Ruby Style Guide

The personal style guide of Xavier Shay. Any potential exceptions must be justified and documented. Code examples can be revealed by clicking ↩ , and these imply more rules than are explicitly labeled. Rules that I often use editor features or scripts to apply are notated with ⚙ and a link to details.

Older code bases should migrate to this style, rather than stay consistent with themselves. Style upgrades should be in separate commits from behaviour changes.

The canonical URL for this document is http://xaviershay.com/ruby-style-guide.

Arrays

Use a trailing comma on final entry of multi-line definitions.

right = [
  1,
  2,
]

wrong = [
  1,
  2
]

Prefer %w[] for construction of string arrays. It is easier to write, and harder to mess up syntactically.

right = %w[apples oranges]
wrong = %w(apples oranges)
wrong = ['apples', 'oranges']

Prefer %i[] for construction of symbol arrays.

right = %i[apples oranges grapes mangoes]
wrong = [:apples, :oranges, :grapes, :mangoes]

Assignment

Prefer using instance variables rather than writer methods in constructors.

class Right
  attr_writer :x, :y

  def initialize
    @x = []
    @y = []
  end
end

class Wrong
  attr_writer :x, :y

  def initialize
    self.x = []
    self.y = []
  end
end

Prefer separating assignment from conditionals.

right = true
42 if right

42 if wrong = true

If using an assignment as a boolean value in a conditional, enclose in parentheses to indicate it's not a = vs == typo.

if (right = big_calculation)
if wrong = big_calculation
if right == big_calculation

Only use parallel assignment for short variable names or when splitting the return value of a method.

x, y = "right", 42
long_name_one, long_name_two = right_method

long_name_one, long_name_two = "wrong", 0

Blocks and Procs

Use curly-braces when block is being chained.

right = [].map {|x|
  x == 2
}.size

wrong = [].map do |x|
  x == 2
end.size

Use stabby syntax with parenthesis.

right = ->(x) { x }
wrong = -> x { x }
wrong = lambda {|x| x }

Omit parenthesis for zero-argument lambdas.

right = ->{ }
wrong = ->() { }

Prefer Symbol#to_proc (the &preztel operator) where applicable.

right = [].map(&:length)
wrong = [].map {|x| x.length }

Use .() to call.

right = ->{ }.()
wrong = ->{ }.call
wrong = ->{ }[]

Classes

Assign empty subclasses rather than inheriting.

Right = Class.new(StandardError)
class Wrong < StandardError; end

Assign struct subclasses so that they can be re-opened.

Right = Struct.new(:value) do
  def squared; value * value end
end

class Wrong < Struct.new(:value)
  def squared; value * value end
end

Alias [] to new for value objects.

right = Number[3]
wrong = Number.new(3)

Nest namespaced definitions.

class Namespace
  class Right; end
end

class Namespace::Wrong; end

Conditionals

Only use ternary operator for very short conditionals. When in doubt, avoid.

right = flag ? 42 : 36
right = if flag && other_flag
  long_name_one
else
  long_name_two
end

wrong = flag && other_flag ? long_name_one : long_name_two

Use && and || in preference to and and or. The latter have looser precedence and often lead to unexpected bugs.

right if a && b
wrong if a and b

Dependencies

Explicitly require third-party code in each file it is used.
Prefer constant stubbing when depending on code that you own.

def my_method; MyCollaborater.does_something end

it do
  collaborator = class_double("MyCollaborator").as_stubbed_const
  collaborator.should_receive(:does_something)

  my_method
end

Prefer constructor or parameter injection when depending on code you do not own. When providing code to others, include fake versions of your classes.

def my_method(client = Service::Client); client.echo('hello') end

it do
  client = Service::FakeClient.new
  my_method(client)

  expect(client.echoes).to eq(['hello'])
end

Hashes

Use a trailing comma on final entry of multi-line definitions.

right = {
  a: 1,
  b: 2,
}

wrong = {
  a: 1,
  b: 2
}

Prefer 1.9 syntax, though stay consistent within a single definition.

right = {
  :a   => 1,
  'ab' => 2,
}

wrong = {
  a: 1,
  'abc' => 2,
}

Construct from an array using each_with_object.

right = (1..10).each_with_object({}) {|x, h| h[x] = x ** 2 }
wrong = Hash[(1..10).map {|x| [x, x ** 2] }]
wrong = (1..10).reduce({}) {|h, x| h.update(x => x ** 2) }

Omit {} when passing as the last argument to a method.

right(a: 1)
wrong({a: 1})

Prefer fetch for providing default values.

right = {}.fetch(:a, 0)
wrong = {}[:a] || 0

Enumeration

Prefer lisp-style enumeration methods.

right.map     { ... }
wrong.collect { ... }

right.reduce { ... }
wrong.inject { ... }

right.find   { ... }
wrong.detect { ... }

right.select   { ... } # Select is an exception for symmetry with `reject`.
wrong.find_all { ... }

right.reject { ... }
wrong.there_is_no_wrong_way_to_reject { ... }

Line Length

80 characters is good enough for anyone. ⚙
Long URLs are exempt, though consider shortening them.
Line up method arguments when they do not fit on one line.

def right_method(long_param_one,
                 long_param_two,
                 long_param_three,
                 long_param_four)
  42
end

def wrong_method(long_param_one, long_param_two, long_param_three, long_param_four)
  42
end

Squeeze multiline strings if you need to remove newlines.

<<-EOS.strip.gsub(/\s+/, ' ')
  But we refuse to believe that the bank of justice is bankrupt. We refuse to
  believe that there are insufficient funds in the great vaults of opportunity
  of this nation. And so we have come to cash this check, a check that will
  give us upon demand the riches of freedom and the security of justice.
EOS

Use sprintf (the % operator) to interpolate variables into long strings.

right = "%s thinks that %s should %s" % [
  long_variable_one,
  long_variable_two,
  long_variable_three
]

wrong = "#{long_variable_one} thinks that #{long_variable_two} should #{long_variable_three}"

One line per method for long chains.

right = (1..10)
  .map {|x| x + 1 }
  .select(&:odd?)

wrong = (1..10).map {|x| x + 1 }.select(&:odd?)

Methods

Use parentheses to enclose parameters in method definitions.

def right(*args)
end

def wrong *args
end

Use a single line for blocks of trivial methods.

def top;    @top    ||= @rect.top    end
def right;  @right  ||= @rect.right  end
def bottom; @bottom ||= @rect.bottom end
def left;   @left   ||= @rect.left   end

def wrong
  @wrong ||= []
end

Use _ for ignored arguments.

def right(x, _); x end
def wrong(x, y); x end

Use * when all arguments are ignored.

def right(*); 42 end
def wrong(_, _); 42 end

Prefer using parentheses to send messages with arguments.

object.right(17, 23)
object.wrong 17, 23

Uses parentheses when using return value of a call.

puts right(42)
puts wrong 42

Omit parentheses when message has no arguments.

object.right
object.wrong()

Omit parentheses for DSLs when the return value is not used.

do_the right thing
do_the(wrong thing)

puts :right
puts(:ymmv)

Naming

Use short variable names for simple and obvious blocks.

right = bird_names.map {|x| x.to_s.length }
wrong = bird_names.map {|bird_name| bird_name.to_s.length }

Use short variable names for math.

right = v * t + 0.5 * a * t ** 2
wrong = velocity * time + 0.5 * acceleration * time ** 2

Use ? suffix for methods rather than has_ or is_ prefix.

def valid?; "right" end
def is_valid?; "wrong" end
def has_valid?; "wrong" end

Use modules or classes for namespacing rather than suffixes, except when necessarily following Rails conventions.

Example::Right
WrongExample

Avoid double namespacing.

right = bird.name
wrong = bird.bird_name

Nil values

Prefer || to ternary for default values.

right(possibly_nil_value || default_value)
wrong(possibly_nil_value ? possibly_nil_value : default_value)

Prefer ||= for memoization.

right ||= big_calculation
wrong = big_calculation unless wrong

Prefer && for nil guards.

right = object && object.name
wrong = object ? object.name : nil
wrong = object.name if object

Regexes

Make with %r{} when escaping of slashes would otherwise be required.

right = %r{/some/path}
wrong = /\/some\/path/

Use \A and \z rather than ^ and $ for start and end of line matching.

right = "hello\nthere" =~ /\Ahello\z/
wrong = "hello\nthere" =~ /^hello$/

Use String#[] for extraction.

right = "http://example.com"[%r{\A(http(s)?)://}, 1]
wrong = %r{(http(s)?)://}.match("http://")[1]

Whitespace

No trailing whitespace. ⚙
Two space indentation.

def my_method
  if rand < 0.5
    if rand > 0.5
      42
    end
  end
end

No indent after private or protected modifiers.

class Right
  private

  def my_method; 42 end
end

class Wrong
  private

    def my_method; 42 end
end

Align if blocks with left indent.

right = if rand < 0.5
  0
else
  1
end

wrong = if rand < 0.5
          0
        else
          1
        end

Align 1.8 style hashes along hash rocket. ⚙

right = {
  'a'   => 1,
  'ab'  => 2,
  'abc' => 3,
}

wrong = {
  'a' => 1,
  'ab' => 2,
  'abc' => 3,
}

Align 1.9 style hashes along right-hand side. ⚙

right = {
  a:   1,
  ab:  2,
  abc: 3,
}

wrong = {
  a: 1,
  ab: 2,
  abc: 3,
}

Align assignment along equals sign. ⚙

a  = "right"
ab = "right"

a = "wrong"
ab = "wrong"

No space between block brace and parameters.

right = [].map {|x| x }
wrong = [].map { |x| x }

No space between method definition name, argument, and bracket.

def right(x); end
def wrong (x); end
def wrong( x ); end

No space between method call name, argument, and bracket.

right(42)
wrong (42)
wrong( 42 )