# bad
  SomeClass::some_method
  some_object::some_method

  # good
  SomeClass.some_method
  some_object.some_method
  SomeModule::SomeClass::SOME_CONST
  SomeModule::SomeClass()

# bad
   def some_method()
    # body omitted
   end

   # good
   def some_method
    # body omitted
   end

   # bad
   def some_method_with_arguments arg1, arg2
    # body omitted
   end

   # good
   def some_method_with_arguments(arg1, arg2)
    # body omitted
   end

  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'

# bad
  if some_condition then
   # body omitted
  end

  # good
  if some_condition
   # body omitted
  end

  # bad
  if
   some_condition
   do_something
   do_something_else
  end

  # good
  if some_condition
   do_something
   do_something_else
  end

  # bad
  result = if some_condition then something else something_else end

  # good
  result = some_condition ? something : something_else

  # 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

  # bad
  result = if some_condition then something else something_else end

  # good
  result = some_condition ? something : something_else

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

  # good
  result =
   if condition
    x
   else
    y
   end

# 差 - 因为操作符有优先级，需要用括号。
  x = (not something)

  # good
  x = !something

  避免使用 !!.

  # 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

 # 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

# bad
  if some_condition
   do_something
  end

  # good
  do_something if some_condition

  # another good option
  some_condition && do_something

 # boolean expression
  if some_condition && some_other_condition
   do_something
  end

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

 # bad
  if some_condition
   do_something
  end

  # good
  do_something if some_condition

  # another good option
  some_condition and do_something

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

  # good
  if success?
   puts 'success'
  else
   puts 'failure'
  end

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

  # good
  if x > 10
   # body omitted
  end

 # 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

  # bad
  while some_condition
   do_something
  end

  # good
  do_something while some_condition

 # bad
  do_something while !some_condition

  # good
  do_something until some_condition

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

  # good
  loop do
   puts val
   val += 1
   break unless val < 0
  end

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

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

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

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

   # good
   validates :name, presence: true, length: { within: 1..10 }
  end

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

  # good
  Kernel.exit!
  2.even?
  fork
  'test'.upcase

  names = ['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 }

  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 parameter and pwd is set: #{dir}"
  end

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

  # good
  def some_method(some_arr)
   some_arr.size
  end

# 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

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

 # 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

 # 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

  # set name to Vozhidar, only if it's nil or false
  name ||= 'Bozhidar'

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

  # good
  enabled = true if enabled.nil?

  # bad
  if something
   something = something.downcase
  end

  # bad
  something = something ? nil : something.downcase

  # ok
  something = something.downcase if something

  # good
  something = something && something.downcase

  # better
  something &&= something.downcase

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

  # good
  something.is_a?(Array)
  (1..100).include?(7)
  some_string =~ /something/

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

  # good
  require 'English'
  $LOAD_PATH.unshift File.dirname(__FILE__)

  从来不要在方法名和（参数）开括号之间使用空格。

  # bad
  f (3+2) + 1

  # good
  f(3 + 2) +1

 # 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

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

  # good
  p = proc { |n| puts n }

  匿名方法 和 块 用 proc.call() 而不是 proc[] 或 proc.()。

  # 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)

# 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

  # 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'

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

  # good
  %w(one two three).join(', ')
  # => 'one, two, three'

  # 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) }

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

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

  # good
  do_something if x.between?(1000, 2000)

 # 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

  # bad

  END { puts 'Goodbye!' }

  # good

  at_exit { puts 'Goodbye!' }

 # 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