Ruby Coding Guidelines

Formatting

Use UTF-8 as the source file encoding.
Use 2 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 this $ 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, before { and }.

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 exactly know 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 multiline if/unless.

# bad
if x.odd? then
  puts "odd"
end

# good
if x.odd?
  puts "odd"
end

Use when x; ... for one-line cases.
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.)
Avoid multiline ?: (the ternary operator), use if/unless instead.

Favor modifier if/unless usage when you have a single-line body.

# bad
if some_condition
  do_something
end

# good
do_something if some_condition

# another good option
some_condition && do_something

Favor unless over if for negative conditions:

# bad
do_something if !some_condition

# good
do_something unless some_condition

# another good option
some_condition || do_something

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. Multiline {...} is fine: having different statement endings (} for blocks, end for if/while/...) makes it easier to see what ends where. But 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 okay:
```
if v = array.grep(/foo/) ...
```

Use ||= freely.

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

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

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 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| (mnemonic: accumulator, element)
When defining binary operators, name the argument "other".
```
def +(other)
  # body omitted
end
```
Prefer map over collect, find over detect, find_all over select, size over length. This is not a hard requirement, though - 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 two spaces 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.

Misc

Write ruby -w safe code.
Avoid hashes-as-optional-parameters. Does the method do too much?
Avoid long methods (longer than 10 LOC). Ideally most methods will be shorter than 5 LOC. Empty line do not contribute to the relevant LOC.
Avoid long parameter lists (more than 3-4 params).

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

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

  # good
  def self.some_other_method
    # body omitted
  end
end

Add "global" methods to Kernel (if you have to) and make them private.
Avoid alias when alias_method will do.
Use OptionParser for parsing complex command line options and *ruby -s for trivial command line options.
Avoid needless metaprogramming.

Design

Code in a functional way, avoid mutation when it 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. See this article for more details.
Keep the code simple (subjective, but still...). Each method should have a single well-defined responsibility.
Avoid more than 3 Level 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 it should not be 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 the points listed here in this guidelines.
Use common sense.

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

(forked from https://github.com/bbatsov/ruby-style-guide/)

antonrogov/gist:1216347