# Ruby

Este guia documenta nosso estilo de código e práticas da linguagem Ruby. Ele foi pego do [Bozhidar Batsov's Ruby Style Guide](https://github.com/bbatsov/ruby-style-guide).

## Table of Contents

* [Source Code Layout](#source-code-layout)
* [Syntax](#syntax)
* [Naming](#naming)
* [Comments](#comments)
  * [Comment Annotations](#comment-annotations)
* [Classes](#classes--modules)
* [Exceptions](#exceptions)
* [Collections](#collections)
* [Strings](#strings)
* [Regular Expressions](#regular-expressions)
* [Percent Literals](#percent-literals)
* [Metaprogramming](#metaprogramming)
* [Misc](#misc)
* [Tools](#tools)

## Source Code Layout

> Nearly everybody is convinced that every style but their own is ugly and unreadable. Leave out the "but their own" and they're probably right... \
>  \-- Jerry Coffin (on indentation)

*  Use `UTF-8` as the source file encoding. \[[link](#utf-8)]
*  Use two **spaces** per indentation level (aka soft tabs). No hard tabs. \[[link](#spaces-indentation)]

  ```ruby
  # bad - four spaces
  def some_method
      do_something
  end

  # good
  def some_method
    do_something
  end
  ```
*  Use Unix-style line endings. (\*BSD/Solaris/Linux/OS X users are covered by default, Windows users have to be extra careful.) \[[link](#crlf)]
  * If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:

    ```bash
    $ git config --global core.autocrlf true
    ```
*  Don't use `;` to separate statements and expressions. As a corollary - use one expression per line. \[[link](#no-semicolon)]

  ```ruby
  # bad
  puts 'foobar'; # superfluous semicolon

  puts 'foo'; puts 'bar' # two expressions on the same line

  # good
  puts 'foobar'

  puts 'foo'
  puts 'bar'

  puts 'foo', 'bar' # this applies to puts in particular
  ```
*  Prefer a single-line format for class definitions with no body. \[[link](#single-line-classes)]

  ```ruby
  # bad
  class FooError < StandardError
  end

  # okish
  class FooError < StandardError; end

  # good
  FooError = Class.new(StandardError)
  ```
* &#x20;Avoid single-line methods. Although they are somewhat popular in the wild, there are a few peculiarities about their definition syntax that make their use undesirable. At any rate - there should be no more than one expression in a single-line method. \[[link](#no-single-line-methods)]

  ```ruby
  # bad
  def too_much; something; something_else; end

  # okish - notice that the first ; is required
  def no_braces_method; body end

  # okish - notice that the second ; is optional
  def no_braces_method; body; end

  # okish - valid syntax, but no ; makes it kind of hard to read
  def some_method() body end

  # good
  def some_method
    body
  end
  ```

  One exception to the rule are empty-body methods.

  ```ruby
  # good
  def no_op; end
  ```
* &#x20;Use spaces around operators, after commas, colons and semicolons, around `{` and before `}`. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code. \[[link](#spaces-operators)]

  ```ruby
  sum = 1 + 2
  a, b = 1, 2
  [1, 2, 3].each { |e| puts e }
  class FooError < StandardError; end
  ```

  The only exception, regarding operators, is the exponent operator:

  ```ruby
  # bad
  e = M * c ** 2

  # good
  e = M * c**2
  ```

  `{` and `}` deserve a bit of clarification, since they are used for block and hash literals, as well as embedded expressions in strings. For hash literals two styles are considered acceptable.

  ```ruby
  # good - space after { and before }
  { one: 1, two: 2 }

  # good - no space after { and before }
  {one: 1, two: 2}
  ```

  The first variant is slightly more readable (and arguably more popular in the Ruby community in general). The second variant has the advantage of adding visual difference between block and hash literals. Whichever one you pick - apply it consistently.

  As far as embedded expressions go, there are also two acceptable options:

  ```ruby
  # good - no spaces
  "string#{expr}"

  # ok - arguably more readable
  "string#{ expr }"
  ```

  The first style is extremely more popular and you're generally advised to stick with it. The second, on the other hand, is (arguably) a bit more readable. As with hashes - pick one style and apply it consistently.
* &#x20;No spaces after `(`, `[` or before `]`, `)`. \[[link](#no-spaces-braces)]

  ```ruby
  some(arg).other
  [1, 2, 3].size
  ```
* &#x20;No space after `!`. \[[link](#no-space-bang)]

  ```ruby
  # bad
  ! something

  # good
  !something
  ```
* &#x20;No space inside range literals. \[[link](#no-space-inside-range-literals)]

  ```ruby
    # bad
    1 .. 3
    'a' ... 'z'

    # good
    1..3
    'a'...'z'
  ```
* &#x20;Indent `when` as deep as `case`. I know that many would disagree with this one, but it's the style established in both "The Ruby Programming Language" and "Programming Ruby". \[[link](#indent-when-to-case)]

  ```ruby
  # bad
  case
    when song.name == 'Misty'
      puts 'Not again!'
    when song.duration > 120
      puts 'Too long!'
    when Time.now.hour > 21
      puts "It's too late"
    else
      song.play
  end

  # good
  case
  when song.name == 'Misty'
    puts 'Not again!'
  when song.duration > 120
    puts 'Too long!'
  when Time.now.hour > 21
    puts "It's too late"
  else
    song.play
  end
  ```
* &#x20;When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches. \[[link](#indent-conditional-assignment)]

  ```ruby
  # bad - pretty convoluted
  kind = case year
  when 1850..1889 then 'Blues'
  when 1890..1909 then 'Ragtime'
  when 1910..1929 then 'New Orleans Jazz'
  when 1930..1939 then 'Swing'
  when 1940..1950 then 'Bebop'
  else 'Jazz'
  end

  result = if some_cond
    calc_something
  else
    calc_something_else
  end

  # good - it's apparent what's going on
  kind = case year
         when 1850..1889 then 'Blues'
         when 1890..1909 then 'Ragtime'
         when 1910..1929 then 'New Orleans Jazz'
         when 1930..1939 then 'Swing'
         when 1940..1950 then 'Bebop'
         else 'Jazz'
         end

  result = if some_cond
             calc_something
           else
             calc_something_else
           end

  # good (and a bit more width efficient)
  kind =
    case year
    when 1850..1889 then 'Blues'
    when 1890..1909 then 'Ragtime'
    when 1910..1929 then 'New Orleans Jazz'
    when 1930..1939 then 'Swing'
    when 1940..1950 then 'Bebop'
    else 'Jazz'
    end

  result =
    if some_cond
      calc_something
    else
      calc_something_else
    end
  ```
* &#x20;Use empty lines between method definitions and also to break up a method into logical paragraphs internally. \[[link](#empty-lines-between-methods)]

  ```ruby
  def some_method
    data = initialize(options)

    data.manipulate!

    data.result
  end

  def some_method
    result
  end
  ```
* &#x20;Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines. \[[link](#no-trailing-params-comma)]

  ```ruby
  # bad - easier to move/add/remove parameters, but still not preferred
  some_method(
               size,
               count,
               color,
             )

  # bad
  some_method(size, count, color, )

  # good
  some_method(size, count, color)
  ```
* &#x20;Use spaces around the `=` operator when assigning default values to method parameters: \[[link](#spaces-around-equals)]

  ```ruby
  # bad
  def some_method(arg1=:default, arg2=nil, arg3=[])
    # do something...
  end

  # good
  def some_method(arg1 = :default, arg2 = nil, arg3 = [])
    # do something...
  end
  ```

  While several Ruby books suggest the first style, the second is much more prominent in practice (and arguably a bit more readable).
* &#x20;Avoid line continuation `\` where not required. In practice, avoid using line continuations for anything but string concatenation. \[[link](#no-trailing-backslash)]

  ```ruby
  # bad
  result = 1 - \
           2

  # good (but still ugly as hell)
  result = 1 \
           - 2

  long_string = 'First part of the long string' \
                ' and second part of the long string'
  ```
* &#x20;Adopt a consistent multi-line method chaining style. There are two popular styles in the Ruby community, both of which are considered good - leading `.` (Option A) and trailing `.` (Option B). \[[link](#consistent-multi-line-chains)]

  * **(Option A)** When continuing a chained method invocation on another line keep the `.` on the second line.

    ```ruby
    # bad - need to consult first line to understand second line
    one.two.three.
      four

    # good - it's immediately clear what's going on the second line
    one.two.three
      .four
    ```
  * **(Option B)** When continuing a chained method invocation on another line, include the `.` on the first line to indicate that the expression continues.

    ```ruby
    # bad - need to read ahead to the second line to know that the chain continues
    one.two.three
      .four

    # good - it's immediately clear that the expression continues beyond the first line
    one.two.three.
      four
    ```

  A discussion on the merits of both alternative styles can be found [here](https://github.com/bbatsov/ruby-style-guide/pull/176).
* &#x20;Align the parameters of a method call if they span more than one line. When aligning parameters is not appropriate due to line-length constraints, single indent for the lines after the first is also acceptable. \[[link](#no-double-indent)]

  ```ruby
  # starting point (line is too long)
  def send_mail(source)
    Mailer.deliver(to: 'bob@example.com', from: 'us@example.com', subject: 'Important message', body: source.text)
  end

  # bad (double indent)
  def send_mail(source)
    Mailer.deliver(
        to: 'bob@example.com',
        from: 'us@example.com',
        subject: 'Important message',
        body: source.text)
  end

  # good
  def send_mail(source)
    Mailer.deliver(to: 'bob@example.com',
                   from: 'us@example.com',
                   subject: 'Important message',
                   body: source.text)
  end

  # good (normal indent)
  def send_mail(source)
    Mailer.deliver(
      to: 'bob@example.com',
      from: 'us@example.com',
      subject: 'Important message',
      body: source.text
    )
  end
  ```
* &#x20;Align the elements of array literals spanning multiple lines. \[[link](#align-multiline-arrays)]

  ```ruby
  # bad - single indent
  menu_item = ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
    'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']

  # good
  menu_item = [
    'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
    'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam'
  ]

  # good
  menu_item =
    ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
     'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']
  ```
* &#x20;Add underscores to large numeric literals to improve their readability. \[[link](#underscores-in-numerics)]

  ```ruby
  # bad - how many 0s are there?
  num = 1000000

  # good - much easier to parse for the human brain
  num = 1_000_000
  ```
* &#x20;Use RDoc and its conventions for API documentation. Don't put an empty line between the comment block and the `def`. \[[link](#rdoc-conventions)]
* &#x20;Limit lines to 80 characters. \[[link](#80-character-limits)]
* &#x20;Avoid trailing whitespace. \[[link](#no-trailing-whitespace)]
* &#x20;End each file with a newline. \[[link](#newline-eof)]
* &#x20;Don't use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments. \[[link](#no-block-comments)]

  ```ruby
  # bad
  =begin
  comment line
  another comment line
  =end

  # good
  # comment line
  # another comment line
  ```

## Syntax

* &#x20;Use `::` only to reference constants(this includes classes and modules) and constructors (like `Array()` or `Nokogiri::HTML()`). Do not use `::` for regular method invocation. \[[link](#double-colons)]

  ```ruby
  # bad
  SomeClass::some_method
  some_object::some_method

  # good
  SomeClass.some_method
  some_object.some_method
  SomeModule::SomeClass::SOME_CONST
  SomeModule::SomeClass()
  ```
* &#x20;Use `def` with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters. \[[link](#method-parens)]

  ```ruby
   # bad
   def some_method()
     # body omitted
   end

   # good
   def some_method
     # body omitted
   end

   # bad
   def some_method_with_parameters param1, param2
     # body omitted
   end

   # good
   def some_method_with_parameters(param1, param2)
     # body omitted
   end
  ```
* &#x20;Do not use `for`, unless you know exactly why. Most of the time iterators should be used instead. `for` is implemented in terms of `each` (so you're adding a level of indirection), but with a twist - `for` doesn't introduce a new scope (unlike `each`) and variables defined in its block will be visible outside it. \[[link](#no-for-loops)]

  ```ruby
  arr = [1, 2, 3]

  # bad
  for elem in arr do
    puts elem
  end

  # note that elem is accessible outside of the for loop
  elem # => 3

  # good
  arr.each { |elem| puts elem }

  # elem is not accessible outside each's block
  elem # => NameError: undefined local variable or method `elem'
  ```
* &#x20;Do not use `then` for multi-line `if/unless`. \[[link](#no-then)]

  ```ruby
  # bad
  if some_condition then
    # body omitted
  end

  # good
  if some_condition
    # body omitted
  end
  ```
* &#x20;Always put the condition on the same line as the `if`/`unless` in a multi-line conditional. \[[link](#same-line-condition)]

  ```ruby
  # bad
  if
    some_condition
    do_something
    do_something_else
  end

  # good
  if some_condition
    do_something
    do_something_else
  end
  ```
* &#x20;Favor the ternary operator(`?:`) over `if/then/else/end` constructs. It's more common and obviously more concise. \[[link](#ternary-operator)]

  ```ruby
  # bad
  result = if some_condition then something else something_else end

  # good
  result = some_condition ? something : something_else
  ```
* &#x20;Use one expression per branch in a ternary operator. This also means that ternary operators must not be nested. Prefer `if/else` constructs in these cases. \[[link](#no-nested-ternary)]

  ```ruby
  # bad
  some_condition ? (nested_condition ? nested_something : nested_something_else) : something_else

  # good
  if some_condition
    nested_condition ? nested_something : nested_something_else
  else
    something_else
  end
  ```
* &#x20;Do not use `if x; ...`. Use the ternary operator instead. \[[link](#no-semicolon-ifs)]

  ```ruby
  # bad
  result = if some_condition; something else something_else end

  # good
  result = some_condition ? something : something_else
  ```
* &#x20;Leverage the fact that `if` and `case` are expressions which return a result. \[[link](#use-if-case-returns)]

  ```ruby
  # bad
  if condition
    result = x
  else
    result = y
  end

  # good
  result =
    if condition
      x
    else
      y
    end
  ```
* &#x20;Use `when x then ...` for one-line cases. The alternative syntax `when x: ...` has been removed as of Ruby 1.9. \[[link](#one-line-cases)]
* &#x20;Do not use `when x; ...`. See the previous rule. \[[link](#no-when-semicolons)]
* &#x20;Use `!` instead of `not`. \[[link](#bang-not-not)]

  ```ruby
  # bad - braces are required because of op precedence
  x = (not something)

  # good
  x = !something
  ```
* &#x20;Avoid the use of `!!`. \[[link](#no-bang-bang)]

  ```ruby
  # bad
  x = 'test'
  # obscure nil check
  if !!x
    # body omitted
  end

  x = false
  # double negation is useless on booleans
  !!x # => false

  # good
  x = 'test'
  unless x.nil?
    # body omitted
  end
  ```
* &#x20;The `and` and `or` keywords are banned. It's just not worth it. Always use `&&` and `||` instead. \[[link](#no-and-or-or)]

  ```ruby
  # bad
  # boolean expression
  if some_condition and some_other_condition
    do_something
  end

  # control flow
  document.saved? or document.save!

  # good
  # boolean expression
  if some_condition && some_other_condition
    do_something
  end

  # control flow
  document.saved? || document.save!
  ```
* &#x20;Avoid multi-line `?:` (the ternary operator); use `if/unless` instead. \[[link](#no-multiline-ternary)]
* &#x20;Favor modifier `if/unless` usage when you have a single-line body. Another good alternative is the usage of control flow `&&/||`. \[[link](#if-as-a-modifier)]

  ```ruby
  # bad
  if some_condition
    do_something
  end

  # good
  do_something if some_condition

  # another good option
  some_condition && do_something
  ```
* &#x20;Avoid modifier `if/unless` usage at the end of a non-trivial multi-line block. \[[link](#no-multiline-if-modifiers)]

  ```ruby
  # bad
  10.times do
    # multi-line body omitted
  end if some_condition

  # good
  if some_condition
    10.times do
      # multi-line body omitted
    end
  end
  ```
* &#x20;Favor `unless` over `if` for negative conditions (or control flow `||`). \[[link](#unless-for-negatives)]

  ```ruby
  # bad
  do_something if !some_condition

  # bad
  do_something if not some_condition

  # good
  do_something unless some_condition

  # another good option
  some_condition || do_something
  ```
* &#x20;Do not use `unless` with `else`. Rewrite these with the positive case first. \[[link](#no-else-with-unless)]

  ```ruby
  # bad
  unless success?
    puts 'failure'
  else
    puts 'success'
  end

  # good
  if success?
    puts 'success'
  else
    puts 'failure'
  end
  ```
* &#x20;Don't use parentheses around the condition of an `if/unless/while/until`. \[[link](#no-parens-if)]

  ```ruby
  # bad
  if (x > 10)
    # body omitted
  end

  # good
  if x > 10
    # body omitted
  end
  ```

Note that there is an exception to this rule, namely [safe assignment in condition](#safe-assignment-in-condition).

* &#x20;Do not use `while/until condition do` for multi-line `while/until`. \[[link](#no-multiline-while-do)]

  ```ruby
  # bad
  while x > 5 do
    # body omitted
  end

  until x > 5 do
    # body omitted
  end

  # good
  while x > 5
    # body omitted
  end

  until x > 5
    # body omitted
  end
  ```
* &#x20;Favor modifier `while/until` usage when you have a single-line body. \[[link](#while-as-a-modifier)]

  ```ruby
  # bad
  while some_condition
    do_something
  end

  # good
  do_something while some_condition
  ```
* &#x20;Favor `until` over `while` for negative conditions. \[[link](#until-for-negatives)]

  ```ruby
  # bad
  do_something while !some_condition

  # good
  do_something until some_condition
  ```
* &#x20;Use `Kernel#loop` instead of `while/until` when you need an infinite loop. \[[link](#infinite-loop)]

  ```ruby
    # bad
    while true
      do_something
    end

    until false
      do_something
    end

    # good
    loop do
      do_something
    end
  ```
* &#x20;Use `Kernel#loop` with `break` rather than `begin/end/until` or `begin/end/while` for post-loop tests. \[[link](#loop-with-break)]

  ```ruby
  # bad
  begin
    puts val
    val += 1
  end while val < 0

  # good
  loop do
    puts val
    val += 1
    break unless val < 0
  end
  ```
* &#x20;Omit parentheses around parameters for methods that are part of an internal DSL (e.g. Rake, Rails, RSpec), methods that have "keyword" status in Ruby (e.g. `attr_reader`, `puts`) and attribute access methods. Use parentheses around the arguments of all other method invocations. \[[link](#no-dsl-parens)]

  ```ruby
  class Person
    attr_reader :name, :age

    # omitted
  end

  temperance = Person.new('Temperance', 30)
  temperance.name

  puts temperance.age

  x = Math.sin(y)
  array.delete(e)

  bowling.score.should == 0
  ```
* &#x20;Omit the outer braces around an implicit options hash. \[[link](#no-braces-opts-hash)]

  ```ruby
  # bad
  user.set({ name: 'John', age: 45, permissions: { read: true } })

  # good
  user.set(name: 'John', age: 45, permissions: { read: true })
  ```
* &#x20;Omit both the outer braces and parentheses for methods that are part of an internal DSL. \[[link](#no-dsl-decorating)]

  ```ruby
  class Person < ActiveRecord::Base
    # bad
    validates(:name, { presence: true, length: { within: 1..10 } })

    # good
    validates :name, presence: true, length: { within: 1..10 }
  end
  ```
* &#x20;Omit parentheses for method calls with no arguments. \[[link](#no-args-no-parens)]

  ```ruby
  # bad
  Kernel.exit!()
  2.even?()
  fork()
  'test'.upcase()

  # good
  Kernel.exit!
  2.even?
  fork
  'test'.upcase
  ```
* &#x20;Prefer `{...}` over `do...end` for single-line blocks. Avoid using `{...}` for multi-line blocks (multiline chaining is always ugly). Always use `do...end` for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs). Avoid `do...end` when chaining. \[[link](#single-line-blocks)]

  ```ruby
  names = %w(Bozhidar Steve Sarah)

  # bad
  names.each do |name|
    puts name
  end

  # good
  names.each { |name| puts name }

  # bad
  names.select do |name|
    name.start_with?('S')
  end.map { |name| name.upcase }

  # good
  names.select { |name| name.start_with?('S') }.map { |name| name.upcase }
  ```

  Some will argue that multiline chaining would look OK with the use of {...}, but they should ask themselves - is this code really readable and can the blocks' contents be extracted into nifty methods?
* &#x20;Consider using explicit block argument to avoid writing block literal that just passes its arguments to another block. Beware of the performance impact, though, as the block gets converted to a Proc. \[[link](#block-argument)]

  ```ruby
  require 'tempfile'

  # bad
  def with_tmp_dir
    Dir.mktmpdir do |tmp_dir|
      Dir.chdir(tmp_dir) { |dir| yield dir }  # block just passes arguments
    end
  end

  # good
  def with_tmp_dir(&block)
    Dir.mktmpdir do |tmp_dir|
      Dir.chdir(tmp_dir, &block)
    end
  end

  with_tmp_dir do |dir|
    puts "dir is accessible as a parameter and pwd is set: #{dir}"
  end
  ```
* &#x20;Avoid `return` where not required for flow of control. \[[link](#no-explicit-return)]

  ```ruby
  # bad
  def some_method(some_arr)
    return some_arr.size
  end

  # good
  def some_method(some_arr)
    some_arr.size
  end
  ```
* &#x20;Avoid `self` where not required. (It is only required when calling a self write accessor.) \[[link](#no-self-unless-required)]

  ```ruby
  # bad
  def ready?
    if self.last_reviewed_at > self.last_updated_at
      self.worker.update(self.content, self.options)
      self.status = :in_progress
    end
    self.status == :verified
  end

  # good
  def ready?
    if last_reviewed_at > last_updated_at
      worker.update(content, options)
      self.status = :in_progress
    end
    status == :verified
  end
  ```
* &#x20;As a corollary, avoid shadowing methods with local variables unless they are both equivalent. \[[link](#no-shadowing)]

  ```ruby
  class Foo
    attr_accessor :options

    # ok
    def initialize(options)
      self.options = options
      # both options and self.options are equivalent here
    end

    # bad
    def do_something(options = {})
      unless options[:when] == :later
        output(self.options[:message])
      end
    end

    # good
    def do_something(params = {})
      unless params[:when] == :later
        output(options[:message])
      end
    end
  end
  ```
* &#x20;Don't use the return value of `=` (an assignment) in conditional expressions unless the assignment is wrapped in parentheses. This is a fairly popular idiom among Rubyists that's sometimes referred to as *safe assignment in condition*. \[[link](#safe-assignment-in-condition)]

  ```ruby
  # bad (+ a warning)
  if v = array.grep(/foo/)
    do_something(v)
    ...
  end

  # good (MRI would still complain, but RuboCop won't)
  if (v = array.grep(/foo/))
    do_something(v)
    ...
  end

  # good
  v = array.grep(/foo/)
  if v
    do_something(v)
    ...
  end
  ```
* &#x20;Use shorthand self assignment operators whenever applicable. \[[link](#self-assignment)]

  ```ruby
  # bad
  x = x + y
  x = x * y
  x = x**y
  x = x / y
  x = x || y
  x = x && y

  # good
  x += y
  x *= y
  x **= y
  x /= y
  x ||= y
  x &&= y
  ```
* &#x20;Use `||=` to initialize variables only if they're not already initialized. \[[link](#double-pipe-for-uninit)]

  ```ruby
  # bad
  name = name ? name : 'Bozhidar'

  # bad
  name = 'Bozhidar' unless name

  # good - set name to Bozhidar, only if it's nil or false
  name ||= 'Bozhidar'
  ```
* &#x20;Don't use `||=` to initialize boolean variables. (Consider what would happen if the current value happened to be `false`.) \[[link](#no-double-pipes-for-bools)]

  ```ruby
  # bad - would set enabled to true even if it was false
  enabled ||= true

  # good
  enabled = true if enabled.nil?
  ```
* &#x20;Use `&&=` to preprocess variables that may or may not exist. Using `&&=` will change the value only if it exists, removing the need to check its existence with `if`. \[[link](#double-amper-preprocess)]

  ```ruby
  # bad
  if something
    something = something.downcase
  end

  # bad
  something = something ? something.downcase : nil

  # ok
  something = something.downcase if something

  # good
  something = something && something.downcase

  # better
  something &&= something.downcase
  ```
* &#x20;Avoid explicit use of the case equality operator `===`. As its name implies it is meant to be used implicitly by `case` expressions and outside of them it yields some pretty confusing code. \[[link](#no-case-equality)]

  ```ruby
  # bad
  Array === something
  (1..100) === 7
  /something/ === some_string

  # good
  something.is_a?(Array)
  (1..100).include?(7)
  some_string =~ /something/
  ```
* &#x20;Do not use `eql?` when using `==` will do. The stricter comparison semantics provided by `eql?` are rarely needed in practice. \[[link](#eql)]

  ```ruby
  # bad - eql? is the same as == for strings
  "ruby".eql? some_str

  # good
  "ruby" == some_str
  1.0.eql? x # eql? makes sense here if want to differentiate between Fixnum and Float 1
  ```
* &#x20;Avoid using Perl-style special variables (like `$:`, `$;`, etc. ). They are quite cryptic and their use in anything but one-liner scripts is discouraged. Use the human-friendly aliases provided by the `English` library. \[[link](#no-cryptic-perlisms)]

  ```ruby
  # bad
  $:.unshift File.dirname(__FILE__)

  # good
  require 'English'
  $LOAD_PATH.unshift File.dirname(__FILE__)
  ```
* &#x20;Do not put a space between a method name and the opening parenthesis. \[[link](#parens-no-spaces)]

  ```ruby
  # bad
  f (3 + 2) + 1

  # good
  f(3 + 2) + 1
  ```
* &#x20;If the first argument to a method begins with an open parenthesis, always use parentheses in the method invocation. For example, write `f((3 + 2) + 1)`. \[[link](#parens-as-args)]
* &#x20;Always run the Ruby interpreter with the `-w` option so it will warn you if you forget either of the rules above! \[[link](#always-warn-at-runtime)]
* &#x20;Use the new lambda literal syntax for single line body blocks. Use the `lambda` method for multi-line blocks. \[[link](#lambda-multi-line)]

  ```ruby
  # bad
  l = lambda { |a, b| a + b }
  l.call(1, 2)

  # correct, but looks extremely awkward
  l = ->(a, b) do
    tmp = a * 7
    tmp * b / 50
  end

  # good
  l = ->(a, b) { a + b }
  l.call(1, 2)

  l = lambda do |a, b|
    tmp = a * 7
    tmp * b / 50
  end
  ```
* &#x20;Prefer `proc` over `Proc.new`. \[[link](#proc)]

  ```ruby
  # bad
  p = Proc.new { |n| puts n }

  # good
  p = proc { |n| puts n }
  ```
* &#x20;Prefer `proc.call()` over `proc[]` or `proc.()` for both lambdas and procs. \[[link](#proc-call)]

  ```ruby
  # bad - looks similar to Enumeration access
  l = ->(v) { puts v }
  l[1]

  # also bad - uncommon syntax
  l = ->(v) { puts v }
  l.(1)

  # good
  l = ->(v) { puts v }
  l.call(1)
  ```
* &#x20;Prefix with `_` unused block parameters and local variables. It's also acceptable to use just `_` (although it's a bit less descriptive). This convention is recognized by the Ruby interpreter and tools like RuboCop and will suppress their unused variable warnings. \[[link](#underscore-unused-vars)]

  ```ruby
  # bad
  result = hash.map { |k, v| v + 1 }

  def something(x)
    unused_var, used_var = something_else(x)
    # ...
  end

  # good
  result = hash.map { |_k, v| v + 1 }

  def something(x)
    _unused_var, used_var = something_else(x)
    # ...
  end

  # good
  result = hash.map { |_, v| v + 1 }

  def something(x)
    _, used_var = something_else(x)
    # ...
  end
  ```
* &#x20;Use `$stdout/$stderr/$stdin` instead of `STDOUT/STDERR/STDIN`. `STDOUT/STDERR/STDIN` are constants, and while you can actually reassign (possibly to redirect some stream) constants in Ruby, you'll get an interpreter warning if you do so. \[[link](#global-stdout)]
* &#x20;Use `warn` instead of `$stderr.puts`. Apart from being more concise and clear, `warn` allows you to suppress warnings if you need to (by setting the warn level to 0 via `-W0`). \[[link](#warn)]
* &#x20;Favor the use of `sprintf` and its alias `format` over the fairly cryptic `String#%` method. \[[link](#sprintf)]

  ```ruby
  # bad
  '%d %d' % [20, 10]
  # => '20 10'

  # good
  sprintf('%d %d', 20, 10)
  # => '20 10'

  # good
  sprintf('%{first} %{second}', first: 20, second: 10)
  # => '20 10'

  format('%d %d', 20, 10)
  # => '20 10'

  # good
  format('%{first} %{second}', first: 20, second: 10)
  # => '20 10'
  ```
* &#x20;Favor the use of `Array#join` over the fairly cryptic `Array#*` with \[[link](#array-join)] a string argument.

  ```ruby
  # bad
  %w(one two three) * ', '
  # => 'one, two, three'

  # good
  %w(one two three).join(', ')
  # => 'one, two, three'
  ```
* &#x20;Use `[*var]` or `Array()` instead of explicit `Array` check, when dealing with a variable you want to treat as an Array, but you're not certain it's an array. \[[link](#splat-arrays)]

  ```ruby
  # bad
  paths = [paths] unless paths.is_a? Array
  paths.each { |path| do_something(path) }

  # good
  [*paths].each { |path| do_something(path) }

  # good (and a bit more readable)
  Array(paths).each { |path| do_something(path) }
  ```
* &#x20;Use ranges or `Comparable#between?` instead of complex comparison logic when possible. \[[link](#ranges-or-between)]

  ```ruby
  # bad
  do_something if x >= 1000 && x <= 2000

  # good
  do_something if (1000..2000).include?(x)

  # good
  do_something if x.between?(1000, 2000)
  ```
* &#x20;Favor the use of predicate methods to explicit comparisons with `==`. Numeric comparisons are OK. \[[link](#predicate-methods)]

  ```ruby
  # bad
  if x % 2 == 0
  end

  if x % 2 == 1
  end

  if x == nil
  end

  # good
  if x.even?
  end

  if x.odd?
  end

  if x.nil?
  end

  if x.zero?
  end

  if x == 0
  end
  ```
* &#x20;Don't do explicit non-`nil` checks unless you're dealing with boolean values. \[[link](#no-non-nil-checks)]

  ```ruby
    # bad
    do_something if !something.nil?
    do_something if something != nil

    # good
    do_something if something

    # good - dealing with a boolean
    def value_set?
      !@some_boolean.nil?
    end
  ```
* &#x20;Avoid the use of `BEGIN` blocks. \[[link](#no-BEGIN-blocks)]
* &#x20;Do not use `END` blocks. Use `Kernel#at_exit` instead. \[[link](#no-END-blocks)]

  ```ruby
  # bad
  END { puts 'Goodbye!' }

  # good
  at_exit { puts 'Goodbye!' }
  ```
* &#x20;Avoid the use of flip-flops. \[[link](#no-flip-flops)]
* &#x20;Avoid use of nested conditionals for flow of control. \[[link](#no-nested-conditionals)]

  Prefer a guard clause when you can assert invalid data. A guard clause is a conditional statement at the top of a function that bails out as soon as it can.

  ```ruby
  # bad
  def compute_thing(thing)
    if thing[:foo]
      update_with_bar(thing)
      if thing[:foo][:bar]
        partial_compute(thing)
      else
        re_compute(thing)
      end
    end
  end

  # good
  def compute_thing(thing)
    return unless thing[:foo]
    update_with_bar(thing[:foo])
    return re_compute(thing) unless thing[:foo][:bar]
    partial_compute(thing)
  end
  ```

  Prefer `next` in loops instead of conditional blocks.

  ```ruby
  # bad
  [0, 1, 2, 3].each do |item|
    if item > 1
      puts item
    end
  end

  # good
  [0, 1, 2, 3].each do |item|
    next unless item > 1
    puts item
  end
  ```
* &#x20;Prefer `map` over `collect`, `find` over `detect`, `select` over `find_all`, `reduce` over `inject` and `size` over `length`. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it. The rhyming methods are inherited from Smalltalk and are not common in other programming languages. The reason the use of `select` is encouraged over `find_all` is that it goes together nicely with `reject` and its name is pretty self-explanatory. \[[link](#map-find-select-reduce-size)]
* &#x20;Don't use `count` as a substitute for `size`. For `Enumerable` objects other than `Array` it will iterate the entire collection in order to determine its size. \[[link](#count-vs-size)]

  ```ruby
  # bad
  some_hash.count

  # good
  some_hash.size
  ```
* &#x20;Use `flat_map` instead of `map` + `flatten`. This does not apply for arrays with a depth greater than 2, i.e. if `users.first.songs == ['a', ['b','c']]`, then use `map + flatten` rather than `flat_map`. `flat_map` flattens the array by 1, whereas `flatten` flattens it all the way. \[[link](#flat-map)]

  ```ruby
  # bad
  all_songs = users.map(&:songs).flatten.uniq

  # good
  all_songs = users.flat_map(&:songs).uniq
  ```
* &#x20;Use `reverse_each` instead of `reverse.each`. `reverse_each` doesn't do a new array allocation and that's a good thing. \[[link](#reverse-each)]

  ```ruby
  # bad
  array.reverse.each { ... }

  # good
  array.reverse_each { ... }
  ```

## Naming

> The only real difficulties in programming are cache invalidation and naming things. \
> &#x20;\-- Phil Karlton

* &#x20;Name identifiers in English. \[[link](#english-identifiers)]

  ```ruby
  # bad - identifier using non-ascii characters
  заплата = 1_000

  # bad - identifier is a Bulgarian word, written with Latin letters (instead of Cyrillic)
  zaplata = 1_000

  # good
  salary = 1_000
  ```
* &#x20;Use `snake_case` for symbols, methods and variables. \[[link](#snake-case-symbols-methods-vars)]

  ```ruby
  # bad
  :'some symbol'
  :SomeSymbol
  :someSymbol

  someVar = 5

  def someMethod
    ...
  end

  def SomeMethod
   ...
  end

  # good
  :some_symbol

  def some_method
    ...
  end
  ```
* &#x20;Use `CamelCase` for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) \[[link](#camelcase-classes)]

  ```ruby
  # bad
  class Someclass
    ...
  end

  class Some_Class
    ...
  end

  class SomeXml
    ...
  end

  # good
  class SomeClass
    ...
  end

  class SomeXML
    ...
  end
  ```
* &#x20;Use `snake_case` for naming files, e.g. `hello_world.rb`. \[[link](#snake-case-files)]
* &#x20;Use `snake_case` for naming directories, e.g. `lib/hello_world/hello_world.rb`. \[[link](#snake-case-dirs)]
* &#x20;Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing CamelCase with snake\_case. \[[link](#one-class-per-file)]
* &#x20;Use `SCREAMING_SNAKE_CASE` for other constants. \[[link](#screaming-snake-case)]

  ```ruby
  # bad
  SomeConst = 5

  # good
  SOME_CONST = 5
  ```
* &#x20;The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e. `Array#empty?`). Methods that don't return a boolean, shouldn't end in a question mark. \[[link](#bool-methods-qmark)]
* &#x20;The names of potentially *dangerous* methods (i.e. methods that modify `self` or the arguments, `exit!` (doesn't run the finalizers like `exit` does), etc.) should end with an exclamation mark if there exists a safe version of that *dangerous* method. \[[link](#dangerous-method-bang)]

  ```ruby
  # bad - there is no matching 'safe' method
  class Person
    def update!
    end
  end

  # good
  class Person
    def update
    end
  end

  # good
  class Person
    def update!
    end

    def update
    end
  end
  ```
* &#x20;Define the non-bang (safe) method in terms of the bang (dangerous) one if possible. \[[link](#safe-because-unsafe)]

  ```ruby
  class Array
    def flatten_once!
      res = []

      each do |e|
        [*e].each { |f| res << f }
      end

      replace(res)
    end

    def flatten_once
      dup.flatten_once!
    end
  end
  ```
* &#x20;When using `reduce` with short blocks, name the arguments `|a, e|` (accumulator, element). \[[link](#reduce-blocks)]
* &#x20;When defining binary operators, name the parameter `other`(`<<` and `[]` are exceptions to the rule, since their semantics are different). \[[link](#other-arg)]

  ```ruby
  def +(other)
    # body omitted
  end
  ```

## Comments

> Good code is its own best documentation. As you're about to add a comment, ask yourself, "How can I improve the code so that this comment isn't needed?" Improve the code and then document it to make it even clearer. \
> &#x20;\-- Steve McConnell

* &#x20;Write self-documenting code and ignore the rest of this section. Seriously! \[[link](#no-comments)]
* &#x20;Write comments in English. \[[link](#english-comments)]
* &#x20;Use one space between the leading `#` character of the comment and the text of the comment. \[[link](#hash-space)]
* &#x20;Comments longer than a word are capitalized and use punctuation. Use [one space](http://en.wikipedia.org/wiki/Sentence_spacing) after periods. \[[link](#english-syntax)]
* &#x20;Avoid superfluous comments. \[[link](#no-superfluous-comments)]

  ```ruby
  # bad
  counter += 1 # Increments counter by one.
  ```
* &#x20;Keep existing comments up-to-date. An outdated comment is worse than no comment at all. \[[link](#comment-upkeep)]

> Good code is like a good joke - it needs no explanation. \
> &#x20;\-- Russ Olsen

* Avoid writing comments to explain bad code. Refactor the code to make it

  self-explanatory. (Do or do not - there is no try. --Yoda)

  \[[link](#refactor-dont-comment)]

### Comment Annotations

* &#x20;Annotations should usually be written on the line immediately above the relevant code. \[[link](#annotate-above)]
* &#x20;The annotation keyword is followed by a colon and a space, then a note describing the problem. \[[link](#annotate-keywords)]
* &#x20;If multiple lines are required to describe the problem, subsequent lines should be indented three spaces after the `#` (one general plus two for indentation purpose). \[[link](#indent-annotations)]

  ```ruby
  def bar
    # FIXME: This has crashed occasionally since v3.2.1. It may
    #   be related to the BarBazUtil upgrade.
    baz(:quux)
  end
  ```
* &#x20;In cases where the problem is so obvious that any documentation would be redundant, annotations may be left at the end of the offending line with no note. This usage should be the exception and not the rule. \[[link](#rare-eol-annotations)]

  ```ruby
  def bar
    sleep 100 # OPTIMIZE
  end
  ```
* &#x20;Use `TODO` to note missing features or functionality that should be added at a later date. \[[link](#todo)]
* &#x20;Use `FIXME` to note broken code that needs to be fixed. \[[link](#fixme)]
* &#x20;Use `OPTIMIZE` to note slow or inefficient code that may cause performance problems. \[[link](#optimize)]
* &#x20;Use `HACK` to note code smells where questionable coding practices were used and should be refactored away. \[[link](#hack)]
* &#x20;Use `REVIEW` to note anything that should be looked at to confirm it is working as intended. For example: `REVIEW: Are we sure this is how the client does X currently?` \[[link](#review)]
* &#x20;Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's `README` or similar. \[[link](#document-annotations)]

## Classes & Modules

* &#x20;Use a consistent structure in your class definitions. \[[link](#consistent-classes)]

  ```ruby
  class Person
    # extend and include go first
    extend SomeModule
    include AnotherModule

    # inner classes
    CustomErrorKlass = Class.new(StandardError)

    # constants are next
    SOME_CONSTANT = 20

    # afterwards we have attribute macros
    attr_reader :name

    # followed by other macros (if any)
    validates :name

    # public class methods are next in line
    def self.some_method
    end

    # followed by public instance methods
    def some_method
    end

    # protected and private methods are grouped near the end
    protected

    def some_protected_method
    end

    private

    def some_private_method
    end
  end
  ```
* &#x20;Don't nest multi line classes within classes. Try to have such nested classes each in their own file in a folder named like the containing class. \[[link](#file-classes)]

  ```ruby
  # bad

  # foo.rb
  class Foo
    class Bar
      # 30 methods inside
    end

    class Car
      # 20 methods inside
    end

    # 30 methods inside
  end

  # good

  # foo.rb
  class Foo
    # 30 methods inside
  end

  # foo/bar.rb
  class Foo
    class Bar
      # 30 methods inside
    end
  end

  # foo/car.rb
  class Foo
    class Car
      # 20 methods inside
    end
  end
  ```
* &#x20;Prefer modules to classes with only class methods. Classes should be used only when it makes sense to create instances out of them. \[[link](#modules-vs-classes)]

  ```ruby
  # bad
  class SomeClass
    def self.some_method
      # body omitted
    end

    def self.some_other_method
    end
  end

  # good
  module SomeModule
    module_function

    def some_method
      # body omitted
    end

    def some_other_method
    end
  end
  ```
* &#x20;Favor the use of `module_function` over `extend self` when you want to turn a module's instance methods into class methods. \[[link](#module-function)]

  ```ruby
  # bad
  module Utilities
    extend self

    def parse_something(string)
      # do stuff here
    end

    def other_utility_method(number, string)
      # do some more stuff
    end
  end

  # good
  module Utilities
    module_function

    def parse_something(string)
      # do stuff here
    end

    def other_utility_method(number, string)
      # do some more stuff
    end
  end
  ```
* &#x20;When designing class hierarchies make sure that they conform to the [Liskov Substitution Principle](http://en.wikipedia.org/wiki/Liskov_substitution_principle). \[[link](#liskov)]
* &#x20;Try to make your classes as [SOLID](http://en.wikipedia.org/wiki/SOLID_\(object-oriented_design\)) as possible. \[[link](#solid-design)]
* &#x20;Always supply a proper `to_s` method for classes that represent domain objects. \[[link](#define-to-s)]

  ```ruby
  class Person
    attr_reader :first_name, :last_name

    def initialize(first_name, last_name)
      @first_name = first_name
      @last_name = last_name
    end

    def to_s
      "#{@first_name} #{@last_name}"
    end
  end
  ```
* &#x20;Use the `attr` family of functions to define trivial accessors or mutators. \[[link](#attr_family)]

  ```ruby
  # bad
  class Person
    def initialize(first_name, last_name)
      @first_name = first_name
      @last_name = last_name
    end

    def first_name
      @first_name
    end

    def last_name
      @last_name
    end
  end

  # good
  class Person
    attr_reader :first_name, :last_name

    def initialize(first_name, last_name)
      @first_name = first_name
      @last_name = last_name
    end
  end
  ```
* &#x20;Avoid the use of `attr`. Use `attr_reader` and `attr_accessor` instead. \[[link](#attr)]

  ```ruby
  # bad - creates a single attribute accessor (deprecated in 1.9)
  attr :something, true
  attr :one, :two, :three # behaves as attr_reader

  # good
  attr_accessor :something
  attr_reader :one, :two, :three
  ```
* &#x20;Consider using `Struct.new`, which defines the trivial accessors, constructor and comparison operators for you. \[[link](#struct-new)]

  ```ruby
  # good
  class Person
    attr_accessor :first_name, :last_name

    def initialize(first_name, last_name)
      @first_name = first_name
      @last_name = last_name
    end
  end

  # better
  Person = Struct.new(:first_name, :last_name) do
  end
  `
  ```
* &#x20;Don't extend an instance initialized by `Struct.new`. Extending it introduces a superfluous class level and may also introduce weird errors if the file is required multiple times. \[[link](#no-extend-struct-new)]

  ```ruby
  # bad
  class Person < Struct.new(:first_name, :last_name)
  end

  # good
  Person = Struct.new(:first_name, :last_name)
  `
  ```
* &#x20;Consider adding factory methods to provide additional sensible ways to create instances of a particular class. \[[link](#factory-methods)]

  ```ruby
  class Person
    def self.create(options_hash)
      # body omitted
    end
  end
  ```
* &#x20;Prefer [duck-typing](http://en.wikipedia.org/wiki/Duck_typing) over inheritance. \[[link](#duck-typing)]

  ```ruby
  # bad
  class Animal
    # abstract method
    def speak
    end
  end

  # extend superclass
  class Duck < Animal
    def speak
      puts 'Quack! Quack'
    end
  end

  # extend superclass
  class Dog < Animal
    def speak
      puts 'Bau! Bau!'
    end
  end

  # good
  class Duck
    def speak
      puts 'Quack! Quack'
    end
  end

  class Dog
    def speak
      puts 'Bau! Bau!'
    end
  end
  ```
* &#x20;Avoid the usage of class (`@@`) variables due to their "nasty" behavior in inheritance. \[[link](#no-class-vars)]

  ```ruby
  class Parent
    @@class_var = 'parent'

    def self.print_class_var
      puts @@class_var
    end
  end

  class Child < Parent
    @@class_var = 'child'
  end

  Parent.print_class_var # => will print "child"
  ```

  As you can see all the classes in a class hierarchy actually share one class variable. Class instance variables should usually be preferred over class variables.
* &#x20;Assign proper visibility levels to methods (`private`, `protected`) in accordance with their intended usage. Don't go off leaving everything `public` (which is the default). After all we're coding in *Ruby* now, not in *Python*. \[[link](#visibility)]
* &#x20;Indent the `public`, `protected`, and `private` methods as much as the method definitions they apply to. Leave one blank line above the visibility modifier and one blank line below in order to emphasize that it applies to all methods below it. \[[link](#indent-public-private-protected)]

  ```ruby
  class SomeClass
    def public_method
      # ...
    end

    private

    def private_method
      # ...
    end

    def another_private_method
      # ...
    end
  end
  ```
* &#x20;Use `def self.method` to define singleton methods. This makes the code easier to refactor since the class name is not repeated. \[[link](#def-self-singletons)]

  ```ruby
  class TestClass
    # bad
    def TestClass.some_method
      # body omitted
    end

    # good
    def self.some_other_method
      # body omitted
    end

    # Also possible and convenient when you
    # have to define many singleton methods.
    class << self
      def first_method
        # body omitted
      end

      def second_method_etc
        # body omitted
      end
    end
  end
  ```
* &#x20;Prefer `alias` when aliasing methods in lexical class scope as the resolution of `self` in this context is also lexical, and it communicates clearly to the user that the indirection of your alias will not be altered at runtime or by any subclass unless made explicit. \[[link](#alias-method-lexically)]

  ```ruby
  class Westerner
    def first_name
      @names.first
    end

    alias given_name first_name
  end
  ```

  Since `alias`, like `def`, is a keyword, prefer bareword arguments over symbols or strings. In other words, do `alias foo bar`, not `alias :foo :bar`.

  Also be aware of how Ruby handles aliases and inheritance: an alias references the method that was resolved at the time the alias was defined; it is not dispatched dynamically.

  ```ruby
  class Fugitive < Westerner
    def first_name
      'Nobody'
    end
  end
  ```

  In this example, `Fugitive#given_name` would still call the original `Westerner#first_name` method, not `Fugitive#first_name`. To override the behavior of `Fugitive#given_name` as well, you'd have to redefine it in the derived class.

  ```ruby
  class Fugitive < Westerner
    def first_name
      'Nobody'
    end

    alias given_name first_name
  end
  ```
* &#x20;Always use `alias_method` when aliasing methods of modules, classes, or singleton classes at runtime, as the lexical scope of `alias` leads to unpredictability in these cases. \[[link](#alias-method)]

  ```ruby
  module Mononymous
    def self.included(other)
      other.class_eval { alias_method :full_name, :given_name }
    end
  end

  class Sting < Westerner
    include Mononymous
  end
  ```

## Exceptions

* &#x20;Signal exceptions using the `fail` method. Use `raise` only when catching an exception and re-raising it (because here you're not failing, but explicitly and purposefully raising an exception). \[[link](#fail-method)]

  ```ruby
  begin
    fail 'Oops'
  rescue => error
    raise if error.message != 'Oops'
  end
  ```
* &#x20;Don't specify `RuntimeError` explicitly in the two argument version of `fail/raise`. \[[link](#no-explicit-runtimeerror)]

  ```ruby
  # bad
  fail RuntimeError, 'message'

  # good - signals a RuntimeError by default
  fail 'message'
  ```
* &#x20;Prefer supplying an exception class and a message as two separate arguments to `fail/raise`, instead of an exception instance. \[[link](#exception-class-messages)]

  ```ruby
  # bad
  fail SomeException.new('message')
  # Note that there is no way to do `fail SomeException.new('message'), backtrace`.

  # good
  fail SomeException, 'message'
  # Consistent with `fail SomeException, 'message', backtrace`.
  ```
* &#x20;Do not return from an `ensure` block. If you explicitly return from a method inside an `ensure` block, the return will take precedence over any exception being raised, and the method will return as if no exception had been raised at all. In effect, the exception will be silently thrown away. \[[link](#no-return-ensure)]

  ```ruby
  def foo
    fail
  ensure
    return 'very bad idea'
  end
  ```
* &#x20;Use *implicit begin blocks* where possible. \[[link](#begin-implicit)]

  ```ruby
  # bad
  def foo
    begin
      # main logic goes here
    rescue
      # failure handling goes here
    end
  end

  # good
  def foo
    # main logic goes here
  rescue
    # failure handling goes here
  end
  ```
* &#x20;Mitigate the proliferation of `begin` blocks by using *contingency methods* (a term coined by Avdi Grimm). \[[link](#contingency-methods)]

  ```ruby
  # bad
  begin
    something_that_might_fail
  rescue IOError
    # handle IOError
  end

  begin
    something_else_that_might_fail
  rescue IOError
    # handle IOError
  end

  # good
  def with_io_error_handling
     yield
  rescue IOError
    # handle IOError
  end

  with_io_error_handling { something_that_might_fail }

  with_io_error_handling { something_else_that_might_fail }
  ```
* &#x20;Don't suppress exceptions. \[[link](#dont-hide-exceptions)]

  ```ruby
  # bad
  begin
    # an exception occurs here
  rescue SomeError
    # the rescue clause does absolutely nothing
  end

  # bad
  do_something rescue nil
  ```
* &#x20;Avoid using `rescue` in its modifier form. \[[link](#no-rescue-modifiers)]

  ```ruby
  # bad - this catches exceptions of StandardError class and its descendant classes
  read_file rescue handle_error($!)

  # good - this catches only the exceptions of Errno::ENOENT class and its descendant classes
  def foo
    read_file
  rescue Errno::ENOENT => ex
    handle_error(ex)
  end
  ```
* &#x20;Don't use exceptions for flow of control. \[[link](#no-exceptional-flows)]

  ```ruby
  # bad
  begin
    n / d
  rescue ZeroDivisionError
    puts 'Cannot divide by 0!'
  end

  # good
  if d.zero?
    puts 'Cannot divide by 0!'
  else
    n / d
  end
  ```
* &#x20;Avoid rescuing the `Exception` class. This will trap signals and calls to `exit`, requiring you to `kill -9` the process. \[[link](#no-blind-rescues)]

  ```ruby
  # bad
  begin
    # calls to exit and kill signals will be caught (except kill -9)
    exit
  rescue Exception
    puts "you didn't really want to exit, right?"
    # exception handling
  end

  # good
  begin
    # a blind rescue rescues from StandardError, not Exception as many
    # programmers assume.
  rescue => e
    # exception handling
  end

  # also good
  begin
    # an exception occurs here

  rescue StandardError => e
    # exception handling
  end
  ```
* &#x20;Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from. \[[link](#exception-ordering)]

  ```ruby
  # bad
  begin
    # some code
  rescue Exception => e
    # some handling
  rescue StandardError => e
    # some handling that will never be executed
  end

  # good
  begin
    # some code
  rescue StandardError => e
    # some handling
  rescue Exception => e
    # some handling
  end
  ```
* &#x20;Release external resources obtained by your program in an `ensure` block. \[[link](#release-resources)]

  ```ruby
  f = File.open('testfile')
  begin
    # .. process
  rescue
    # .. handle error
  ensure
    f.close if f
  end
  ```
* &#x20;Use versions of resource obtaining methods that do automatic resource cleanup when possible. \[[link](#auto-release-resources)]

  ```ruby
  # bad - you need to close the file descriptor explicitly
  f = File.open('testfile')
    # ...
  f.close

  # good - the file descriptor is closed automatically
  File.open('testfile') do |f|
    # ...
  end
  ```
* &#x20;Favor the use of exceptions for the standard library over introducing new exception classes. \[[link](#standard-exceptions)]

## Collections

* &#x20;Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is). \[[link](#literal-array-hash)]

  ```ruby
  # bad
  arr = Array.new
  hash = Hash.new

  # good
  arr = []
  hash = {}
  ```
* &#x20;Prefer `%w` to the literal array syntax when you need an array of words (non-empty strings without spaces and special characters in them). Apply this rule only to arrays with two or more elements. \[[link](#percent-w)]

  ```ruby
  # bad
  STATES = ['draft', 'open', 'closed']

  # good
  STATES = %w(draft open closed)
  ```
* &#x20;Prefer `%i` to the literal array syntax when you need an array of symbols (and you don't need to maintain Ruby 1.9 compatibility). Apply this rule only to arrays with two or more elements. \[[link](#percent-i)]

  ```ruby
  # bad
  STATES = [:draft, :open, :closed]

  # good
  STATES = %i(draft open closed)
  ```
* &#x20;Avoid comma after the last item of an `Array` or `Hash` literal, especially when the items are not on separate lines. \[[link](#no-trailing-array-commas)]

  ```ruby
  # bad - easier to move/add/remove items, but still not preferred
  VALUES = [
             1001,
             2020,
             3333,
           ]

  # bad
  VALUES = [1001, 2020, 3333, ]

  # good
  VALUES = [1001, 2020, 3333]
  ```
* &#x20;Avoid the creation of huge gaps in arrays. \[[link](#no-gappy-arrays)]

  ```ruby
  arr = []
  arr[100] = 1 # now you have an array with lots of nils
  ```
* &#x20;When accessing the first or last element from an array, prefer `first` or `last` over `[0]` or `[-1]`. \[[link](#first-and-last)]
* &#x20;Use `Set` instead of `Array` when dealing with unique elements. `Set` implements a collection of unordered values with no duplicates. This is a hybrid of `Array`'s intuitive inter-operation facilities and `Hash`'s fast lookup. \[[link](#set-vs-array)]
* &#x20;Prefer symbols instead of strings as hash keys. \[[link](#symbols-as-keys)]

  ```ruby
  # bad
  hash = { 'one' => 1, 'two' => 2, 'three' => 3 }

  # good
  hash = { one: 1, two: 2, three: 3 }
  ```
* &#x20;Avoid the use of mutable objects as hash keys. \[[link](#no-mutable-keys)]
* &#x20;Use the Ruby 1.9 hash literal syntax when your hash keys are symbols. \[[link](#hash-literals)]

  ```ruby
  # bad
  hash = { :one => 1, :two => 2, :three => 3 }

  # good
  hash = { one: 1, two: 2, three: 3 }
  ```
* &#x20;Don't mix the Ruby 1.9 hash syntax with hash rockets in the same hash literal. When you've got keys that are not symbols stick to the hash rockets syntax. \[[link](#no-mixed-hash-syntaces)]

  ```ruby
  # bad
  { a: 1, 'b' => 2 }

  # good
  { :a => 1, 'b' => 2 }
  ```
* &#x20;Use `Hash#key?` instead of `Hash#has_key?` and `Hash#value?` instead of `Hash#has_value?`. As noted [here](http://blade.nagaokaut.ac.jp/cgi-bin/scat.rb/ruby/ruby-core/43765) by Matz, the longer forms are considered deprecated. \[[link](#hash-key)]

  ```ruby
  # bad
  hash.has_key?(:test)
  hash.has_value?(value)

  # good
  hash.key?(:test)
  hash.value?(value)
  ```
* &#x20;Use `Hash#fetch` when dealing with hash keys that should be present. \[[link](#hash-fetch)]

  ```ruby
  heroes = { batman: 'Bruce Wayne', superman: 'Clark Kent' }
  # bad - if we make a mistake we might not spot it right away
  heroes[:batman] # => "Bruce Wayne"
  heroes[:supermann] # => nil

  # good - fetch raises a KeyError making the problem obvious
  heroes.fetch(:supermann)
  ```
* &#x20;Introduce default values for hash keys via `Hash#fetch` as opposed to using custom logic. \[[link](#hash-fetch-defaults)]

  ```ruby
  batman = { name: 'Bruce Wayne', is_evil: false }

  # bad - if we just use || operator with falsy value we won't get the expected result
  batman[:is_evil] || true # => true

  # good - fetch work correctly with falsy values
  batman.fetch(:is_evil, true) # => false
  ```
* &#x20;Prefer the use of the block instead of the default value in `Hash#fetch`. \[[link](#use-hash-blocks)]

  ```ruby
  batman = { name: 'Bruce Wayne' }

  # bad - if we use the default value, we eager evaluate it
  # so it can slow the program down if done multiple times
  batman.fetch(:powers, get_batman_powers) # get_batman_powers is an expensive call

  # good - blocks are lazy evaluated, so only triggered in case of KeyError exception
  batman.fetch(:powers) { get_batman_powers }
  ```
* &#x20;Use `Hash#values_at` when you need to retrieve several values consecutively from a hash. \[[link](#hash-values-at)]

  ```ruby
  # bad
  email = data['email']
  username = data['nickname']

  # good
  email, username = data.values_at('email', 'nickname')
  ```
* &#x20;Rely on the fact that as of Ruby 1.9 hashes are ordered. \[[link](#ordered-hashes)]
* &#x20;Do not modify a collection while traversing it. \[[link](#no-modifying-collections)]
* &#x20;When accessing elements of a collection, avoid direct access via `[n]` by using an alternate form of the reader method if it is supplied. This guards you from calling `[]` on `nil`. \[[link](#accessing-elements-directly)]

  ```ruby
  # bad
  Regexp.last_match[1]

  # good
  Regexp.last_match(1)
  ```
* &#x20;When providing an accessor for a collection, provide an alternate form to save users from checking for `nil` before accessing an element in the collection. \[[link](#provide-alternate-accessor-to-collections)]

  ```ruby
  # bad
  def awesome_things
    @awesome_things
  end

  # good
  def awesome_things(index = nil)
    if index && @awesome_things
      @awesome_things[index]
    else
      @awesome_things
    end
  end
  ```

## Strings

* &#x20;Prefer string interpolation and string formatting instead of string concatenation: \[[link](#string-interpolation)]

  ```ruby
  # bad
  email_with_name = user.name + ' <' + user.email + '>'

  # good
  email_with_name = "#{user.name} <#{user.email}>"

  # good
  email_with_name = format('%s <%s>', user.name, user.email)
  ```
* &#x20;Consider padding string interpolation code with space. It more clearly sets the code apart from the string. \[[link](#pad-string-interpolation)]

  ```ruby
  "#{ user.last_name }, #{ user.first_name }"
  ```
* &#x20;Adopt a consistent string literal quoting style. There are two popular styles in the Ruby community, both of which are considered good - single quotes by default (Option A) and double quotes by default (Option B). \[[link](#consistent-string-literals)]

  * **(Option A)** Prefer single-quoted strings when you don't need string interpolation or special symbols such as `\t`, `\n`, `'`, etc.

    ```ruby
    # bad
    name = "Bozhidar"

    # good
    name = 'Bozhidar'
    ```
  * **(Option B)** Prefer double-quotes unless your string literal contains `"` or escape characters you want to suppress.

    ```ruby
    # bad
    name = 'Bozhidar'

    # good
    name = "Bozhidar"
    ```

  The string literals in this guide are aligned with the first style.
* &#x20;Don't use the character literal syntax `?x`. Since Ruby 1.9 it's basically redundant - `?x` would interpreted as `'x'` (a string with a single character in it). \[[link](#no-character-literals)]

  ```ruby
  # bad
  char = ?c

  # good
  char = 'c'
  ```
* &#x20;Don't leave out `{}` around instance and global variables being interpolated into a string. \[[link](#curlies-interpolate)]

  ```ruby
  class Person
    attr_reader :first_name, :last_name

    def initialize(first_name, last_name)
      @first_name = first_name
      @last_name = last_name
    end

    # bad - valid, but awkward
    def to_s
      "#@first_name #@last_name"
    end

    # good
    def to_s
      "#{@first_name} #{@last_name}"
    end
  end

  $global = 0
  # bad
  puts "$global = #$global"

  # good
  puts "$global = #{$global}"
  ```
* &#x20;Don't use `Object#to_s` on interpolated objects. It's invoked on them automatically. \[[link](#no-to-s)]

  ```ruby
  # bad
  message = "This is the #{result.to_s}."

  # good
  message = "This is the #{result}."
  ```
* &#x20;Avoid using `String#+` when you need to construct large data chunks. Instead, use `String#<<`. Concatenation mutates the string instance in-place and is always faster than `String#+`, which creates a bunch of new string objects. \[[link](#concat-strings)]

  ```ruby
  # good and also fast
  html = ''
  html << '<h1>Page title</h1>'

  paragraphs.each do |paragraph|
    html << "<p>#{paragraph}</p>"
  end
  ```
* &#x20;Don't use `String#gsub` in scenarios in which you can use a faster more specialized alternative. \[[link](#dont-abuse-gsub)]

  ```ruby
    url = 'http://example.com'
    str = 'lisp-case-rules'

    # bad
    url.gsub("http://", "https://")
    str.gsub("-", "_")

    # good
    url.sub("http://", "https://")
    str.tr("-", "_")
  ```
* &#x20;When using heredocs for multi-line strings keep in mind the fact that they preserve leading whitespace. It's a good practice to employ some margin based on which to trim the excessive whitespace. \[[link](#heredocs)]

  ```ruby
  code = <<-END.gsub(/^\s+\|/, '')
    |def test
    |  some_method
    |  other_method
    |end
  END
  # => "def test\n  some_method\n  other_method\nend\n"
  ```

## Regular Expressions

> Some people, when confronted with a problem, think "I know, I'll use regular expressions." Now they have two problems.\
> &#x20;\-- Jamie Zawinski

* &#x20;Don't use regular expressions if you just need plain text search in string: `string['text']` \[[link](#no-regexp-for-plaintext)]
* &#x20;For simple constructions you can use regexp directly through string index. \[[link](#regexp-string-index)]

  ```ruby
  match = string[/regexp/]             # get content of matched regexp
  first_group = string[/text(grp)/, 1] # get content of captured group
  string[/text (grp)/, 1] = 'replace'  # string => 'text replace'
  ```
* &#x20;Use non-capturing groups when you don't use captured result of parentheses. \[[link](#non-capturing-regexp)]

  ```ruby
  /(first|second)/   # bad
  /(?:first|second)/ # good
  ```
* &#x20;Don't use the cryptic Perl-legacy variables denoting last regexp group matches (`$1`, `$2`, etc). Use `Regexp.last_match(n)` instead. \[[link](#no-perl-regexp-last-matchers)]

  ```ruby
  /(regexp)/ =~ string
  ...

  # bad
  process $1

  # good
  process Regexp.last_match(1)
  ```
* &#x20;Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead. \[[link](#no-numbered-regexes)]

  ```ruby
  # bad
  /(regexp)/ =~ string
  ...
  process Regexp.last_match(1)

  # good
  /(?<meaningful_var>regexp)/ =~ string
  ...
  process meaningful_var
  ```
* &#x20;Character classes have only a few special characters you should care about: `^`, `-`, `\`, `]`, so don't escape `.` or brackets in `[]`. \[[link](#limit-escapes)]
* &#x20;Be careful with `^` and `$` as they match start/end of line, not string endings. If you want to match the whole string use: `\A` and `\z` (not to be confused with `\Z` which is the equivalent of `/\n?\z/`). \[[link](#caret-and-dollar-regexp)]

  ```ruby
  string = "some injection\nusername"
  string[/^username$/]   # matches
  string[/\Ausername\z/] # doesn't match
  ```
* &#x20;Use `x` modifier for complex regexps. This makes them more readable and you can add some useful comments. Just be careful as spaces are ignored. \[[link](#comment-regexes)]

  ```ruby
  regexp = /
    start         # some text
    \s            # white space char
    (group)       # first group
    (?:alt1|alt2) # some alternation
    end
  /x
  ```
* &#x20;For complex replacements `sub`/`gsub` can be used with block or hash. \[[link](#gsub-blocks)]

## Percent Literals

* &#x20;Use `%()`(it's a shorthand for `%Q`) for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs. \[[link](#percent-q-shorthand)]

  ```ruby
  # bad (no interpolation needed)
  %(<div class="text">Some text</div>)
  # should be '<div class="text">Some text</div>'

  # bad (no double-quotes)
  %(This is #{quality} style)
  # should be "This is #{quality} style"

  # bad (multiple lines)
  %(<div>\n<span class="big">#{exclamation}</span>\n</div>)
  # should be a heredoc.

  # good (requires interpolation, has quotes, single line)
  %(<tr><td class="name">#{name}</td>)
  ```
* &#x20;Avoid `%q` unless you have a string with both `'` and `"` in it. Regular string literals are more readable and should be preferred unless a lot of characters would have to be escaped in them. \[[link](#percent-q)]

  ```ruby
  # bad
  name = %q(Bruce Wayne)
  time = %q(8 o'clock)
  question = %q("What did you say?")

  # good
  name = 'Bruce Wayne'
  time = "8 o'clock"
  question = '"What did you say?"'
  ```
* &#x20;Use `%r` only for regular expressions matching *at least* one '/' character. \[[link](#percent-r)]

  ```ruby
  # bad
  %r{\s+}

  # good
  %r{^/(.*)$}
  %r{^/blog/2011/(.*)$}
  ```
* &#x20;Avoid the use of `%x` unless you're going to invoke a command with backquotes in it(which is rather unlikely). \[[link](#percent-x)]

  ```ruby
  # bad
  date = %x(date)

  # good
  date = `date`
  echo = %x(echo `date`)
  ```
* &#x20;Avoid the use of `%s`. It seems that the community has decided `:"some string"` is the preferred way to create a symbol with spaces in it. \[[link](#percent-s)]
* &#x20;Prefer `()` as delimiters for all `%` literals, except `%r`. Since parentheses often appear inside regular expressions in many scenarios a less common character like `{` might be a better choice for a delimiter, depending on the regexp's content. \[[link](#percent-literal-braces)]

  ```ruby
  # bad
  %w[one two three]
  %q{"Test's king!", John said.}

  # good
  %w(one two three)
  %q("Test's king!", John said.)
  ```

## Metaprogramming

* &#x20;Avoid needless metaprogramming. \[[link](#no-needless-metaprogramming)]
* &#x20;Do not mess around in core classes when writing libraries. (Do not monkey-patch them.) \[[link](#no-monkey-patching)]
* &#x20;The block form of `class_eval` is preferable to the string-interpolated form. - when you use the string-interpolated form, always supply `__FILE__` and `__LINE__`, so that your backtraces make sense: \[[link](#block-class-eval)]

  ```ruby
  class_eval 'def use_relative_model_naming?; true; end', __FILE__, __LINE__
  ```

  * `define_method` is preferable to `class_eval{ def ... }`
* &#x20;When using `class_eval` (or other `eval`) with string interpolation, add a comment block showing its appearance if interpolated (a practice used in Rails code): \[[link](#eval-comment-docs)]

  ```ruby
  # from activesupport/lib/active_support/core_ext/string/output_safety.rb
  UNSAFE_STRING_METHODS.each do |unsafe_method|
    if 'String'.respond_to?(unsafe_method)
      class_eval <<-EOT, __FILE__, __LINE__ + 1
        def #{unsafe_method}(*params, &block)       # def capitalize(*params, &block)
          to_str.#{unsafe_method}(*params, &block)  #   to_str.capitalize(*params, &block)
        end                                       # end

        def #{unsafe_method}!(*params)              # def capitalize!(*params)
          @dirty = true                           #   @dirty = true
          super                                   #   super
        end                                       # end
      EOT
    end
  end
  ```
* &#x20;Avoid using `method_missing` for metaprogramming because backtraces become messy, the behavior is not listed in `#methods`, and misspelled method calls might silently work, e.g. `nukes.launch_state = false`. Consider using delegation, proxy, or `define_method` instead. If you must use `method_missing`: \[[link](#no-method-missing)]
  * Be sure to [also define `respond_to_missing?`](http://blog.marc-andre.ca/2010/11/methodmissing-politely.html)
  * Only catch methods with a well-defined prefix, such as `find_by_*` -- make your code as assertive as possible.
  * Call `super` at the end of your statement
  * Delegate to assertive, non-magical methods:

    ```ruby
    # bad
    def method_missing?(meth, *params, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        # ... lots of code to do a find_by
      else
        super
      end
    end

    # good
    def method_missing?(meth, *params, &block)
      if /^find_by_(?<prop>.*)/ =~ meth
        find_by(prop, *params, &block)
      else
        super
      end
    end

    # best of all, though, would to define_method as each findable attribute is declared
    ```
* &#x20;Prefer `public_send` over `send` so as not to circumvent `private`/`protected` visibility. \[[link](#prefer-public-send)]

## Misc

* &#x20;Write `ruby -w` safe code. \[[link](#always-warn)]
* &#x20;Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule). \[[link](#no-optional-hash-params)]
* &#x20;Avoid methods longer than 10 LOC (lines of code). Ideally, most methods will be shorter than 5 LOC. Empty lines do not contribute to the relevant LOC. \[[link](#short-methods)]
* &#x20;Avoid parameter lists longer than three or four parameters. \[[link](#too-many-params)]
* &#x20;If you really need "global" methods, add them to Kernel and make them private. \[[link](#private-global-methods)]
* &#x20;Use module instance variables instead of global variables. \[[link](#instance-vars)]

  ```ruby
  # bad
  $foo_bar = 1

  # good
  module Foo
    class << self
      attr_accessor :bar
    end
  end

  Foo.bar = 1
  ```
* &#x20;Use `OptionParser` for parsing complex command line options and `ruby -s` for trivial command line options. \[[link](#optionparser)]
* &#x20;Prefer `Time.now` over `Time.new` when retrieving the current system time. \[[link](#time-now)]
* &#x20;Code in a functional way, avoiding mutation when that makes sense. \[[link](#functional-code)]
* &#x20;Do not mutate parameters unless that is the purpose of the method. \[[link](#no-param-mutations)]
* &#x20;Avoid more than three levels of block nesting. \[[link](#three-is-the-number-thou-shalt-count)]
* &#x20;Be consistent. In an ideal world, be consistent with these guidelines. \[[link](#be-consistent)]
* &#x20;Use common sense. \[[link](#common-sense)]

## Tools

Here's some tools to help you automatically check Ruby code against this guide.

### RuboCop

\[RuboCop]\[] is a Ruby code style checker based on this style guide. RuboCop already covers a significant portion of the Guide, supports both MRI 1.9 and MRI 2.0 and has good Emacs integration.

### RubyMine

[RubyMine](http://www.jetbrains.com/ruby/)'s code inspections are [partially based](http://confluence.jetbrains.com/display/RUBYDEV/RubyMine+Inspections) on this guide.