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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]

• Use one tab per indentation level (aka hard tabs). No soft tabs. ^[link]

   # bad - two spaces
   def some_method
     do_something
   end
  
   # good - tabs
   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]

  • If you're using Git you might want to add the following configuration setting to protect your project from Windows line endings creeping in:

     $ git config --global core.autocrlf true

• Don't use ; to separate statements and expressions. As a corollary—use one expression per line. ^[link]

   # 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]

   # bad
   class FooError < StandardError
   end
  
   # okish
   class FooError < StandardError; end
  
   # good
   FooError = Class.new(StandardError)

• 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]

   # 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

• Use spaces around operators, after commas, colons and semicolons. Whitespace might be (mostly) irrelevant to the Ruby interpreter, but its proper use is the key to writing easily readable code. ^[link]

   sum = 1 + 2
   a, b = 1, 2
   class FooError < StandardError; end

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

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

• No spaces after (, [ or before ], ). Use spaces around { and before }. ^[link]

   # bad
   some( arg ).other
   [ 1, 2, 3 ].each{|e| puts e}
  
   # good
   some(arg).other
   [1, 2, 3].each { |e| puts e }

  { and } deserve a bit of clarification, since they are used for block and hash literals, as well as string interpolation.

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

  With interpolated expressions, there should be no padded-spacing inside the braces.

   # bad
   "From: #{ user.first_name }, #{ user.last_name }"
  
   # good
   "From: #{user.first_name}, #{user.last_name}"

• No space after !. ^[link]

   # bad
   ! something
  
   # good
   !something

• No space inside range literals. ^[link]

   # bad
   1 .. 3
   'a' ... 'z'
  
   # good
   1..3
   'a'...'z'

• Indent when one level deeper than case. ^[link]

   # 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

• When assigning the result of a conditional expression to a variable, preserve the usual alignment of its branches. Do not use extra indentation for additional lines. ^[link]

   # 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
   		'Blues'
   	when 1890..1909
   		'Ragtime'
   	when 1910..1929
   		'New Orleans Jazz'
   	when 1930..1939
   		'Swing'
   	when 1940..1950
   		'Bebop'
   	else
   		'Jazz'
   end
  
   result = if some_cond
   	calc_something
   else
   	calc_something_else
   end

• Use two empty lines between method definitions and also to break up methods into logical paragraphs internally. ^[link]

   def some_method
   	data = initialize(options)
  
   	data.manipulate!
  
   	data.result
   end
  
  
   def some_method
   	result
   end

• Don't use several empty lines in a row. ^[link]

   # bad - It has two empty lines.
   some_method
  
  
   some_method
  
   # good
   some_method
  
   some_method

• Use empty lines around access modifiers. ^[link]

   # bad
   class Foo
   	attr_reader :foo
   	def foo
   		# do something...
   	end
   end
  
   # good
   class Foo
   	attr_reader :foo
  
   	def foo
   		# do something...
   	end
   end

• Don't use empty lines around method, class, module, block bodies. ^[link]

   # bad
   class Foo
  
   	def foo
  
   		begin
  
   			do_something do
  
   				something
  
   			end
  
   		rescue
  
   			something
  
   		end
  
   	end
  
   end
  
   # good
   class Foo
   	def foo
   		begin
   			do_something do
   				something
   			end
   		rescue
   			something
   		end
   	end
   end

• Avoid comma after the last parameter in a method call, especially when the parameters are not on separate lines. ^[link]

   # 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)

• Use spaces around the = operator when assigning default values to method parameters: ^[link]

   # 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).

• Avoid line continuation \. ^[link]

   # bad
   result = 1 - \
   	2
  
   long_string = 'First part of the long string' \
   	' and second part of the long string'

• Place dots on the first line for method chaining across lines. ^[link]

   # 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

• Align the parameters of a method call if they span more than one line with a single additional indent. ^[link]

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

• Align the elements of array literals spanning multiple lines with a single indent. ^[link]

   # bad, multi indent
   menu_item =
   	['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
   	 'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']
  
   # okish - single indent
   menu_item = ['Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
   	'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam']
  
   # good - opening/closing brackets on separate lines
   menu_item = [
   	'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam',
   	'Baked beans', 'Spam', 'Spam', 'Spam', 'Spam', 'Spam'
   ]

• Add underscores to large numeric literals to improve their readability. ^[link]

   # bad - how many 0s are there?
   num = 1000000
  
   # good - much easier to parse for the human brain
   num = 1_000_000

• Prefer smallcase letters for numeric literal prefixes. 0o for octal, 0x for hexadecimal and 0b for binary. Do not use 0d prefix for decimal literals. ^[link]

   # bad
   num = 01234
   num = 0O1234
   num = 0X12AB
   num = 0B10101
   num = 0D1234
   num = 0d1234
  
   # good - easier to separate digits from the prefix
   num = 0o1234
   num = 0x12AB
   num = 0b10101
   num = 1234

• Use [Rdoc][rdoc] and its conventions for API documentation. Don't put an empty line between the comment block and the def. ^[link]

• Limit lines to 180 characters. ^[link]

• Avoid trailing whitespace. ^[link]

• End each file with a single newline. ^[link]

• Don't use block comments. They cannot be preceded by whitespace and are not as easy to spot as regular comments. ^[link]

   # bad
   =begin
   comment line
   another comment line
   =end
  
   # good
   # comment line
   # another comment line

• 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]

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

• Use def with parentheses when there are parameters. Omit the parentheses when the method doesn't accept any parameters. ^[link]

   # 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

• Use parentheses around the arguments of method invocations only when multiple method invocations on the same line have parameters. ^[link]

   # bad
   x = Math.sin(y)
   # good
   x = Math.sin y
  
   # bad
   array.delete(e)
   # good
   array.delete e
  
   # bad
   temperance = Person.new('Temperance', 30)
   # good
   temperance = Person.new 'Temperance', 30
   
   # required for the right-hand invocation
   result = some_calculator sub_step(44)

• Define optional arguments at the end of the list of arguments. Ruby has some unexpected results when calling methods that have optional arguments at the front of the list. ^[link]

   # bad
   def some_method(a = 1, b = 2, c, d)
   	puts "#{a}, #{b}, #{c}, #{d}"
   end
  
   some_method 'w', 'x' # => '1, 2, w, x'
   some_method 'w', 'x', 'y' # => 'w, 2, x, y'
   some_method 'w', 'x', 'y', 'z' # => 'w, x, y, z'
  
   # good
   def some_method(c, d, a = 1, b = 2)
   	puts "#{a}, #{b}, #{c}, #{d}"
   end
  
   some_method 'w', 'x' # => '1, 2, w, x'
   some_method 'w', 'x', 'y' # => 'y, 2, w, x'
   some_method 'w', 'x', 'y', 'z' # => 'y, z, w, x'

• Avoid the use of unnecessary trailing underscore variables during parallel assignment. Named underscore variables are to be preferred over underscore variables because of the context that they provide. Trailing underscore variables are necessary when there is a splat variable defined on the left side of the assignment, and the splat variable is not an underscore. ^[link]

   # bad
   foo = 'one,two,three,four,five'
   # Unnecessary assignment that does not provide useful information
   first, second, _ = foo.split(',')
   first, _, _ = foo.split(',')
   first, *_ = foo.split(',')
  
  
   # good
   foo = 'one,two,three,four,five'
   # The underscores are needed to show that you want all elements
   # except for the last number of underscore elements
   *beginning, _ = foo.split(',')
   *beginning, something, _ = foo.split(',')
  
   a, = foo.split(',')
   a, b, = foo.split(',')
   # Unnecessary assignment to an unused variable, but the assignment
   # provides us with useful information.
   first, _second = foo.split(',')
   first, _second, = foo.split(',')
   first, *_ending = foo.split(',')

• 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]

   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'

• Do not use then for multi-line if/unless. ^[link]

   # bad
   if some_condition then
   	# body omitted
   end
  
   # good
   if some_condition
   	# body omitted
   end

• Always put the condition on the same line as the if/unless in a multi-line conditional. ^[link]

   # bad
   if
   	some_condition
   	do_something
   	do_something_else
   end
  
   # good
   if some_condition
   	do_something
   	do_something_else
   end

• Favor the ternary operator(?:) over if/then/else/end constructs for single-line evaluations. It's more common and obviously more concise. ^[link]

   # bad
   result = if some_condition then something else something_else end
  
   # good
   result = some_condition ? something : something_else

• 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]

   # 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

• Do not use if x; .... Use the ternary operator instead. ^[link]

   # bad
   result = if some_condition; something else something_else end
  
   # good
   result = some_condition ? something : something_else

• Leverage the fact that if and case are expressions which return a result. ^[link]

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

• Use when x then ... for one-line cases. The alternative syntax when x: ... has been removed as of Ruby 1.9. ^[link]

• Do not use when x; .... See the previous rule. ^[link]

• Use ! instead of not. ^[link]

   # bad - parentheses are required because of op precedence
   x = (not something)
  
   # good
   x = !something

• Avoid the use of !! in expressions, except for explicitly coercing to boolean. ^[link]

  !! converts a value to boolean, but you don't need this explicit conversion in the condition of a control expression; using it only obscures your intention. If you want to do a nil check, use nil? instead.

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

• The and and or keywords are banned. The minimal added readability is just not worth the high probability of introducing subtle bugs. For boolean expressions, always use && and || instead. For flow control, use if and unless; && and || are also acceptable but less clear. ^[link]

   # bad
   # boolean expression
   ok = got_needed_arguments and arguments_are_valid
  
   # control flow
   document.save or fail(RuntimeError, "Failed to save document!")
  
   # good
   # boolean expression
   ok = got_needed_arguments && arguments_are_valid
  
   # control flow
   fail(RuntimeError, "Failed to save document!") unless document.save
  
   # ok
   # control flow
   document.save || fail(RuntimeError, "Failed to save document!")

• Avoid multi-line ?: (the ternary operator); use if/unless instead. ^[link]

• Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow &&/||. ^[link]

   # bad
   if some_condition
   	do_something
   end
  
   # good
   do_something if some_condition
  
   # another good option
   some_condition && do_something

• Avoid modifier if/unless usage at the end of a non-trivial multi-line block. ^[link]

   # 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

• Avoid nested modifier if/unless/while/until usage. Favor &&/|| if appropriate. ^[link]

   # bad
   do_something if other_condition if some_condition
  
   # good
   do_something if some_condition && other_condition

• Favor unless over if for negative conditions (or control flow ||). ^[link]

   # 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

• Do not use unless with else. Rewrite these with the positive case first. ^[link]

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

• Don't use parentheses around the condition of a control expression. ^[link]

   # 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.

• Do not use while/until condition do for multi-line while/until. ^[link]

   # 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

• Favor modifier while/until usage when you have a single-line body. ^[link]

   # bad
   while some_condition
   	do_something
   end
  
   # good
   do_something while some_condition

• Favor until over while for negative conditions. ^[link]

   # bad
   do_something while !some_condition
  
   # good
   do_something until some_condition

• Use Kernel#loop instead of while/until when you need an infinite loop. ^[link]

    ```ruby
    # bad
    while true
    	do_something
    end
  
    until false
    	do_something
    end
  
    # good
    loop do
    	do_something
    end
    ```
  

• Use Kernel#loop with break rather than begin/end/until or begin/end/while for post-loop tests. ^[link]

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

• Omit the outer braces around an implicit options hash. ^[link]

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

• Omit both the outer braces and parentheses for methods that are part of an internal DSL. ^[link]

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

• Use the proc invocation shorthand when the invoked method is the only operation of a block. ^[link]

   # bad
   names.map { |name| name.upcase }
  
   # good
   names.map &:upcase

• Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks (multi-line 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]

   names = %w[Bozhidar Steve Sarah]
  
   # bad
   names.select do |name|
   	name.start_with?('S')
   end.map { |name| name.upcase }
  
   # good
   names.each do |name|
   	puts name
   end
  
   # good
   names.each { |name| puts name }
  
   # good
   names.select { |name| name.start_with?('S') }.map &:upcase

  Some will argue that multi-line 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?

• 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]

   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

• Avoid return where not required for flow of control. ^[link]

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

• Avoid self where not required. (It is only required when calling a self write accessor, methods named after reserved words, or overloadable operators.) ^[link]

   # 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

• As a corollary, avoid shadowing methods with local variables unless they are both equivalent. ^[link]

   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

• 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]

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

• Use shorthand self assignment operators whenever applicable. ^[link]

   # 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

• Use ||= to initialize variables only if they're not already initialized. ^[link]

   # bad
   name = name ? name : 'Bozhidar'
  
   # bad
   name = 'Bozhidar' unless name
  
   # good - set name to 'Bozhidar', only if it's nil or false
   name ||= 'Bozhidar'

• Don't use ||= to initialize boolean variables. (Consider what would happen if the current value happened to be false.) ^[link]

   # bad - would set enabled to true even if it was false
   enabled ||= true
  
   # good
   enabled = true if enabled.nil?

• 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]

   # 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

• 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]

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

• Do not use eql? when using == will do. The stricter comparison semantics provided by eql? are rarely needed in practice. ^[link]

   # 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 Integer and Float 1

• 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]

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

• Do not put a space between a method name and the opening parenthesis. ^[link]

   # bad
   f (3 + 2) + 1
  
   # good
   f(3 + 2) + 1

• Always run the Ruby interpreter with the -w option so it will warn you if you forget either of the rules above! ^[link]

• Do not use nested method definitions, use lambda instead. Nested method definitions actually produce methods in the same scope (e.g. class) as the outer method. Furthermore, the "nested method" will be redefined every time the method containing its definition is invoked. ^[link]

   # bad
   def foo(x)
   	def bar(y)
   		# body omitted
   	end
  
   	bar(x)
   end
  
   # good - the same as the previous, but no bar redefinition on every foo call
   def bar(y)
   	# body omitted
   end
  
   def foo(x)
   	bar x
   end
  
   # also good
   def foo(x)
   	bar = ->(y) { ... }
   	bar.call x
   end

• Use the new lambda literal syntax for single line body blocks. Use the lambda method for multi-line blocks. ^[link]

   # 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

• Don't omit the parameter parentheses when defining a stabby lambda with parameters. ^[link]

   # bad
   l = ->x, y { something(x, y) }
  
   # good
   l = ->(x, y) { something(x, y) }

• Omit the parameter parentheses when defining a stabby lambda with no parameters. ^[link]

   # bad
   l = ->() { something }
  
   # good
   l = -> { something }

• Prefer proc over Proc.new. ^[link]

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

• Prefer proc.call() over proc[] or proc.() for both lambdas and procs. ^[link]

   # 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

• 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]

   # bad
   result = hash.map { |k, v| v + 1 }
  
   def something(x)
   	unused_var, used_var = something_else x
   	# some code
   end
  
   # good
   result = hash.map { |_k, v| v + 1 }
  
   def something(x)
   	_unused_var, used_var = something_else x
   	# some code
   end
  
   # good
   result = hash.map { |_, v| v + 1 }
  
   def something(x)
   	_, used_var = something_else x
   	# some code
   end

• 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]

• 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]

• Favor the use of sprintf and its alias format over the fairly cryptic String#% method. ^[link]

   # 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'

• Favor the use of Array#join over the fairly cryptic Array#* with a string argument. ^[link]

   # bad
   %w[one two three] * ', '
   # => 'one, two, three'
  
   # good
   %w[one two three].join ', '
   # => 'one, two, three'

• Use Array() instead of explicit Array check or [*var], when dealing with a variable you want to treat as an Array, but you're not certain it's an array. ^[link]

   # bad
   paths = [paths] unless paths.is_a? Array
   paths.each { |path| do_something(path) }
  
   # bad (always creates a new Array instance)
   [*paths].each { |path| do_something(path) }
  
   # good (and a bit more readable)
   Array(paths).each { |path| do_something(path) }

• Use ranges or Comparable#between? instead of complex comparison logic when possible. ^[link]

   # bad
   do_something if x >= 1000 && x <= 2000
  
   # good
   do_something if (1000..2000).include?(x)
  
   # good
   do_something if x.between?(1000, 2000)

• Favor the use of predicate methods to explicit comparisons with ==. Numeric comparisons are OK. ^[link]

   # 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

• Don't do explicit non-nil checks unless you're dealing with boolean values. ^[link]

   # 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

• Avoid the use of BEGIN blocks. ^[link]

• Do not use END blocks. Use Kernel#at_exit instead. ^[link]

   # bad
   END { puts 'Goodbye!' }
  
   # good
   at_exit { puts 'Goodbye!' }

• Avoid the use of flip-flops. ^[link]

• Avoid use of nested conditionals for flow of control. ^[link]

  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.

   # bad
   def compute_thing(thing)
   	if thing[:foo]
   		update_with_bar(thing[:foo])
   		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_computething) unless thing[:foo][:bar]
   	partial_compute thing
   end

  Prefer next in loops instead of conditional blocks.

   # 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

• 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]

• 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]

   # bad
   some_hash.count
  
   # good
   some_hash.size

• 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]

   # bad
   all_songs = users.map(&:songs).flatten.uniq
  
   # good
   all_songs = users.flat_map(&:songs).uniq

• Prefer reverse_each to reverse.each because some classes that include Enumerable will provide an efficient implementation. Even in the worst case where a class does not provide a specialized implementation, the general implementation inherited from Enumerable will be at least as efficient as using reverse.each. ^[link]

   # bad
   array.reverse.each { ... }
  
   # good
   array.reverse_each { ... }

The only real difficulties in programming are cache invalidation and naming things.
-- Phil Karlton

• Name identifiers in English. ^[link]

   # 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

• Use snake_case for symbols, methods and variables. ^[link]

   # bad
   :'some symbol'
   :SomeSymbol
   :someSymbol
  
   someVar = 5
   var_10	= 10
  
   def someMethod
   	# some code
   end
  
   def SomeMethod
   	# some code
   end
  
   # good
   :some_symbol
  
   some_var = 5
   var10 = 10
  
   def some_method
   	# some code
   end

• Do not separate numbers from letters on symbols, methods and variables. ^[link]

   # bad
   :some_sym_1
  
   some_var_1 = 1
  
   def some_method_1
   	# some code
   end
  
   # good
   :some_sym1
  
   some_var1 = 1
  
   def some_method1
   	# some code
   end

• Use TamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.) ^[link]

   # bad
   class Someclass
   	# some code
   end
  
   class Some_Class
   	# some code
   end
  
   class SomeXml
   	# some code
   end
  
   class XmlSomething
   	# some code
   end
  
   # good
   class SomeClass
   	# some code
   end
  
   class SomeXML
   	# some code
   end
  
   class XMLSomething
   	# some code
   end

• Use snake_case for naming files, e.g. hello_world.rb. ^[link]

• Use snake_case for naming directories, e.g. lib/hello_world/hello_world.rb. ^[link]

• Aim to have just a single class/module per source file. Name the file name as the class/module, but replacing TitleCase with snake_case. ^[link]

• Use SCREAMING_SNAKE_CASE for other constants. ^[link]

   # bad
   SomeConst = 5
  
   # good
   SOME_CONST = 5

• 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]

• Avoid prefixing predicate methods with the auxiliary verbs such as is, does, or can. These words are redundant and inconsistent with the style of boolean methods in the Ruby core library, such as empty? and include?. ^[link]

   # bad
   class Person
   	def is_tall?
   		true
   	end
  
   	def can_play_basketball?
   		false
   	end
  
   	def does_like_candy?
   		true
   	end
   end
  
   # good
   class Person
   	def tall?
   		true
   	end
  
   	def basketball_player?
   		false
   	end
  
   	def likes_candy?
   		true
   	end
   end

• 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]

   # 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

• Define the non-bang (safe) method in terms of the bang (dangerous) one if possible. ^[link]

   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

• When defining binary operators, name the parameter other(<< and [] are exceptions to the rule, since their semantics are different). ^[link]

   def +(other)
   	# body omitted
   end

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.
-- Steve McConnell

• Write self-documenting code and ignore the rest of this section. Seriously! ^[link]

• Write comments in English. ^[link]

• Use one space between the leading # character of the comment and the text of the comment. ^[link]

• Comments longer than a word are capitalized and use punctuation. Use one space after periods. ^[link]

• Avoid superfluous comments. ^[link]

   # bad
   counter += 1 # Increments counter by one.

• Keep existing comments up-to-date. An outdated comment is worse than no comment at all. ^[link]

Good code is like a good joke: it needs no explanation.
— old programmers maxim, through 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]

• Annotations should usually be written on the line immediately above the relevant code. ^[link]

• The annotation keyword is followed by a colon and a space, then a note describing the problem. ^[link]

• 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]

   def bar
   	sleep 100 # OPTIMIZE
   end

• Use TODO to note missing features or functionality that should be added at a later date. ^[link]

• Use FIXME to note broken code that needs to be fixed. ^[link]

• Use OPTIMIZE to note slow or inefficient code that may cause performance problems. ^[link]

• Use HACK to note code smells where questionable coding practices were used and should be refactored away. ^[link]

• 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]

• Use other custom annotation keywords if it feels appropriate, but be sure to document them in your project's README or similar. ^[link]

• Place magic comments above all code and documentation. Magic comments should only go below shebangs if they are needed in your source file. ^[link]

   # bad
   # Some documentation about Person
   # frozen_string_literal: true
   class Person
   end
  
   # good
   # frozen_string_literal: true
   # Some documentation about Person
   class Person
   end

   # bad
   # frozen_string_literal: true
   #!/usr/bin/env ruby
   App.parse(ARGV)
  
   # good
   #!/usr/bin/env ruby
   # frozen_string_literal: true
   App.parse ARGV

• Use one magic comment per line if you need multiple. ^[link]

   # bad
   # -*- frozen_string_literal: true; encoding: ascii-8bit -*-
  
   # good
   # frozen_string_literal: true
   # encoding: ascii-8bit

• Separate magic comments from code and documentation with a blank line. ^[link]

   # bad
   # frozen_string_literal: true
   # Some documentation for Person
   class Person
   	# Some code
   end
  
   # good
   # frozen_string_literal: true
  
   # Some documentation for Person
   class Person
   	# Some code
   end

• Use a consistent structure in your class definitions. ^[link]

   class Person
   	# extend and include go first
   	extend SomeModule
   	include AnotherModule
  
   	# 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
  
   	# initialization goes between class methods and other instance methods
   	def initialize
   	end
  
   	# followed by other 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

• Split multiple mixins into separate statements. ^[link]

   # bad
   class Person
   	include Foo, Bar
   end
  
   # good
   class Person
   	# multiple mixins go in separate statements
   	include Foo
   	include Bar
   end

• 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]

   # 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

• Prefer classes to modules with only module methods. ^[link]

   # bad
   module SomeModule
   	module_function
  
   	def some_method
   		# body omitted
   	end
  
   	def some_other_method
   		# body omitted
   	end
   end
  
   # good
   class SomeClass
   	def self.some_method
   		# body omitted
   	end
  
   	def self.some_other_method
   		# body omitted
   	end
   end

• When designing class hierarchies make sure that they conform to the Liskov Substitution Principle. ^[link]

• Try to make your classes as SOLID as possible. ^[link]

• Always supply a proper to_s method for classes that represent domain objects. ^[link]

   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

• Use the attr family of functions to define trivial accessors or mutators. ^[link]

   # 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

• For accessors and mutators, avoid prefixing method names with get_ and set_. It is a Ruby convention to use attribute names for accessors (readers) and attr_name= for mutators (writers). ^[link]

   # bad
   class Person
   	def get_name
   		"#{@first_name} #{@last_name}"
   	end
  
   	def set_name(name)
   		@first_name, @last_name = name.split(' ')
   	end
   end
  
   # good
   class Person
   	def name
   		"#{@first_name} #{@last_name}"
   	end
  
   	def name=(name)
   		@first_name, @last_name = name.split ' '
   	end
   end

• Avoid the use of attr. Use attr_reader and attr_accessor instead. ^[link]

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

• Consider using Struct.new, which defines the trivial accessors, constructor and comparison operators for you. ^[link]

   # 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

• 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]

   # bad
   class Person < Struct.new(:first_name, :last_name)
   end
  
   # good
   Person = Struct.new(:first_name, :last_name)

• Consider adding factory methods to provide additional sensible ways to create instances of a particular class. ^[link]

   class Person
   	def self.create(options_hash)
   		# body omitted
   	end
   end

• Prefer duck-typing over inheritance. ^[link]

   # 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

• Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance. ^[link]

   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.

• 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]

• 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]

   class SomeClass
   	def public_method
   		# some code
   	end
  
   	private
  
   	def private_method
   		# some code
   	end
  
   	def another_private_method
   		# some code
   	end
   end

• Use def self.method to define class methods. This makes the code easier to refactor since the class name is not repeated. ^[link]

   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 class methods.
   	class << self
   		def first_method
   			# body omitted
   		end
  
   		def second_method_etc
   			# body omitted
   		end
   	end
   end

• 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]

   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.

   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.

   class Fugitive < Westerner
   	def first_name
   		'Nobody'
   	end
  
   	alias given_name first_name
   end

• 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]

   module Mononymous
   	def self.included(other)
   		other.class_eval { alias_method :full_name, :given_name }
   	end
   end
  
   class Sting < Westerner
   	include Mononymous
   end

• When class (or module) methods call other such methods, omit the use of a leading self or own name followed by a . when calling other such methods. This is often seen in "service classes" or other similar concepts where a class is treated as though it were a function. This convention tends to reduce repetitive boilerplate in such classes. ^[link]

   class TestClass
   	# bad -- more work when class renamed/method moved
   	def self.call(param1, param2)
   		TestClass.new(param1).call(param2)
   	end
  
   	# bad -- more verbose than necessary
   	def self.call(param1, param2)
   		self.new(param1).call(param2)
   	end
  
   	# good
   	def self.call(param1, param2)
   		new(param1).call(param2)
   	end
  
   	# ...other methods...
   end

• Prefer raise over fail for exceptions. ^[link]

   # bad
   fail SomeException, 'message'
  
   # good
   raise SomeException, 'message'

• Don't specify RuntimeError explicitly in the two argument version of raise. ^[link]

   # bad
   raise RuntimeError, 'message'
  
   # good - signals a RuntimeError by default
   raise 'message'

• Prefer supplying an exception class and a message as two separate arguments to raise, instead of an exception instance. ^[link]

   # bad
   raise SomeException.new('message')
   # Note that there is no way to do `raise SomeException.new('message'), backtrace`.
  
   # good
   raise SomeException, 'message'
   # Consistent with `raise SomeException, 'message', backtrace`.

• 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]

   # bad
   def foo
   	raise
   ensure
   	return 'very bad idea'
   end

• Use implicit begin blocks where possible. ^[link]

   # 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

• Mitigate the proliferation of begin blocks by using contingency methods (a term coined by Avdi Grimm). ^[link]

   # 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 }

• Don't suppress exceptions. ^[link]

   # bad
   begin
   	# an exception occurs here
   rescue SomeError
   	# the rescue clause does absolutely nothing
   end
  
   # bad
   do_something rescue nil

• Avoid using rescue in its modifier form. ^[link]

   # 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

• Don't use exceptions for flow of control. ^[link]

   # 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

• Avoid rescuing the Exception class. This will trap signals and calls to exit, requiring you to kill -9 the process. ^[link]

   # 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

• Put more specific exceptions higher up the rescue chain, otherwise they'll never be rescued from. ^[link]

   # bad
   begin
   	# some code
   rescue StandardError => e
   	# some handling
   rescue IOError => e
   	# some handling that will never be executed
   end
  
   # good
   begin
   	# some code
   rescue IOError => e
   	# some handling
   rescue StandardError => e
   	# some handling
   end

• Release external resources obtained by your program in an ensure block. ^[link]

   f = File.open('testfile')
   begin
   	# .. process
   rescue
   	# .. handle error
   ensure
   	f.close if f
   end

• Use versions of resource obtaining methods that do automatic resource cleanup when possible. ^[link]

   # bad - you need to close the file descriptor explicitly
   f = File.open('testfile')
   # some action on the file
   f.close
  
   # good - the file descriptor is closed automatically
   File.open('testfile') do |f|
   	# some action on the file
   end

• Favor the use of exceptions from the standard library over introducing new exception classes. ^[link]

• Prefer literal array and hash creation notation (unless you need to pass parameters to their constructors, that is). ^[link]

   # bad
   arr = Array.new
   hash = Hash.new
  
   # good
   arr = []
   hash = {}

• 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]

   # bad
   STATES = ['draft', 'open', 'closed']
  
   # good
   STATES = %w[draft open closed]

• 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]

   # bad
   STATES = [:draft, :open, :closed]
  
   # good
   STATES = %i[draft open closed]

• Always use a comma after the last item of an Array or Hash literal, especially when the items are on separate lines. ^[link]

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

• Avoid the creation of huge gaps in arrays. ^[link]

   arr = []
   arr[100] = 1 # now you have an array with lots of nils

• When accessing the first or last element from an array, prefer first or last over [0] or [-1]. ^[link]

• 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]

• Prefer symbols instead of strings as hash keys. ^[link]

   # bad
   hash = { 'one' => 1, 'two' => 2, 'three' => 3 }
  
   # good
   hash = { one: 1, two: 2, three: 3 }

• Avoid the use of mutable objects as hash keys. ^[link]

• Use the Ruby 1.9 hash literal syntax when your hash keys are symbols. ^[link]

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

• 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]

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

• Use Hash#has_key? instead of Hash#key? and Hash#has_value? instead of Hash#value?. ^[link]

   # bad
   hash.key?(:test)
   hash.value?(value)
  
   # good
   hash.has_key? :test
   hash.has_value? value

• Use Hash#each_key instead of Hash#keys.each and Hash#each_value instead of Hash#values.each. ^[link]

   # bad
   hash.keys.each { |k| p k }
   hash.values.each { |v| p v }
   hash.each { |k, _v| p k }
   hash.each { |_k, v| p v }
  
   # good
   hash.each_key { |k| p k }
   hash.each_value { |v| p v }

• Use Hash#fetch when dealing with hash keys that should be present. ^[link]

   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)

• Introduce default values for hash keys via Hash#fetch as opposed to using custom logic. ^[link]

   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

• Prefer the use of the block instead of the default value in Hash#fetch if the code that has to be evaluated may have side effects or be expensive. ^[link]

   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, obtain_batman_powers) # obtain_batman_powers is an expensive call
  
   # good - blocks are lazy evaluated, so only triggered in case of KeyError exception
   batman.fetch(:powers) { obtain_batman_powers }

• Use Hash#values_at when you need to retrieve several values consecutively from a hash. ^[link]

   # bad
   email = data['email']
   username = data['nickname']
  
   # good
   email, username = data.values_at('email', 'nickname')

• Rely on the fact that as of Ruby 1.9 hashes are ordered. ^[link]

• Do not modify a collection while traversing it. ^[link]

• 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]

   # bad
   Regexp.last_match[1]
  
   # good
   Regexp.last_match(1)

• 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]

   # 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

• Use Integer check type of an integer number. Since Fixnum is platform-dependent, checking against it will return different results on 32-bit and 64-bit machines. ^[link]

   timestamp = Time.now.to_i
  
   # bad
   timestamp.is_a? Fixnum
   timestamp.is_a? Bignum
  
   # good
   timestamp.is_a? Integer

• Prefer to use ranges when generating random numbers instead of integers with offsets, since it clearly states your intentions. Imagine simulating a role of a dice: ^[link]

   # bad
   rand(6) + 1
  
   # good
   rand(1..6)

• Prefer string interpolation and string formatting instead of string concatenation: ^[link]

   # bad
   email_with_name = user.name + ' <' + user.email + '>'
  
   # better
   email_with_name = "#{user.name} <#{user.email}>"
  
   # best
   email_with_name = format '%s <%s>', user.name, user.email

• Prefer single-quoted strings unless interpolation is employed. ^[link]

   # bad
   name = "Bozhidar"
  
   # good
   name = 'Bozhidar'

• 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]

   # bad
   char = ?c
  
   # good
   char = 'c'

• Don't leave out {} around instance and global variables being interpolated into a string. ^[link]

   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}"

• Don't use Object#to_s on interpolated objects. It's invoked on them automatically. ^[link]

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

• 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]

   # bad
   html = ''
   html += '<h1>Page title</h1>'
  
   paragraphs.each do |paragraph|
   	html += "<p>#{paragraph}</p>"
   end
  
   # good and also fast
   html = ''
   html << '<h1>Page title</h1>'
  
   paragraphs.each do |paragraph|
   	html << "<p>#{paragraph}</p>"
   end

• Don't use String#gsub in scenarios in which you can use a faster more specialized alternative. ^[link]

   url = 'http://example.com'
   str = 'lisp-case-rules'
  
   # bad
   url.gsub('http://', 'https://')
   str.gsub('-', '_')
  
   # good
   url.sub('http://', 'https://')
   str.tr('-', '_')

• 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]

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

• Use Ruby 2.3's squiggly heredocs for nicely indented multi-line strings. ^[link]

   # bad - using Powerpack String#strip_margin
   code = <<-END.strip_margin('|')
   	|def test
   	|	some_method
   	|	other_method
   	|end
   END
  
   # also bad
   code = <<-END
   def test
   	some_method
   	other_method
   end
   END
  
   # good
   code = <<~END
   	def test
   		some_method
   		other_method
   	end
   END

• Prefer Time.now over Time.new when retrieving the current system time. ^[link]

• Don't use DateTime unless you need to account for historical calendar reform -- and if you do, explicitly specify the start argument to clearly state your intentions. ^[link]

   # bad - uses DateTime for current time
   DateTime.now
  
   # good - uses Time for current time
   Time.now
  
   # bad - uses DateTime for modern date
   DateTime.iso8601('2016-06-29')
  
   # good - uses Date for modern date
   Date.iso8601('2016-06-29')
  
   # good - uses DateTime with start argument for historical date
   DateTime.iso8601('1751-04-23', Date::ENGLAND)

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

• Don't use regular expressions if you just need plain text search in string: string['text'] ^[link]

• For simple constructions you can use regexp directly through string index. ^[link]

   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'

• Use non-capturing groups when you don't use the captured result. ^[link]

   # bad
   /(first|second)/
  
   # good
   /(?:first|second)/

• Don't use the cryptic Perl-legacy variables denoting last regexp group matches ($1, $2, etc). Use Regexp.last_match(n) instead. ^[link]

   /(regexp)/ =~ string
   ...
  
   # bad
   process $1
  
   # good
   process Regexp.last_match(1)

• Avoid using numbered groups as it can be hard to track what they contain. Named groups can be used instead. ^[link]

   # bad
   /(regexp)/ =~ string
   # some code
   process Regexp.last_match(1)
  
   # good
   /(?<meaningful_var>regexp)/ =~ string
   # some code
   process meaningful_var

• Character classes have only a few special characters you should care about: ^, -, \, ], so don't escape . or brackets in []. ^[link]

• 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]

   string = "some injection\nusername"
   string[/^username$/]	 # matches
   string[/\Ausername\z/] # doesn't match

• 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]

   regexp = /
   	start				 # some text
   	\s						# white space char
   	(group)			 # first group
   	(?:alt1|alt2) # some alternation
   	end
   /x

• For complex replacements sub/gsub can be used with a block or a hash. ^[link]

   words = 'foo bar'
   words.sub(/f/, 'f' => 'F') # => 'Foo bar'
   words.gsub(/\w+/) { |word| word.capitalize } # => 'Foo Bar'

• 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]

   # 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>)

• Avoid %() or the equivalent %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]

   # 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?"'
   quote = %q(<p class='quote'>"What did you say?"</p>)

• Use %r only for regular expressions matching at least one '/' character. ^[link]

   # bad
   %r{\s+}
  
   # good
   %r{^/(.*)$}
   %r{^/blog/2011/(.*)$}

• Always use %x instead of backticks. ^[link]

   # bad
   date = `date`
   echo = %x(echo `date`)
  
   # good
   date = %x(date)

• 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]

• Use the braces that are the most appropriate for the various kinds of percent literals. ^[link]

  • () for string literals(%q, %Q).
  • [] for array literals(%w, %i, %W, %I) as it is aligned with the standard array literals.
  • {} for regexp literals(%r) since parentheses often appear inside regular expressions. That's why a less common character with { is usually the best delimiter for %r literals.
  • () for all other literals (e.g. %s, %x)

   # bad
   %q{"Test's king!", John said.}
  
   # good
   %q("Test's king!", John said.)
  
   # bad
   %w(one two three)
   %i(one two three)
  
   # good
   %w[one two three]
   %i[one two three]
  
   # bad
   %r((\w+)-(\d+))
   %r{\w{1,2}\d{2,5}}
  
   # good
   %r{(\w+)-(\d+)}
   %r|\w{1,2}\d{2,5}|

• Avoid needless metaprogramming. ^[link]

• Do not mess around in core classes when writing libraries. (Do not monkey-patch them.) ^[link]

• The block form of class_eval is preferable to the string-interpolated form. ^[link]

  • when you use the string-interpolated form, always supply __FILE__ and __LINE__, so that your backtraces make sense:

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

  • define_method is preferable to class_eval{ def ... }

• 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]

   # 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

• 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]

  • Be sure to also define respond_to_missing?

  • 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:

     # 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

• Prefer public_send over send so as not to circumvent private/protected visibility. ^[link]

   # We have	an ActiveModel Organization that includes concern Activatable
   module Activatable
   	extend ActiveSupport::Concern
  
   	included do
   		before_create :create_token
   	end
  
   	private
  
   	def reset_token
   		# some code
   	end
  
   	def create_token
   		# some code
   	end
  
   	def activate!
   		# some code
   	end
   end
  
   class Organization < ActiveRecord::Base
   	include Activatable
   end
  
   linux_organization = Organization.find(...)
   # BAD - violates privacy
   linux_organization.send(:reset_token)
   # GOOD - should throw an exception
   linux_organization.public_send(:reset_token)

• Prefer __send__ over send, as send may overlap with existing methods. ^[link]

   require 'socket'
  
   u1 = UDPSocket.new
   u1.bind('127.0.0.1', 4913)
   u2 = UDPSocket.new
   u2.connect('127.0.0.1', 4913)
   # Won't send a message to the receiver obj.
   # Instead it will send a message via UDP socket.
   u2.send :sleep, 0
   # Will actually send a message to the receiver obj.
   u2.__send__ ...

• Write ruby -w safe code. ^[link]

• Avoid hashes as optional parameters. Does the method do too much? (Object initializers are exceptions for this rule). ^[link]

• Avoid methods longer than 20 LOC (lines of code). Ideally, most methods will be shorter than 10 LOC. Empty lines do not contribute to the relevant LOC. ^[link]

• Avoid parameter lists longer than three or four parameters. ^[link]

• If you really need "global" methods, add them to Kernel and make them private. ^[link]

• Use module instance variables instead of global variables. ^[link]

   # bad
   $foo_bar = 1
  
   # good
   module Foo
   	class << self
   		attr_accessor :bar
   	end
   end
  
   Foo.bar = 1

• Use OptionParser for parsing complex command line options and ruby -s for trivial command line options. ^[link]

• Code in a functional way, avoiding mutation when that makes sense. ^[link]

• Do not mutate parameters unless that is the purpose of the method. ^[link]

• Avoid more than three levels of block nesting. ^[link]

• Be consistent. In an ideal world, be consistent with these guidelines. ^[link]

• Use common sense. ^[link]