.price # => 50

However, you have to be aware that in some situations, using attr_accessor is a code smell, read more here.

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A Fluent interface is an object oriented API that aims to improve the readability of the source code by using method chaining.

While there can be some contexts, frequently builder objects, where this pattern reduces the verbosity of the code (e.g., ActiveRecord queries), more often it comes at some costs:

1. Breaks Encapsulation
2. Breaks Decorators
3. Is harder to mock in a test suite
4. Makes diffs of commits harder to read

For more information you can read the full blog post on this topic written by Marco Pivetta.

Bad:

class Car
  def initialize(make, model, color)
    @make = make
    @model = model
    @color = color
    # NOTE: Returning self for chaining
    self
  end

  def set_make(make)
    @make = make
    # NOTE: Returning self for chaining
    self
  end

  def set_model(model)
    @model = model
    # NOTE: Returning self for chaining
    self
  end

  def set_color(color)
    @color = color
    # NOTE: Returning self for chaining
    self
  end

  def save
    # save object...
    # NOTE: Returning self for chaining
    self
  end
end

car = Car.new('Ford','F-150','red')
  .set_color('pink')
  .save

Good:

class Car
  attr_accessor :make, :model, :color

  def initialize(make, model, color)
    @make = make
    @model = model
    @color = color
  end

  def save
    # Save object...
  end
end

car = Car.new('Ford', 'F-150', 'red')
car.color = 'pink'
car.save

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As stated famously in Design Patterns by the Gang of Four, you should prefer composition over inheritance where you can. There are lots of good reasons to use inheritance and lots of good reasons to use composition. The main point for this maxim is that if your mind instinctively goes for inheritance, try to think if composition could model your problem better. In some cases, it can.

You might be wondering then, "when should I use inheritance?" It depends on your problem at hand, but this is a decent list of when inheritance makes more sense than composition:

1. Your inheritance represents an "is-a" relationship and not a "has-a" relationship (Human->Animal vs. User->UserDetails).
2. You can reuse code from the base classes (Humans can move like all animals).
3. You want to make global changes to derived classes by changing a base class. (Change the caloric expenditure of all animals when they move).

Bad:

class Employee
  def initialize(name, email)
    @name = name
    @email = email
  end

  # ...
end

# Bad because Employees "have" tax data. EmployeeTaxData is not a type of Employee
class EmployeeTaxData < Employee
  def initialize(ssn, salary)
    @ssn = ssn
    @salary = salary
  end

  # ...
end

Good:

class EmployeeTaxData
  def initialize(ssn, salary)
    @ssn = ssn
    @salary = salary
  end

  # ...
end

class Employee
  def initialize(name, email)
    @name = name
    @email = email
  end

  def set_tax_data(ssn, salary)
    @tax_data = EmployeeTaxData.new(ssn, salary)
  end
  # ...
end

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As stated in Clean Code, "There should never be more than one reason for a class to change". It's tempting to jam-pack a class with a lot of functionality, like when you can only take one suitcase on your flight. The issue with this is that your class won't be conceptually cohesive and it will give it many reasons to change. Minimizing the number of times you need to change a class is important. It's important because if too much functionality is in one class and you modify a piece of it, it can be difficult to understand how that will affect other dependent modules in your codebase.

Bad:

class UserSettings
  def initialize(user)
    @user = user
  end

  def change_settings(settings)
    return unless valid_credentials?
    # ...
  end

  def valid_credentials?
    # ...
  end
end

Good:

class UserAuth
  def initialize(user)
    @user = user
  end

  def valid_credentials?
    # ...
  end
end

class UserSettings
  def initialize(user)
    @user = user
    @auth = UserAuth.new(user)
  end

  def change_settings(settings)
    return unless @auth.valid_credentials?
    # ...
  end
end

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As stated by Bertrand Meyer, "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification." What does that mean though? This principle basically states that you should allow users to add new functionalities without changing existing code.

In the "bad" example below adding another adapter would require changing HttpRequester class. This violates OCP.

Bad:

class AjaxAdapter
  attr_reader :name

  def initialize
    @name = 'ajaxAdapter'
  end
end

class NodeAdapter
  attr_reader :name

  def initialize
    @name = 'nodeAdapter'
  end
end

class HttpRequester
  def initialize(adapter)
    @adapter = adapter
  end

  def fetch(url)
    case @adapter.name
    when 'ajaxAdapter'
      make_ajax_call(url)
    when 'nodeAdapter'
      make_http_call(url)
    end
  end

  def make_ajax_call(url)
    # ...
  end

  def make_http_call(url)
    # ...
  end
end

Good:

class AjaxAdapter
  def request(url)
    # ...
  end
end

class NodeAdapter
  def request(url)
    # ...
  end
end

class HttpRequester
  def initialize(adapter)
    @adapter = adapter
  end

  def fetch(url)
    @adapter.request(url)
  end
end

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This is a scary term for a very simple concept. It's formally defined as "If S is a subtype of T, then objects of type T may be replaced with objects of type S (i.e., objects of type S may substitute objects of type T) without altering any of the desirable properties of that program (correctness, task performed, etc.)." That's an even scarier definition.

The best explanation for this is if you have a parent class and a child class, then the base class can always be replaced by the child class without getting incorrect results. This might still be confusing, so let's take a look at the classic Square-Rectangle example. Mathematically, a square is a rectangle, but if you model it using the "is-a" relationship via inheritance, you quickly get into trouble.

Bad:

class Rectangle
  def initialize
    @width = 0
    @height = 0
  end

  def color=(color)
    # ...
  end

  def render(area)
    # ...
  end

  def width=(width)
    @width = width
  end

  def height=(height)
    @height = height
  end

  def area
    @width * @height
  end
end

class Square < Rectangle
  def width=(width)
    @width = width
    @height = width
  end

  def height=(height)
    @width = height
    @height = height
  end
end

def render_large_rectangles(rectangles)
  rectangles.each do |rectangle|
    rectangle.width = 4
    rectangle.height = 5
    area = rectangle.area # BAD: Returns 25 for Square. Should be 20.
    rectangle.render(area)
  end
end

rectangles = [Rectangle.new, Rectangle.new, Square.new]
render_large_rectangles(rectangles)

Good:

class Shape
  def color=(color)
    # ...
  end

  def render(area)
    # ...
  end
end

class Rectangle < Shape
  def initialize(width, height)
    @width = width
    @height = height
  end

  def area
    @width * @height
  end
end

class Square < Shape
  def initialize(length)
    @length = length
  end

  def area
    @length * @length
  end
end

def render_large_shapes(shapes)
  shapes.each do |shape|
    area = shape.area
    shape.render(area)
  end
end

shapes = [Rectangle.new(4, 5), Rectangle.new(4, 5), Square.new(5)]
render_large_shapes(shapes)

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Ruby doesn't have interfaces so this principle doesn't apply as strictly as others. However, it's important and relevant even with Ruby's lack of type system.

ISP states that "Clients should not be forced to depend upon interfaces that they do not use." Interfaces are implicit contracts in Ruby because of duck typing.

When a client depends upon a class that contains interfaces that the client does not use, but that other clients do use, then that client will be affected by the changes that those other clients force upon the class.

The following example is taken from here.

Bad:

class Car
  # used by Driver
  def open
    # ...
  end

  # used by Driver
  def start_engine
    # ...
  end

  # used by Mechanic
  def change_engine
    # ...
  end
end

class Driver
  def drive
    @car.open
    @car.start_engine
  end
end

class Mechanic
  def do_stuff
    @car.change_engine
  end
end

Good:

# used by Driver only
class Car
  def open
    # ...
  end

  def start_engine
    # ...
  end
end

# used by Mechanic only
class CarInternals
  def change_engine
    # ...
  end
end

class Driver
  def drive
    @car.open
    @car.start_engine
  end
end

class Mechanic
  def do_stuff
    @car_internals.change_engine
  end
end

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This principle states two essential things:

1. High-level modules should not depend on low-level modules. Both should depend on abstractions.
2. Abstractions should not depend upon details. Details should depend on abstractions.

Simply put, DIP keeps high-level modules from knowing the details of its low-level modules and setting them up. It can accomplish this through dependency injection. A huge benefit of this is that it reduces the coupling between modules. Coupling is a very bad development pattern because it makes your code hard to refactor.

As stated previously, Ruby doesn't have interfaces so the abstractions that are depended upon are implicit contracts. That is to say, the methods and properties that an object/class exposes to another object/class. In the example below, the implicit contract is that any Request module for an InventoryTracker will have a request_items method.

Bad:

class InventoryRequester
  def initialize
    @req_methods = ['HTTP']
  end

  def request_item(item)
    # ...
  end
end

class InventoryTracker
  def initialize(items)
    @items = items

    # BAD: We have created a dependency on a specific request implementation.
    @requester = InventoryRequester.new
  end

  def request_items
    @items.each do |item|
      @requester.request_item(item)
    end
  end
end

inventory_tracker = InventoryTracker.new(['apples', 'bananas'])
inventory_tracker.request_items

Good:

class InventoryTracker
  def initialize(items, requester)
    @items = items
    @requester = requester
  end

  def request_items
    @items.each do |item|
      @requester.request_item(item)
    end
  end
end

class InventoryRequesterV1
  def initialize
    @req_methods = ['HTTP']
  end

  def request_item(item)
    # ...
  end
end

class InventoryRequesterV2
  def initialize
    @req_methods = ['WS']
  end

  def request_item(item)
    # ...
  end
end

# By constructing our dependencies externally and injecting them, we can easily
# substitute our request module for a fancy new one that uses WebSockets.
inventory_tracker = InventoryTracker.new(['apples', 'bananas'], InventoryRequesterV2.new)
inventory_tracker.request_items

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Testing is more important than shipping. If you have no tests or an inadequate amount, then every time you ship code you won't be sure that you didn't break anything. Deciding on what constitutes an adequate amount is up to your team, but having 100% coverage (all statements and branches) is how you achieve very high confidence and developer peace of mind. This means that in addition to having a great testing framework, you also need to use a good coverage tool.

There's no excuse to not write tests. Aim to always write tests for every new feature/module you introduce. If your preferred method is Test Driven Development (TDD), that is great, but the main point is to just make sure you are reaching your coverage goals before launching any feature, or refactoring an existing one.

Bad:

require 'rspec'

describe 'Calculator' do
  let(:calculator) { Calculator.new }

  it 'performs addition, subtraction, multiplication and division' do
    expect(calculator.calculate('1 + 2')).to eq(3)
    expect(calculator.calculate('4 - 2')).to eq(2)
    expect(calculator.calculate('2 * 3')).to eq(6)
    expect(calculator.calculate('6 / 2')).to eq(3)
  end
end

Good:

require 'rspec'

describe 'Calculator' do
  let(:calculator) { Calculator.new }

  it 'performs addition' do
    expect(calculator.calculate('1 + 2')).to eq(3)
  end

  it 'performs subtraction' do
    expect(calculator.calculate('4 - 2')).to eq(2)
  end

  it 'performs multiplication' do
    expect(calculator.calculate('2 * 3')).to eq(6)
  end

  it 'performs division' do
    expect(calculator.calculate('6 / 2')).to eq(3)
  end
end

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Thrown errors are a good thing! They mean the runtime has successfully identified when something in your program has gone wrong and it's letting you know by stopping method execution on the current stack, killing the process, and notifying you in the logs with a stack trace.

Doing nothing with a caught error doesn't give you the ability to ever fix or react to said error. Logging the error isn't much better as often times it can get lost in a sea of other logs. If you wrap any bit of code in a begin/rescue it means you think an error may occur there and therefore you should have a plan, or create a code path, for when it occurs.

Bad:

require 'logger'

logger = Logger.new(STDOUT)

begin
  method_that_might_throw()
rescue StandardError => err
  logger.info(err)
end

Good:

require 'logger'

logger = Logger.new(STDOUT)
# Change the logger level to ERROR to output only logs with ERROR level and above
logger.level = Logger::ERROR

begin
  method_that_might_throw()
rescue StandardError => err
  # Option 1: Only log errors
  logger.error(err)
  # Option 2: Notify end-user via an interface
  notify_user_of_error(err)
  # Option 3: Report error to a third-party service like Honeybadger
  report_error_to_service(err)
  # OR do all three!
end

Use a descriptive error class name and a message when you raise an error. That way you know why the error occurred and you can rescue the specific type of error.

Bad:

def initialize(user)
  fail unless user
  ...
end

Good:

def initialize(user)
  fail ArgumentError, 'Missing user' unless user
  ...
end

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Formatting is subjective. Like many rules herein, there is no hard and fast rule that you must follow. The main point is DO NOT ARGUE over formatting. There are tons of tools like RuboCop to automate this. Use one! It's a waste of time and money for engineers to argue over formatting.

For things that don't fall under the purview of automatic formatting (indentation, tabs vs. spaces, double vs. single quotes, etc.) look here for some guidance.

Ruby is dynamically typed, so capitalization tells you a lot about your variables, methods, etc. These rules are subjective, so your team can choose whatever they want. The point is, no matter what you all choose, just be consistent.

Bad:

DAYS_IN_WEEK = 7
daysInMonth = 30

songs = ['Back In Black', 'Stairway to Heaven', 'Hey Jude']
Artists = ['ACDC', 'Led Zeppelin', 'The Beatles']

def eraseDatabase; end

def restore_database; end

class ANIMAL; end
class Alpaca; end

Good:

DAYS_IN_WEEK = 7
DAYS_IN_MONTH = 30

SONGS = ['Back In Black', 'Stairway to Heaven', 'Hey Jude'].freeze
ARTISTS = ['ACDC', 'Led Zeppelin', 'The Beatles'].freeze

def erase_database; end

def restore_database; end

class Animal; end
class Alpaca; end

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If a method calls another, keep those methods vertically close in the source file. Ideally, keep the caller right above the callee. We tend to read code from top-to-bottom, like a newspaper. Because of this, make your code read that way.

Bad:

class PerformanceReview
  def initialize(employee)
    @employee = employee
  end

  def lookup_peers
    db.lookup(@employee, :peers)
  end

  def lookup_manager
    db.lookup(@employee, :manager)
  end

  def peer_reviews
    peers = lookup_peers
    # ...
  end

  def perf_review
    peer_reviews
    manager_review
    self_review
  end

  def manager_review
    manager = lookup_manager
    # ...
  end

  def self_review
    # ...
  end
end

review = PerformanceReview.new(employee)
review.perf_review

Good:

class PerformanceReview
  def initialize(employee)
    @employee = employee
  end

  def perf_review
    peer_reviews
    manager_review
    self_review
  end

  def peer_reviews
    peers = lookup_peers
    # ...
  end

  def lookup_peers
    db.lookup(@employee, :peers)
  end

  def manager_review
    manager = lookup_manager
    # ...
  end

  def lookup_manager
    db.lookup(@employee, :manager)
  end

  def self_review
    # ...
  end
end

review = PerformanceReview.new(employee)
review.perf_review

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Version control exists for a reason. Leave old code in your history.

Bad:

do_stuff
# do_other_stuff
# do_some_more_stuff
# do_so_much_stuff

Good:

do_stuff

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Remember, use version control! There's no need for dead code, commented code, and especially journal comments. Use git log to get history!

Bad:

# 2016-12-20: Removed monads, didn't understand them (RM)
# 2016-10-01: Improved using special monads (JP)
# 2016-02-03: Removed type-checking (LI)
# 2015-03-14: Added combine with type-checking (JR)
def combine(a, b)
  a + b
end

Good:

def combine(a, b)
  a + b
end

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This is also available in other languages:

• Brazilian Portuguese
• Simplified Chinese

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