Abstract

... 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)

This document was created when I, as the Technical Lead of the company which I work for, was asked by the CTO to create some internal documents describing good style and best practices for Ruby programming. I started off by building upon this existing style guide, since I concurred with most of the points in it. I hope it will be useful to other people as well and I hope that I'll get a lot of feedback and suggestions on how to improve the guide for the benefit of the entire Ruby community.

Formatting

• Use UTF-8 as the source file encoding.

• Use two-space indent, no tabs. (Your editor/IDE should have a setting to help you with that.)

• Use Unix-style line endings. (Linux/OSX users are covered by default, Windows users have to be extra careful.)

  • If you're using Git you might want to do $ git config --global core.autocrlf true to protect your project from Windows line endings creeping into your project.

• Use spaces around operators, after commas, colons and semicolons, around { and before }.

  sum = 1 + 2
  a, b = 1, 2
  1 > 2 ? true : false; puts "Hi"
  [1, 2, 3].each { |e| puts e }

• No spaces after (, [ or before ], ).

  some(arg).other
  [1, 2, 3].length

• Indent when as deep as case. (As suggested in the Pickaxe.)

  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
  
  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

• Use an empty line before the return value of a method (unless it only has one line), and an empty line between defs.

  def some_method
    do_something
    do_something_else
  
    result
  end
  
  def some_method
    result
  end

• Use RDoc and its conventions for API documentation. Don't put an empty line between the comment block and the def.

• Use empty lines to break up a method into logical paragraphs.

• Keep lines fewer than 80 characters. (Emacs users should really have a look at whitespace-mode.)

• Avoid trailing whitespace. (Emacs users: Whitespace-mode again comes to the rescue.)

Syntax

• Use def with parentheses when there are arguments. Omit the parentheses when the method doesn't accept any arguments.

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

• Never use for, unless you know exactly why. Most of the time iterators should be used instead.

  arr = [1, 2, 3]
  
  # bad
  for elem in arr do
    puts elem
  end
  
  # good
  arr.each { |elem| puts elem }

• Never use then for multi-line if/unless.

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

• Favor the ternary operator over if/then/else/end constructs. It's more common and obviously more concise.

  # 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.

  # 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

• Never use if x; ... - it is deprecated in Ruby 1.9. Use the ternary operator instead.

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

• Use when x then ... for one-line cases. The alternative syntax when x; ... is deprecated in Ruby 1.9.

• Use &&/|| for boolean expressions, and/or for control flow. (Rule of thumb: If you have to use outer parentheses, you are using the wrong operators.)

  # boolean expression
  if some_condition && some_other_condition
    do_something
  end
  
  # control flow
  document.saved? or document.save!

• Avoid multi-line ?: (the ternary operator), use if/unless instead.

• Favor modifier if/unless usage when you have a single-line body. Another good alternative is the usage of control flow and/or.

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

• Favor unless over if for negative conditions (or control flow or).

  # bad
  do_something if !some_condition
  
  # good
  do_something unless some_condition
  
  # another good option
  some_condition or do_something

• Never use unless with else. Rewrite these with the positive case first.

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

• Suppress superfluous parentheses when calling methods, but keep them when calling "functions", i.e. when you use the return value in the same line.

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

• Prefer {...} over do...end for single-line blocks. Avoid using {...} for multi-line blocks. Always use do...end for "control flow" and "method definitions" (e.g. in Rakefiles and certain DSLs.) Avoid do...end when chaining.

• Avoid return where not required.

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

• Avoid line continuation (\) where not required. In practice, avoid using line continuations at all.

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

• Using the return value of = is ok.

  if v = array.grep(/foo/) ...

• Use ||= freely.

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

• Avoid using Perl-style special variables (like $0-9, $`, ...).

• Out-dent public, protected, and private keywords by two spaces, and leave one blank line above and below.

  class SomeClass
    def public_method
      # ...
    end
  
    private
  
      def private_method
        # ...
      end
  end

• Never put a space between a method name and the opening parenthesis.

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

• 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).

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

Naming

• Use snake_case for methods and variables.

• Use CamelCase for classes and modules. (Keep acronyms like HTTP, RFC, XML uppercase.)

• Use SCREAMING_SNAKE_CASE for other constants.

• The names of predicate methods (methods that return a boolean value) should end in a question mark. (i.e. Array#empty?).

• The names of potentially "dangerous" methods (i.e. methods that modify self or the arguments, exit!, etc.) should end with an exclamation mark.

• The length of an identifier determines its scope. Use one-letter variables for short block/method parameters, according to this scheme:

    a,b,c: any object
    d: directory names
    e: elements of an Enumerable
    ex: rescued exceptions
    f: files and file names
    i,j: indexes
    k: the key part of a hash entry
    m: methods
    o: any object
    r: return values of short methods
    s: strings
    v: any value
    v: the value part of a hash entry
    x,y,z: numbers
  

  And in general, the first letter of the class name if all objects are of that type.

• When using inject with short blocks, name the arguments |a, e| (accumulator, element).

• When defining binary operators, name the argument other.

  def +(other)
    # body omitted
  end

• Prefer map over collect, find over detect, select over find_all, size over length. This is not a hard requirement; if the use of the alias enhances readability, it's ok to use it.

Comments

• Write self-documenting code and ignore the rest of this section. "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)

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

• Avoid superfluous comments.

  # bad
  counter += 1 # increments counter by one

• Keep existing comments up-to-date. No comment is better than an outdated comment.

• Avoid writing comments to explain bad code. Try to refactor the code to make it self-explanatory.

Annotations

• Annotations should usually be written on the line immediately above the relevant code.

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

• If multiple lines are required to describe the problem, subsequent lines should be indented two spaces after the #.

  def bar
    # FIXME: This has crashed occasionally since v3.2.1. It may
    #   be related to the BarBazUtil upgrade.
    baz(:quux)
  end

• 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.

  def bar
    sleep 100 # OPTIMIZE
  end

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

• Use FIXME to note broken code that needs to be fixed.

• Use OPTIMIZE to note slow or inefficient code that may cause performance problems.

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

• 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?

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

Classes

• Always supply a proper to_s method.
• Use the attr family of functions to define trivial accessors or mutators.
• Consider adding factory methods to provide additional sensible ways to create instances of a particular class.
• Prefer duck-typing over inheritance.
• Avoid the usage of class (@@) variables due to their "nasty" behavior in inheritance.
• 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.

Exceptions

• Don't suppress exceptions.
• Don't use exceptions for flow of control.
• Avoid rescuing the Exception class.

Strings

• Prefer string interpolation instead of string concatenation:

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

• Prefer single-quoted strings when you don't need string interpolation or special symbols such as "\t", "\n", etc.

• 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.

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

Percent Literals

• Use %w freely.

  STATES = %w(draft open closed)

• Use %() for single-line strings which require both interpolation and embedded double-quotes. For multi-line strings, prefer heredocs.

  # 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>)

• Use %r only for regular expressions matching more than one '/' character.

  # bad
  %r(\s+)
  
  # still bad
  %r(^/(.*)$)
  # should be /^\/(.*)$/
  
  # good
  %r(^/blog/2011/(.*)$)

• Avoid %q, %Q, %x, %s, and %W.

• Prefer () as delimiters for all % literals.

Misc

• Write ruby -w safe code.

• Avoid hashes as optional parameters. Does the method do too much?

• 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.

• Avoid parameter lists longer than three or four parameters.

• Use def self.method to define singleton methods. This makes the methods more resistant to refactoring changes.

  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

• If you really have to, add "global" methods to Kernel and make them private.

• Use class instance variables instead of global variables.

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

• Avoid alias when alias_method will do.

• Use OptionParser for parsing complex command line options and ruby -s for trivial command line options.

• Write for Ruby 1.9. Don't use legacy Ruby 1.8 constructs.

  • Use the new JavaScript literal hash syntax.

  • Use the new lambda syntax.

  • Methods like inject now accept method names as arguments.

    [1, 2, 3].inject(:+)

• Avoid needless metaprogramming.

Design

• Code in a functional way, avoiding mutation when that makes sense.
• Do not mutate arguments unless that is the purpose of the method.
• Do not mess around in core classes when writing libraries. (Do not monkey patch them.)
• Do not program defensively.
• Keep the code simple and subjective. Each method should have a single, well-defined responsibility.
• Avoid more than three levels of block nesting.
• Don't overdesign. Overly complex solutions tend to be brittle and hard to maintain.
• Don't underdesign. A solution to a problem should be as simple as possible, but no simpler than that. Poor initial design can lead to a lot of problems in the future.
• Be consistent. In an ideal world, be consistent with these guidelines.
• Use common sense.

Contributing

Nothing written in this guide is set in stone. It's my desire to work together with everyone interested in Ruby coding style, so that we could ultimately create a resource that will be beneficial to the entire Ruby community.

Feel free to open tickets or send pull requests with improvements. Thanks in advance for your help!